After A Decade, Time To Rethink Microsoft’s Vulnerability Ratings?

Security Experts will tell you that one year is a lifetime in the world of online threats and attacks. But eight years after Microsoft introduced its innovative severity rating system for software vulnerability, the company says its original definitions of what makes a software hole important still apply. Security experts aren’t so sure.

Security Experts will tell you that one year is a lifetime in the world of online threats and attacks. But eight years after Microsoft introduced its innovative severity rating system for software vulnerability, the company says its original definitions of what makes a software hole important still apply. Security experts aren’t so sure. Microsoft today released its final monthly patch release of 2010. The Redmond software giant went out in style with a large release of 17 bulletins containing fixes for some 40 vulnerabilities in Microsoft Windows, Office, Internet Explorer, SharePoint and Exchange.

The “Patch Tuesday” release will set off a scramble within enterprises large and small to sort out which patches are the most important to apply, and then to install them on affected systems. As they’ve done for almost a decade, IT pros will rely on Microsoft’s Severity Rating System for software vulnerabilities to help answer those questions. But are the labels that Microsoft puts on its security bulletins – ratings that declare patches “critical,” “important,” “moderate” or “low” – accurate? And does Microsoft’s definition of what makes a security hole “severe” still reflect the computer security threats facing organizations?

With the Severity Rating system entering its tenth year, Threatpost decided to take a fresh look at the way Microsoft ranks security holes in its products.  We spoke with Microsoft’s Security Response Center and interviewed some of the top vulnerability researchers and security experts in the industry to get their thoughts on the strengths and weaknesses of Microsoft’s current rating system. We wanted to know whether changes are needed to the way Microsoft rates security holes in its products. Here’s who we talked to, and here are their five recommendations for making Microsoft’s Severity Ratings system better:

 

Recommendation 1: Ditch Outdated Assumptions

Recommendation 2: Make Exploitability Count

Recommendation 3: Get with the (CVSS) Program

Recommendation 4: Embrace Complexity

Recommendation 5: Last, Do No Harm

Who we talked to

  • Cesar Cerrudo: Cesar is the co-founder and CEO of Argeniss Information Security & Software in Parana, Argentina. Read more about Cesar.
  • Jeremiah Grossman: Jeremiah is the founder and Chief Technology Officer of WhiteHat Security, a Web application security testing firm. Read more about Jeremiah.
  • Charlie Miller: Charlie is a Principal Analyst for Software  Security with the consulting firm Independent Security Evaluators. Read more about Charlie. Read more about Charlie.
  • Chris Wysopal: Chris is Chief Technology Officer at Veracode, a software application testing firm. Read more about Chris.

 

Ditch “Outdated Assumptions”

The experts we consulted about the current state of Microsoft’s Severity Rating System mostly had good things to say. But all agreed that the system is due for renovation and that there was room for improvement to the definitions that describe the severity of vulnerabilities.
First and foremost, the experts we consulted agreed, Microsoft’s Severity Ratings are based on what most agreed were “outdated” assumptions about threats and attacks.
“The threat space has changed greatly in the last eight years,” wrote Chris Wysopal, CTO at Veracode. First, attacks shifted away from worms and viruses – noisy and intrusive threats that were easy to isolate. This, Wysopal notes, reflected shift in the motivation of malware authors from “proving a point” to “monetizing vulnerable systems.” “If you are monetizing vulnerable systems you don’t want to do anything to call attention to yourself, especially with the network traffic a worm generates.”
 The second big change has been in the kinds of attacks Microsoft‘s customers are being targeted with. Within the past couple years, those have shifted from a focus on core Windows components and vulnerable networks to what Wysopal described as a “target rich environment” in client software such as Adobe’s Acrobat Reader and Web browsers like Internet Explorer and Firefox top threats. 
Alas, these important changes aren’t reflected in Microsoft’s Severity Rating System which, reflecting the time at which it was created, still considers “Critical” holes as those that, when exploited, “could allow the propagation of an Internet worm without user action.” “Important” vulnerabilities – the next level down – could be any vulnerability “whose exploitation could result in compromise of the confidentiality, integrity, or availability of user’s data, or of the integrity or availability of processing resources.”
The problem isn’t so much that worms don’t matter, anymore. It’s that they’re no longer as big of a concern as they were in 2002 and, by no means, are they the only concern.
Charlie Miller, Principal Analyst at consulting firm Independent Security Evaluators, observed that few companies or consumers directly connect their computers to the Internet these days, which has reduced the impact of worms. Even lowly home routers are remarkably effective at stopping worms, even though they weren’t designed for that purpose, Miller said.
Add it all up and it means that worms – while still a threat – don’t eclipse other dangers in the way they might have eight or ten years ago.
“Worms and widespread malware are indeed a health problem for the Internet ecosystem,” wrote Wysopal of Veracode. “They are like epidemics that should be treated (as such) even if they are rare.  But since they are rare they shouldn’t take up the whole top ranking of the severity rating system anymore.”
Cesar Cerrudo, CEO of Argeniss Information Security and Software, agreed that Microsoft’s definition of a “Critical” vulnerability no longer jives with the threats and attacks that organizations are contending with. “For most ‘critical’ vulnerabilities right now, the exploitation consists of a user visiting a Web page or opening a (Microsoft) Office document, etc….There aren’t Internet worms being propagated,” Cerrudo wrote.

Broadening the concept of severity to pull in more factors than just the ability to create worms is key to modernizing the Severity Rating System, our experts agreed.

< back | next: make exploitability count >

  Make  Exploitability Count

How might the idea of Severity be broadened? One idea all our experts agreed on was the need to make the ease with which vulnerability can be exploited an element of the severity rating. While “wormability” matters, the ease with which vulnerability might produce a self-replicating threat is just one factor, behind the ease with which any hole might be used – “exploited” – to take control of a remote system.
Cerrudo,  who has discovered prominent holes in Microsoft products including Windows and SQL Server, said the increasing use of exploits for previously unknown (or “zero day”) vulnerabilities makes exploitability more relevant now than it was ten years ago, but that Microsoft’s existing Severity Rating System fails to sufficiently account for it.
“Critical for me should be any vulnerability that can be exploited remotely without needing user name and password and with minimal user interaction,” he wrote. ”Important should be used for vulnerabilities that require local access or remote access with a user name and password, or that require complex user interaction or specific configurations to work.”
Miller agrees.
“I think it’s important to …make a distinction between those vulnerabilities that are known to have an exploit ‘in the wild’ and those that are just known vulnerabilities,” he wrote
That might sound like common sense. But, as it stands, Microsoft tracks vulnerability severity and exploitability separately, according to Jerry Bryan, Group Manager for Response Communications at Microsoft.
Severity is a measure of what Microsoft calls the “maximum impact” of vulnerability. That’s defined as “the highest impact vulnerability may have.” Bryan described “maximum impact” as a way of making sure that Microsoft errs on the conservative side with its Severity Ratings. For example, Bryan explained in an email, if a vulnerability could be used for remote code execution, but might also cause a denial of service, Microsoft would consider the more severe case (remote code execution) when assigning a severity rating (in this case, a critical rating).
Information on how easy it is for attacker to use (or exploit) security vulnerabilities, however, is tracked in what Microsoft calls the ”Exploitability Index” a separate rating system introduced in 2008 in response to customer requests for more context around vulnerabilities. The Exploitability Index measures the ease with which a vulnerability could be exploited, on a scale from 1 (“consistent exploit code likely”) to 3 (“functioning exploit code unlikely”).
“Together, these two pieces of information help organizations prioritize the deployment of Microsoft updates for their environment by indicating the risk of an issue being exploited and the impact if the exploit is successful,” Bryan said in a statement.
Of course, how “easy” something is to exploit is a subjective measure that’s based, in part, on the researcher’s level of skill and expertise, Miller and others acknowledge. Other vulnerability scoring systems, notably the Common Vulnerability Scoring System (CVSS) that is maintained by the Forum of Incident Response and Security Teams (FIRST) blend exploitability data with other factors, including environmental considerations and “impact” – the way the vulnerability might expose sensitive data or be used to compromise or disable key systems. While not exact, the elements of CVSS ratings are simple enough to allow the creation of automated tools (http://nvd.nist.gov/cvss.cfm?calculator&version=2) for generating consistent severity scores as well as impact and exploitability sub scores.
That’s not a bad model for Microsoft, says Miller.
Sub ratings that indicated whether a vulnerability didn’t require user interaction to trigger and for which a public exploit was circulating, or – barring a public exploit – whether “mad skillz” would be required to create one or  ”my grandma could write the exploit,” would help shade an individual vulnerability in a way that would be useful for IT pros, Miller said.

< back | next: get with the (CVSS) program >

Get with the (CVSS) Program

One problem with Microsoft’s Severity Rating system is that the system, itself, is proprietary and doesn’t play nice with other vendor-neutral industry standards.  Bulletins might link out to independent vulnerability tracking services like MITRE’s Common Vulnerabilities and Exposures List, but don’t necessarily track to them.
At the same time, IT professionals have a wealth of information from the company on any vulnerability, but have to cobble it together from different sources, including advance bulletin notifications, security advisories, the MSRC blog, Microsoft’s exploitability index, and so on.
That complexity is enough that popular understanding of the Severity Ratings has drifted from their actual definitions. After almost a decade, he argues, the Ratings have become little more than “rules of thumb” internalized by harried IT administrators rather than pinpoint accurate declarations about the potential harm that could result from a specific hole going unpatched, Cerrudo said.
“I think what Microsoft has accomplished over the years (is) people associating a ‘Critical’ severity rating with ‘very dangerous,’ ‘patch should be applied ASAP,’ (and) ‘I’m going to be hacked if I don’t patch,’” Cerrudo wrote.  However, lower ratings tend to fall off the map. “Important severity level means: ‘I can patch later’, ‘There are more critical things I should care about.’” Moderate and Low vulnerabilities, he said, often are interpreted as ‘We will patch someday’ or ‘Not dangerous at all,’ Cerrudo wrote.
And that becomes problematic when “Important” vulnerabilities are understood to be holes that lack urgency, even though a close reading would show that they could facilitate remote compromise of vulnerable systems.
As an example of just this problem, Wysopal notes MS10-088, a recent Microsoft bulletin issued regarding two recent Powerpoint vulnerabilities. Microsoft assigned an “Important” severity rating to the bulletin, despite the fact that they affected a common component of the widely used Office Suite to the hole and that each of the vulnerabilities that were identified would allow an attacker to “take complete control of an affected system…” installing their own programs or user accounts and modifying, copying or deleting data.  Both could also be leveraged in Web based attacks by “a Web site that contains a Web page that is used to exploit this vulnerability” or “Web sites and Web sites that accept or host user-provided content… (and) contain specially crafted content that could exploit this vulnerability.”
CVSS, on the other hand, rates the MS10-088 vulnerabilities “HIGH” with a score of 9.3 out of 10 – with an exploitability sub score of 8.6 out of 10 and impact subscore of 10 out of 10 contributing to the overall rating.  In that case, as others, Wysopal argues, Microsoft’s bias towards “wormability” dilutes the severity of vulnerabilities that are the most commonly used tools for compromising individuals and organizations today: those sent via the Web and aimed at client side software. 
Wysopal suggests that Microsoft ditch its proprietary severity rating system altogether in favor of the CVSS system, which he calls a “more realistic and common standard” that has also been adopted as the basis for NIST’s National Vulnerability Database (NVD)( http://nvd.nist.gov/).

< back | next: embrace complexity >

Embrace Complexity

Ultimately, any kind of rating system will oversimplify the complex issues that surround vulnerabilities, but researchers say that’s OK. In fact, Microsoft needs to embrace complexity if it is to reform its existing and flawed Severity Rating System.
“If I were writing a rating system, it would be much more refined,” wrote Miller. “As someone who finds bugs for a living, I have to assess the ease of exploitation of bugs all the time, and it is difficult, and subjective, but I have to do it in order to move on.” 
Microsoft could help by providing more contexts for exploitability that helps shade different vulnerabilities and also supports the ratings that are given.
“Consider the difference between an off-by-one error that allows you to write a null one byte past a buffer compared with a bug that allows you to write an arbitrary amount of arbitrary data past a buffer,” Miller offered, as an example. “I think it’d be really interesting if they tried to make a scale from 1-10 based on this criteria,” he said.
Jeremiah Grossman, of application security firm WhiteHat Security thinks that severity ratings need to embrace complexity, even as they strive to simplify information about vulnerabilities.
“There are several variables, of varying degrees of real importance, worth considering in a final actionable ‘severity’ formula,” he writes. Among them: system severity, exploit difficulty, wormability, local or remote accessibility, industry knowledge and pervasiveness.
While there might not be an easy or consistent way to weight all those different factors, Grossman believes that its worth trying, especially if Microsoft can mine its knowledge of exploits and use it to help rank a decade of vulnerability data. “They might create a new formula, apply it to old and patched issues, and see if the resulting severity matches what would be reasonable accurate (in light of passed exploits),” Grossman suggested.

< back | next: do no harm >

Last: Do No Harm

Despite their criticisms of elements of Microsoft’s Severity Ratings System, there was broad agreement among the experts we contacted that Microsoft should keep some kind of rating system in place – even if it decides not to fix its flaws.
While the Severity Rating System may be more a reflection of the security landscape at the time it was created than an accurate measure of severity in today’s threat environment, its still much better than what preceded it:  a ponderous, erratic and overly technical security bulletin system that focused on technical descriptions of vulnerabilities and patches, without providing much in the way of guidance to IT practitioners.
After close to a decade, IT staff has developed a familiarity and understanding of the rating system: building patch processes around the company’s monthly Patch Tuesday bulk release, and prioritizing activities based on the Severity Ratings of those patches.
“I think one of the successes is the predictable patching schedule,” wrote Grossman of WhiteHat. “From the statistics I read on actual breaches, what’s most important in patch management is not necessarily the speed, but the coverage. Predictable schedules, I think, encourage more broad patching coverage.”
In short: ten years later, the Severity Rating System, even in its current incarnation, accomplishes its core objective: communicating information about the relative seriousness of security vulnerabilities and the need to patch to a broad audience, our experts agreed. Most said that they’d be loath to throw it out altogether.
“I think the way they do it is meaningful in the way it is intended,” wrote Miller. “The rating system is designed for enterprise customers to help them decide how quickly they need to patch,” he wrote.
Cerrudo of Argeniss agreed. “The Severity Rating System is how Microsoft communicates to people about what vulnerabilities should be patched ASAP or not,” he wrote.
The question, these experts say, is whether Microsoft is, in the next decade, willing to commit equivalent resources to secure development and improved communication about product vulnerabilities as they have committed in the past decade.
Some, like Grossman, worry that the company’s attention may be wandering now that software security isn’t perceived of as much of a liability for Microsoft.
“This is no longer the era of (Microsoft) trying to look more secure than Linux,” wrote Grossman, who has blogged on the topic (http://jeremiahgrossman.blogspot.com/2010/06/microsoft-security-is-good-enough-and.html). ”Microsoft won the security PR war long ago. Right or wrong, Adobe and Apple are the new whipping boys now.”
And that may be a lost opportunity for the community, as a whole, he said.
“We’ve learned a great deal as an industry since 2002. A modern system should take much more into consideration.”

<back

Cesar Cerrudo, CEO and Founder, Argeniss Information Security and Software

A security researcher and consultant, Cesar specializes in application
security and has discovered dozens of vulnerabilities in applications
including Microsoft SQL Server, Oracle database server, IBM DB2,
Microsoft BizTalk Server, Microsoft Commerce Server, Microsoft Windows
and Internet Explorer Web Browser.
He has published research on application security attacks and
exploitation techniques, including the much-publicized Token Kidnapping on Windows
and techniques for bypassing Windows Service Isolation .

< back

Jeremiah Grossman, founder and Chief Technology Officer at WhiteHat Security.

A widely recognized expert on Web application security, Jeremiah was
among the first to call attention to the dangers caused by so-called
“Clickjacking” attacks (a term he helped coin) and has discovered
prominent holes in common browser platforms including Apple’s Safari.
Jeremiah has been a frequent contributor to Threatpost on issues
related to Web security
and application development.
He’s also the top related search result on Google when you search for
“Jeremiah,” beating out the eponymous Luke Perry star vehicle and the
biblical prophet.

<back

Charlie Miller, Principal Analyst, Software Security Independent
Security
Evaluators

.

Charlie’s background includes five years working for the National
Security Agency. He’s one of the top vulnerability researchers in the
industry, winner of a $10,000 cash prize at the hacker conference
Pwn2Own in Vancouver Canada for being the first to find a critical bug
in the ultrathin MacBook Air—deploying an exploit in 2 minutes. His
other work includes discovery of critical vulnerabilities in Adobe
Reader, Apple’s iPhone and other platforms. Charlie holds a Ph.D. in
mathematics from the University of Notre Dame. He is a contributor to
Threatpost, where he has spoken on the importance of vulnerability
research
and written on the art of writing exploits and security holes in online gaming and virtual worlds.

< back

Chris Wysopal, Chief Technology Officer at Veracode.

Chris is a recognized expert on application and software security and
Chief Technology Officer of Veracode, an application security testing
firm. Chris started his IT security career as a software engineer at
Lotus Development Corp. and, later, as a founding member of the early
hacker think tank the L0pht Heavy Industries in Boston. Chris later went
to work for security consultancy @Stake, which was acquired by
Symantec. Chris was an early and important contributor to industry
guidelines for so-called “responsible disclosure” of security
vulnerabilities and co-author of the IETF RFC “Responsible Vulnerability
Disclosure Process” in 2002, which became a basis for the Organization
of Internet Safety (OID) of which Chris is a founding member.  Chris is a
frequent contributor to Threatpost, having written about topics
including “The Pitfalls of Website Vulnerability Research and
Disclosure

and Cyberwarfare.

< back

HED: After A Decade, Time To Rethink Microsoft’s Vulnerability Ratings?
DEK: Security Experts will tell you that one year is a lifetime in the world of online threats and

attacks. But eight years after Microsoft introduced its innovative severity rating system for software

vulnerability, the company says its original definitions of what makes a software hole important still

apply. Security experts aren’t so sure.
Microsoft today released its final monthly patch release of 2010. The Redmond software giant went out in

style with a large release of 17 bulletins containing fixes for some 40 vulnerabilities in Microsoft

Windows, Office, Internet Explorer, SharePoint and Exchange. (https://threatpost.com/en_us/blogs/ms-

patch-tuesday-heads-17-bulletins-40-vulnerabilities-120910) As is the case on every Patch Tuesday, the

release will set off a scramble within enterprises large and small to sort out which patches are the most

important to apply, and then to install them on affected systems. As they’ve done for almost a decade, IT

pros will rely on Microsoft’s Severity Rating System for software vulnerabilities to help answer those

questions. But are the labels that Microsoft puts on its security bulletins – ratings that declare patches

“critical,” “important,” “moderate” or “low” – accurate? And does Microsoft’s definition of what makes a

security hole “severe” still reflect the computer security threats facing organizations? With the Severity

Rating system entering its tenth year, is it time for Redmond to take a fresh look at the way it ranks

security holes in its products?
To help answer that question, Threatpost spoke with experts at Microsoft’s Security Response Center and

interviewed some of the top vulnerability researchers and security experts in the industry to get their

thoughts on the strengths and weaknesses of Microsoft’s decade old rating system, and whether changes are

needed to the way Microsoft rates security holes in its products.
Who we talked to
Cesar Cerrudo: Cesar is the co-founder and CEO of Argeniss Information Security & Software in Parana,

Argentina. A security researcher and consultant, Cesar specializes in application security and has

discovered dozens of vulnerabilities in applications including Microsoft SQL Server, Oracle database

server, IBM DB2, Microsoft BizTalk Server, Microsoft Commerce Server, Microsoft Windows and Internet

Explorer Web Browser (https://threatpost.com/en_us/blogs/ie-default-settings-exposes-intranet-attacks-

041409). He has published research on application security attacks and exploitation techniques, including

Token Kidnapping on Windows (https://threatpost.com/en_us/blogs/ms-windows-token-kidnapping-problems-

resurface-071610) and techniques for bypassing Windows Service Isolation

(https://threatpost.com/researcher-will-demo-bypass-windows-service-isolation-feature-090210/).

Jeremiah Grossman: Jeremiah is the founder and Chief Technology Officer of WhiteHat Security, a Web

application security testing firm. A widely recognized expert on Web application security, Jeremiah was

among the first to call attention to the dangers caused by so-called “Clickjacking” attacks (a term he

helped coin) and has discovered prominent holes in common browser platforms including Apple’s Safari

(https://threatpost.com/apple-fixes-autofill-flaw-massive-safari-update-072810/). Jeremiah has

been a frequent contributor to Threatpost on issues related to Web security

(https://threatpost.com/web-won-t-be-safe-let-alone-secure-unless-we-break-it-020410/) and

application development (http://threatpost.com/en_us/blogs/jeremiah-grossman-web-app-security-secure-

development-and-when-web-security-will-improve-05190). He’s also the top related search result on Google

when you search for “Jeremiah,” beating out the eponymous Luke Perry star vehicle and the biblical

prophet.

Charlie Miller: Charlie is a Principal Analyst for Software  Security with the consulting firm Independent

Security Evaluators. Charlie’s background includes five years working for the National Security Agency.

He’s one of the top vulnerability researchers in the industry, winner of a $10,000 cash prize at the

hacker conference Pwn2Own in Vancouver Canada for being the first to find a critical bug in the ultrathin

MacBook Air—deploying an exploit in 2 minutes. His other work includes discovery of critical

vulnerabilities in Adobe Reader, Apple’s iPhone and other platforms. Charlie holds a Ph.D. in mathematics

from the University of Notre Dame. He is a contributor to Threatpost, where he has spoken on the

importance of vulnerability research (http://threatpost.com/en_us/blogs/owning-virtual-worlds-fun-and-

profit-081810) and written on the art of writing exploits (http://threatpost.com/en_us/blogs/charlie-

miller-mac-os-x-pwn2own-and-writing-exploits-031810 ) and security holes in online gaming and virtual

worlds (https://threatpost.com/owning-virtual-worlds-fun-and-profit-081810/).

Chris Wysopal: Chris is a recognized expert on application and software security and Chief Technology

Officer of Veracode, an application security testing firm. Chris started his IT security career as a

software engineer at Lotus Development Corp. and, later, as a founding member of the early hacker think

tank the L0pht Heavy Industries in Boston. Chris later went to work for security consultancy @Stake, which

was acquired by Symantec. Chris was an early and important contributor to industry guidelines for so-

called “responsible disclosure” of security vulnerabilities and co-author of the IETF RFC “Responsible

Vulnerability Disclosure Process” in 2002, which became a basis for the Organization of Internet Safety

(OID) of which Chris is a founding member.  Chris is a frequent contributor to Threatpost, having written

about topics including “The Pitfalls of Website Vulnerability Research and Disclosure”(

https://threatpost.com/pitfalls-website-vulnerability-research-and-disclosure-061510/) and

Cyberwarfare (https://threatpost.com/cyberwarfare-reality-check-070909/)
Ditch “Outdated Assumptions”
The experts we consulted about the current state of Microsoft’s Severity Rating System mostly had good

things to say. But all agreed that the system is due for renovation and that there was room for

improvement to the definitions that describe the severity of vulnerabilities.
First and foremost, the experts we consulted agreed, Microsoft’s Severity Ratings are based on what most

agreed were “outdated” assumptions about threats and attacks.
“The threat space has changed greatly in the last eight years,” wrote Chris Wysopal, CTO at Veracode.

First, attacks shifted away from worms and viruses – noisy and intrusive threats that were easy to

isolate. This, Wysopal notes, reflected shift in the motivation of malware authors from “proving a point”

to “monetizing vulnerable systems.” “If you are monetizing vulnerable systems you don’t want to do

anything to call attention to yourself, especially with the network traffic a worm generates.”
 The second big change has been in the kinds of attacks Microsoft‘s customers are being targeted with.

Within the past couple years, those have shifted from a focus on core Windows components and vulnerable

networks to what Wysopal described as a “target rich environment” in client software such as Adobe’s

Acrobat Reader and Web browsers like Internet Explorer and Firefox top threats.  
Alas, these important changes aren’t reflected in Microsoft’s Severity Rating System which, reflecting the

time at which it was created, still considers “Critical” holes as those that, when exploited, “could allow

the propagation of an Internet worm without user action.” “Important” vulnerabilities – the next level

down – could be any vulnerability “whose exploitation could result in compromise of the confidentiality,

integrity, or availability of user’s data, or of the integrity or availability of processing resources.”
The problem isn’t so much that worms don’t matter, anymore. It’s that they’re no longer as big of a

concern as they were in 2002 and, by no means, are they the only concern.
Charlie Miller, Principal Analyst at consulting firm Independent Security Evaluators, observed that few

companies or consumers directly connect their computers to the Internet these days, which has reduced the

impact of worms. Even lowly home routers are remarkably effective at stopping worms, even though they

weren’t designed for that purpose, Miller said.
Add it all up and it means that worms – while still a threat – don’t eclipse other dangers in the way they

might have eight or ten years ago.
“Worms and widespread malware are indeed a health problem for the Internet ecosystem,” wrote Wysopal of

Veracode. “They are like epidemics that should be treated (as such) even if they are rare.  But since they

are rare they shouldn’t take up the whole top ranking of the severity rating system anymore.”
Cesar Cerrudo, CEO of Argeniss Information Security and Software, agreed that Microsoft’s definition of a

“Critical” vulnerability no longer jives with the threats and attacks that organizations are contending

with. “For most ‘critical’ vulnerabilities right now, the exploitation consists of a user visiting a Web

page or opening a (Microsoft) Office document, etc….There aren’t Internet worms being propagated,” Cerrudo

wrote.
Broadening the concept of severity to pull in more factors than just the ability to create worms is key to

modernizing the Severity Rating System, our experts agreed.  
Make  Exploitability Count
How might the idea of Severity be broadened? One idea all our experts agreed on was the need to make the

ease with which vulnerability can be exploited an element of the severity rating. While “wormability”

matters, the ease with which vulnerability might produce a self-replicating threat is just one factor,

behind the ease with which any hole might be used – “exploited” – to take control of a remote system.
Cerrudo,  who has discovered prominent holes in Microsoft products including Windows and SQL Server, said

the increasing use of exploits for previously unknown (or “zero day”) vulnerabilities makes exploitability

more relevant now than it was ten years ago, but that Microsoft’s existing Severity Rating System fails to

sufficiently account for it.
“Critical for me should be any vulnerability that can be exploited remotely without needing user name and

password and with minimal user interaction,” he wrote. ”Important should be used for vulnerabilities that

require local access or remote access with a user name and password, or that require complex user

interaction or specific configurations to work.”
Miller agrees.
“I think it’s important to …make a distinction between those vulnerabilities that are known to have an

exploit ‘in the wild’ and those that are just known vulnerabilities,” he wrote
That might sound like common sense. But, as it stands, Microsoft tracks vulnerability severity and

exploitability separately, according to Jerry Bryan, Group Manager for Response Communications at

Microsoft.
Severity is a measure of what Microsoft calls the “maximum impact” of vulnerability. That’s defined as

“the highest impact vulnerability may have.” Bryan described “maximum impact” as a way of making sure that

Microsoft errs on the conservative side with its Severity Ratings. For example, Bryan explained in an

email, if a vulnerability could be used for remote code execution, but might also cause a denial of

service, Microsoft would consider the more severe case (remote code execution) when assigning a severity

rating (in this case, a critical rating).
Information on how easy it is for attacker to use (or exploit) security vulnerabilities, however, is

tracked in what Microsoft calls the ”Exploitability Index” a separate rating system introduced in 2008 in

response to customer requests for more context around vulnerabilities. The Exploitability Index measures

the ease with which a vulnerability could be exploited, on a scale from 1 (“consistent exploit code

likely”) to 3 (“functioning exploit code unlikely”).
“Together, these two pieces of information help organizations prioritize the deployment of Microsoft

updates for their environment by indicating the risk of an issue being exploited and the impact if the

exploit is successful,” Bryan said in a statement.
Of course, how “easy” something is to exploit is a subjective measure that’s based, in part, on the

researcher’s level of skill and expertise, Miller and others acknowledge. Other vulnerability scoring

systems, notably the Common Vulnerability Scoring System (CVSS) that is maintained by the Forum of

Incident Response and Security Teams (FIRST) blend exploitability data with other factors, including

environmental considerations and “impact” – the way the vulnerability might expose sensitive data or be

used to compromise or disable key systems. While not exact, the elements of CVSS ratings are simple enough

to allow the creation of automated tools (http://nvd.nist.gov/cvss.cfm?calculator&version=2) for

generating consistent severity scores as well as impact and exploitability sub scores.
That’s not a bad model for Microsoft, says Miller.
Sub ratings that indicated whether a vulnerability didn’t require user interaction to trigger and for

which a public exploit was circulating, or – barring a public exploit – whether “mad skillz” would be

required to create one or  ”my grandma could write the exploit,” would help shade an individual

vulnerability in a way that would be useful for IT pros, Miller said.
Get with the (CVSS) Program
One problem with Microsoft’s Severity Rating system is that the system, itself, is proprietary and doesn’t

play nice with other vendor-neutral industry standards.  Bulletins might link out to independent

vulnerability tracking services like MITRE’s Common Vulnerabilities and Exposures List, but don’t

necessarily track to them.
At the same time, IT professionals have a wealth of information from the company on any vulnerability, but

have to cobble it together from different sources, including advance bulletin notifications, security

advisories, the MSRC blog, Microsoft’s exploitability index, and so on.
That complexity is enough that popular understanding of the Severity Ratings has drifted from their actual

definitions. After almost a decade, he argues, the Ratings have become little more than “rules of thumb”

internalized by harried IT administrators rather than pinpoint accurate declarations about the potential

harm that could result from a specific hole going unpatched, Cerrudo said.
“I think what Microsoft has accomplished over the years (is) people associating a ‘Critical’ severity

rating with ‘very dangerous,’ ‘patch should be applied ASAP,’ (and) ‘I’m going to be hacked if I don’t

patch,’” Cerrudo wrote.  However, lower ratings tend to fall off the map. “Important severity level means:

‘I can patch later’, ‘There are more critical things I should care about.’” Moderate and Low

vulnerabilities, he said, often are interpreted as ‘We will patch someday’ or ‘Not dangerous at all,’

Cerrudo wrote.
And that becomes problematic when “Important” vulnerabilities are understood to be holes that lack

urgency, even though a close reading would show that they could facilitate remote compromise of vulnerable

systems.
As an example of just this problem, Wysopal notes MS10-088, a recent Microsoft bulletin issued regarding

two recent Powerpoint vulnerabilities. Microsoft assigned an “Important” severity rating to the bulletin,

despite the fact that they affected a common component of the widely used Office Suite to the hole and

that each of the vulnerabilities that were identified would allow an attacker to “take complete control of

an affected system…” installing their own programs or user accounts and modifying, copying or deleting

data.  Both could also be leveraged in Web based attacks by “a Web site that contains a Web page that is

used to exploit this vulnerability” or “Web sites and Web sites that accept or host user-provided content…

(and) contain specially crafted content that could exploit this vulnerability.”
CVSS, on the other hand, rates the MS10-088 vulnerabilities “HIGH” with a score of 9.3 out of 10 – with an

exploitability sub score of 8.6 out of 10 and impact subscore of 10 out of 10 contributing to the overall

rating.  In that case, as others, Wysopal argues, Microsoft’s bias towards “wormability” dilutes the

severity of vulnerabilities that are the most commonly used tools for compromising individuals and

organizations today: those sent via the Web and aimed at client side software.  
Wysopal suggests that Microsoft ditch its proprietary severity rating system altogether in favor of the

CVSS system, which he calls a “more realistic and common standard” that has also been adopted as the basis

for NIST’s National Vulnerability Database (NVD)( http://nvd.nist.gov/).
Embrace Complexity
Ultimately, any kind of rating system will oversimplify the complex issues that surround vulnerabilities,

but researchers say that’s OK. In fact, Microsoft needs to embrace complexity if it is to reform its

existing and flawed Severity Rating System.
“If I were writing a rating system, it would be much more refined,” wrote Miller. “As someone who finds

bugs for a living, I have to assess the ease of exploitation of bugs all the time, and it is difficult,

and subjective, but I have to do it in order to move on.”  
Microsoft could help by providing more contexts for exploitability that helps shade different

vulnerabilities and also supports the ratings that are given.
“Consider the difference between an off-by-one error that allows you to write a null one byte past a

buffer compared with a bug that allows you to write an arbitrary amount of arbitrary data past a buffer,”

Miller offered, as an example. “I think it’d be really interesting if they tried to make a scale from 1-10

based on this criteria,” he said.
Jeremiah Grossman, of application security firm WhiteHat Security thinks that severity ratings need to

embrace complexity, even as they strive to simplify information about vulnerabilities.
“There are several variables, of varying degrees of real importance, worth considering in a final

actionable ‘severity’ formula,” he writes. Among them: system severity, exploit difficulty, wormability,

local or remote accessibility, industry knowledge and pervasiveness.
While there might not be an easy or consistent way to weight all those different factors, Grossman

believes that its worth trying, especially if Microsoft can mine its knowledge of exploits and use it to

help rank a decade of vulnerability data. “They might create a new formula, apply it to old and patched

issues, and see if the resulting severity matches what would be reasonable accurate (in light of passed

exploits),” Grossman suggested.
Last: Do No Harm
Despite their criticisms of elements of Microsoft’s Severity Ratings System, there was broad agreement

among the experts we contacted that Microsoft should keep some kind of rating system in place – even if it

decides not to fix its flaws.
While the Severity Rating System may be more a reflection of the security landscape at the time it was

created than an accurate measure of severity in today’s threat environment, its still much better than

what preceded it:  a ponderous, erratic and overly technical security bulletin system that focused on

technical descriptions of vulnerabilities and patches, without providing much in the way of guidance to IT

practitioners.
After close to a decade, IT staff has developed a familiarity and understanding of the rating system:

building patch processes around the company’s monthly Patch Tuesday bulk release, and prioritizing

activities based on the Severity Ratings of those patches.
“I think one of the successes is the predictable patching schedule,” wrote Grossman of WhiteHat. “From the

statistics I read on actual breaches, what’s most important in patch management is not necessarily the

speed, but the coverage. Predictable schedules, I think, encourage more broad patching coverage.”
In short: ten years later, the Severity Rating System, even in its current incarnation, accomplishes its

core objective: communicating information about the relative seriousness of security vulnerabilities and

the need to patch to a broad audience, our experts agreed. Most said that they’d be loath to throw it out

altogether.
“I think the way they do it is meaningful in the way it is intended,” wrote Miller. “The rating system is

designed for enterprise customers to help them decide how quickly they need to patch,” he wrote.
Cerrudo of Argeniss agreed. “The Severity Rating System is how Microsoft communicates to people about what

vulnerabilities should be patched ASAP or not,” he wrote.
The question, these experts say, is whether Microsoft is, in the next decade, willing to commit equivalent

resources to secure development and improved communication about product vulnerabilities as they have

committed in the past decade.
Some, like Grossman, worry that the company’s attention may be wandering now that software security isn’t

perceived of as much of a liability for Microsoft.
“This is no longer the era of (Microsoft) trying to look more secure than Linux,” wrote Grossman, who has

blogged on the topic (http://jeremiahgrossman.blogspot.com/2010/06/microsoft-security-is-good-enough-

and.html). ”Microsoft won the security PR war long ago. Right or wrong, Adobe and Apple are the new

whipping boys now.”
And that may be a lost opportunity for the community, as a whole, he said.
“We’ve learned a great deal as an industry since 2002. A modern system should take much more into

consideration.”

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