The attackers behind the Flame malware used a collision attack against a cryptographic algorithm as part of the method for gaining a forged certificate to sign specific components of the attack tool. Microsoft officials said on Tuesday that it’s imperative for customers to install the update issued for the problem on Sunday, as it’s possible for other attackers to exploit the same vulnerability without using the collision attack.

Cryptographic hash algorithms are designed to produce unique results for each input. If an attacker is able to find two separate inputs that produce the same hash as outputs, he has found a collision. Two of the more popular hash algorithms, MD5 and SHA-1, both have been found to be vulnerable to collisions. SSL certificates, like the one that the Flame attackers forged to sign the malware, use digital signatures, which can be vulnerable to hash collisions.

Microsoft officials said that there is still quite a bit of danger to customers, outside of the Flame malware itself.

“The Flame malware used a cryptographic collision attack in combination with the terminal server licensing service certificates to sign code as if it came from Microsoft. However, code-signing without performing a collision is also possible. This is an avenue for compromise that may be used by additional attackers on customers not originally the focus of the Flame malware. In all cases, Windows Update can only be spoofed with an unauthorized certificate combined with a man-in-the-middle attack,” Mike Reavey of the Microsoft Security Response Center, said.

The Flame attackers used the forged Microsoft digital certificate to perform a man-in-the-middle attack against victims, impersonating the Windows Update mechanism and installing malicious code instead. Reavey said Microsoft is preparing to change the way that Windows Update works in response to the attack.

“To increase protection for customers, the next action of our mitigation strategy is to further harden Windows Update as a defense-in-depth precaution. We will begin this update following broad adoption of Security Advisory 2718704 in order not to interfere with that update’s worldwide deployment. We will provide more information on the timing of the additional hardening to Windows Update in the near future,” Reavey said.

The possibility of attacks against Windows Update have been a serious concern for Microsoft officials and customers for many years now. Real-world attacks had not surfaced until the information about the Flame mechanism surfaced. But the way that the Flame attackers used their forged certificate was interesting. They used it to create a fake update server inside an organization that’s been compromised, and then downloading the malicious code to other machines, spreading the malware.

The way that Flame spread among machines had been a mystery until researchers discovered the use of the forged certificate.

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Comments (3)

  1. Anonymous

    Hard to believe that a collision attack was used. Is there a proof?

    If a collision attack was used, the consequence is that any certificate can be faked, no certificate can be trusted. That would lead to the end of CAs.

  2. Anonymous

    The industry has moved away from MD5, except for legacy support, for this very reason. If you obtain a certificate today, the hash algorithm used will be SHA-1 or SHA-256. It is not the end of CAs, just the end of MD5 hashing in certificates. 

    SHA-1 also has a collision attack (not yet demonstrated to be practical) and this is why SHA-256 will be replacing it over the next few years in all CAs.

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