Malware authors quickly adopt SHA-2 through stolen code-signing certificates

Malware pushers have adapted to new Windows restrictions on files signed with SHA-1-based digital certificates

As the IT industry is working to phase out the aging SHA-1 hashing algorithm it's not just website owners and software developers who are scrambling to replace their digital certificates: Cybercriminals are following suit too.

Researchers from Symantec have recently found new samples of the Carberp.B online banking Trojan that were digitally signed with not one, but two stolen certificates: one using a SHA-1 signature and one using a SHA-2 signature.

"It can be safely surmised that the malware author used certificates containing differing algorithms with the hope of thwarting detection," the Symantec researchers said in a blog post.

The SHA-1 hashing function is in the process of being retired because it is theoretically vulnerable to attacks that could result in forged digital certificates and it's only a matter of time before someone gains the capability to do this.

Last year Microsoft made changes to its crypto libraries so that Windows 7 and higher and Windows Server will no longer trust code signed with a SHA-1 based certificate if its timestamp is later than Jan. 1, 2016. The restriction applies only to files that Windows flags as being obtained from the Internet, which is the case for most malware.

Digitally-signed malware used to be a rare occurrence, but over the past couple of years the number of such programs has increased in response to operating systems making it harder to run non-signed files.

For example, Windows displays User Account Control (UAC) security warnings for unsigned executable files that try to gain administrator privileges while the latest versions of Apple's Mac OS X only allow applications to run if they have been downloaded from the Mac App Store or if they have been signed with a developer certificate obtained from Apple.

Aside from bypassing these restrictions, cybercriminals have also figured out that having multiple digital signatures on a single file can have other benefits.

Since the certificates used to sign malware are typically stolen from legitimate organizations and not bought by the attackers themselves, the likelihood of those thefts being discovered and the certificates being revoked by their legitimate owners is high. Having two signatures on a single file ensures that even if one certificate is revoked, the file will still appear as trusted thanks to the other signature.

"The attacks with Carberp also point to a shift towards using digital certificates with SHA-2," the Symantec researchers said. "While the move from SHA-1 to SHA-2 may not be instant because legacy systems do not support the newer algorithm, these attacks do indicate that change is on the way."

Users shouldn't allow files to run just because they appear to be signed with a valid certificate. How the file was obtained is always more important than its signature, because here's always the possibility that a legitimate developer was compromised and had his code-signing certificate stolen.

As digital certificates have become valuable targets for cybercriminals, it is important for owners of such certificates to maintain strong cybersecurity practices and make sure that they are stored securely.

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Lucian Constantin

IDG News Service
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