Symantec recently noted on its blog that it has seen a disturbing trend in malware attacks: the use of stolen SHA-2 certificates to make the malware appear valid.

SHA-1 Is Insecure

This change in the certs used by the malware is part of an evolving strategy because of security concerns about SHA-1. The malware wants the infected system to believe it is valid code. If the system does, there is a higher probability that the malware will be executed without detection.

Microsoft has already announced the discontinuation of support for files digitally signed with a SHA-1 signature after Jan. 1, 2016, in certain scenarios. Malware creators responded to this by adding stolen SHA-2 certificates to the SHA-1 certificates they may be using.

Symantec described how an older banking Trojan, Trojan.Carberp.B, has been modified to use this approach. The vector used for infection is usually the poisoned attachment embedded in an email document, which has “ATTN 00890” as its subject. The email and the attachment use language targeting people who work in accounting departments.

The infected attachment contains a malicious macro that uses a ROT13 +13/-13 cipher. The macro causes the download of a malicious signed binary, sexit.exe, from a server hosted in Mauritius. Sexit.exe is then installed and completes the infiltration.

SHA-2 Could Have Its Own Problems

Researchers found that sexit.exe was signed by two certificates: One is SHA-1-based, the other SHA-2-based. The goal of using both is to escape detection by the operating system. The use of a SHA-1 cert alone might not be accepted by newer OSs, but the SHA-2 certificate alone would not be accepted by older systems, such as those prior to Windows XP SP3.

There is an additional benefit to malware creators using a SHA-2 certificate: The SHA-2 cert can provide backup if the primary SHA-1 certificate is discovered to be bogus and revoked by the signing authority.

The rise of this technique shows how malware creators are adapting to the new certificate rules. This threat won’t show up immediately in your organization since the creators need to be able to target legacy systems, but it will be coming.