Researcher Proven Right, but It Took 10 Years

It took ten years, but security researcher Thierry Zoller has finally seen the generalized vulnerability he foundin some AV products patched.

The general concept of the bug was not new, even at the time Zoller published. Holler admits that it was known since 2004/2005. He also says that even when notified “not every Anti-virus vendor cared enough to close these apparent problems, or add a logic for gateway products to allow them to block bypasses on request.”

The evasion happens if a format is manipulated in such a way that the scanner thinks it is of another filetype. This causes the AV product to execute another branch of code (and hence logic) that can result in a complete bypass of the protector code necessary to detect the original malware.

The bug can affect multiple types of archive formats including ISO, ZIP and Bz2.

The manipulated archive can be unpacked at the client but not at the server level. An AV engine cannot extract an archive, but the user can. That is the crux of the problem. If AV can’t extract the archive, they have no idea if it is safe to run or not.

This “feature” is also a problem for AV clients if they are being run in an environment where files are not executed. That includes gateway products like fileservers, mail and webservers.

It took until 2017 for Zoller to once again sound the alarm about the problem at Cansecwest. The presentation served as a marker of what had been done by the AV vendors to that point about the problem --basically nothing. Even when notified by Zoller, they came up with reasons why that the vulnerability would not affect routine use.

Zoller says that “one vendor argued that this could not be called a vulnerability because it would not impact Integrity, Availability or Confidentiality. Another Vendor argued that this cannot pose a "risk" to their customers because of XYZ (assumptions).”

But in November 2019, Trustwave discovered that a normal but specially crafted ZIP file was being used in a phishing email to distribute the NanoCore Remote Access Trojan (RAT). The attachment would totally bypass the secured email gateway that was in use and showed as normal.

Uh-oh. Looks like Zoller was right. Things started to get fixed by the AV vendors.

Kaspersky ended up saying in its security advisory that, “We also have fixed three bugs in one of anti-virus (AV) engine components that is responsible for work with ZIP archives. The fix for this component corrects its behaviour in situation of antivirus scanning specially crafted ZIP archives. These malformed archives could be used to circumvent our antivirus scan process. The bugs affected Kaspersky products with antivirus databases.”

ESET’s advisory was more discreet, only saying that “Fixed unpacking of some malformed archives, reported by Thierry Zoller”.

Bitdefender and Avira seemed to have issued “silent” patches that improved the archive handling, but didn’t call attention to the long lag in mitigating the situation.

— Larry Loeb has written for many of the last century's major "dead tree" computer magazines, having been, among other things, a consulting editor for BYTE magazine and senior editor for the launch of WebWeek.