Researchers detail FireEye bug that let hackers whitelist malware

FireEye customers should update their security appliance after researchers have explained how it can be made to temporarily turn a blind-eye to malware.

Researchers at German security Blue Frost Security on Monday published details about a critical “analysis engine evasion” bug that some installations of FireEye’s FX, AX, NX, and EX security appliances could still be exposed to.

FireEye actually released a patch for the bug in October and acknowledged Blue Frost’s find in its December quarterly update for the FireEye Operating System (FEOS), briefly noting that a high severity issue allowed files to bypass the FireEye detection engine.

Blue Frost was set to publish details in-mid January, however FireEye asked it to defer the post for a month due to a “too high” percentage of customers that had not yet updated affected products.

The bug is specific to FireEye’s malware detection capabilities on Windows and could be a highly sought after flaw by the advanced attackers that FireEye’s products are reputed to block.

“The analysis engine evasion allows an attacker to completely bypass FireEye's virtualization-based dynamic analysis on Windows and add arbitrary binaries to the internal white list of binaries for which the analysis will be skipped until the white list entry is wiped after a day,” Blue Frost researcher Moritz Jodeit explained.

Jodeit notes FireEye uses its Virtual Execution Engine to analyse the behaviour of a binary in a virtual machine. However, a flaw in the script used to copy and rename a binary prior to analysis allowed an attacker to abort the copy process, leaving the engine looking at a virtual machine with nothing in it.

This would have a knock-on effect for the product’s behavioural detection methods since it can wrongly attribute an absence of bad behaviour to a malicious binary. It would also give the attackers a 24 hour window to launch a malware attack that would go undetected by FireEye .

Jodeit explains: “Since the binary was not started in the virtual machine in the first place, an empty virtual machine will be analysed and no malicious behaviour will be detected. Once a binary was analysed and did not show any malicious behaviour, its MD5 hash is added to an internal list of binaries already analysed. If a future binary which is to be analysed matches an MD5 hash in this list, the analysis will be skipped for that file. The MD5 hash will stay in the white list until it is wiped after day.”

The flaw stems from the Windows script FireEye used to copy and rename binaries for analysis. It should have prevented Windows environment variables from being used inside the binary’s original filename, which would render the filename invalid and consequently cause the copy operation to fail.

An attacker could exploit this by sending a target an email with such a binary hidden in a ZIP file with several other benign files, explained Jodeit.

“Once this ZIP file was downloaded or sent via email a single time, the MD5 hash of the embedded malware would be whitelisted and the binary could then be used with an arbitrary file name without detection,” wrote Jodeit.

Blue Frost urged FireEye customers to update their systems if they have not already.

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