Enlarge (credit: Project Zero)

When you're a Fortune 500 company that's a favorite target of sophisticated hackers, it often makes sense to install security appliances at the outer edges of your network to stop attacks before they get far. Now, researchers say they have uncovered a vulnerability in such a product from security firm FireEye that can give attackers full network access.

The vulnerability, which is on by default in the NX, EX, AX, FX series of FireEye products, was recently patched by FireEye after researchers from Google's Project Zero privately reported it. It made it possible for attackers to penetrate a network by sending one of its members a single malicious e-mail, even if it's never opened. It's not uncommon for outsiders to find such critical flaws in a security product. Still, the proof-of-concept exploit underscores that such game-over threats often extend to some of a network's most critical equipment. As Google employee Tavis Ormandy explained in a blog post published Tuesday:

For networks with deployed FireEye devices, a vulnerability that can be exploited via the passive monitoring interface would be a nightmare scenario. This would mean an attacker would only have to send an email to a user to gain access to a persistent network tap—the recipient wouldn’t even have to read the email, just receiving it would be enough.

A network tap is one of the most privileged machines on the network, with access to employee’s email, passwords, downloads, browsing history, confidential attachments, everything. In some deployment configurations* an attacker could tamper with traffic, inserting backdoors or worse. Because FireEye devices typically have a secondary internet-connected interface for updates and management, the issue could even be wormable across the internet.

The devices are supposed to passively monitor network traffic from HTTP, FTP, SMTP connections. In instances where there's a file transfer, the security appliance will scan it for malware. Ormandy and fellow Project Zero researcher Natalie Silvanovich found a vulnerability that can be exploited through such a passive monitoring interface. The researchers used the JODE Java decompiler to reverse engineer Java Archive files used by the FireEye devices. They then figured out a way to get the appliance to execute a malicious archive file by mimicking some of the same features found in legitimate ones.

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