PSIRT Advisory

AV Engine evasion via malformed RAR file

Summary

FortiClient and FortiOS AV engines may not immediately detect certain types of malformed or non-standard RAR archives, potentially containing malicious files.

Based on the samples provided, FortiClient will detect the malicious files upon trying extraction by real-time scanning and FortiGate will detect the malicious archive if Virus Outbreak Prevention is enabled.

Impact

Bypass Protection Mechanism

Affected Products

FortiOS 6.2 running AV engine version 6.00142 and below.

FortiOS 6.4 running AV engine version 6.00144 and below.

FortiClient 6.2 running AV engine version 6.00137 and below.

Solutions

FortiOS 6.2 running AV engine version 6.00145 or later.
FortiOS 6.4 running AV engine version 6.00145 or later.

FortiClient 6.2 running AV engine version 6.00145 or later.
FortiClient 6.4 running AV engine version 6.00243 or later.

Workaround for FortiGate: 

Enable Virus Outbreak Prevention feature.

Acknowledgement

Fortinet is pleased to thank Thierry Zoller for reporting this vulnerability under responsible disclosure.