Sophos has revealed that its antivirus products may let worms including MyDoom pass undetected under certain conditions.

The antivirus company said that its Anti-Virus engine version 3.78, when used in qmail servers, may pass Delivery Status Notification messages with the worm body intact, which the AV software will not detect.

Under other circumstances, the software can enter an infinite loop when scanning some emails.

The company has released patches for several affected products.