Antivirus firm Beyond Security has announced a serious vulnerability within Outlook Express that can bypass the SMTP (simple mail transfer protocol) filtering engines used by many companies, including gateway virus scanners, content filters and firewalls.
Outlook Express allows users to split up sent emails, which in turn enables a connecting user to send smaller segments of larger messages in a multiple email.
But the Message Fragment and Reassembly command within Outlook allows anyone, according to Beyond Security, to bypass most of the security restrictions placed on email messages.
As the messages have been fragmented into smaller segments, virus scanners cannot detect them and the recipient will simply receive one large file containing many smaller potentially dangerous emails.
"This generic flaw affects all SMTP content filtering software," said Aviram Jenik, Beyond Security CEO. "As virus writers seek increasingly sophisticated methods to disseminate their viruses, email exploits will become a more popular means for doing so."
Security firm GFI has developed an email security testing engine to allow users to test their systems for the vulnerability.
While several antivirus software vendors have warned users that they could be affected.
"We have confirmed that our product InterScan VirusWall 3.5x is affected by the vulnerability," said a TrendMicro spokesperson. "In order to resolve this problem we have released a patch, which can be downloaded from the FTP server." Users should look for the ISNT Hotfix_build1494_v352_Smtp_case6593.zip hotfix.
Software vendor Symantec admitted it had been aware of the potential malicious use of this feature for some time.
"All currently supported Symantec gateway products, by default, block multipart MIME messages at the gateway. It can be disabled if a multipart email is required," said a spokesman at Symantec. A formal response is due from Symantec today and will be posted here.
"The rejection of segmented emails should be part of a company's comprehensive security policy to restrict potential harmful content from the internal network," he added.
More information about the vulnerability can be found here.