It was discovered that bsd-mailx, an implementation of the mail command, had an undocumented feature which treats syntactically valid email addresses as shell commands to execute.
Users who need this feature can re-enable it using the expandaddr in an appropriate mailrc file. This update also removes the obsolete -T option. An older security vulnerability, CVE-2004-2771, had already been addressed in the Debian's bsd-mailx package.
Note that this security update does not remove all mailx facilities for command execution, though. Scripts which send mail to addresses obtained from an untrusted source (such as a web form) should use the "--" separator before the email addresses (which was fixed to work properly in this update), or they should be changed to invoke "mail -t" or "sendmail -i -t" instead, passing the recipient addresses as part of the mail header.
For the oldstable distribution (squeeze), this problem has been fixed in version 8.1.2-0.20100314cvs-1+deb6u1.
We recommend that you upgrade your bsd-mailx packages.