Thor Larholm discovered that libphp-phpmailer, an email transfer class for PHP, performs insufficient input validition if configured to use Sendmail. This allows the execution of arbitrary shell commands.
The oldstable distribution (sarge) doesn't include libphp-phpmailer.
For the stable distribution (etch) this problem has been fixed in version 1.73-2etch1.
For the unstable distribution (sid) this problem has been fixed in version 1.73-4.
We recommend that you upgrade your libphp-phpmailer package.
MD5 checksums of the listed files are available in the original advisory.