Max Vozeler discovered that the lockmail program from maildrop, a simple mail delivery agent with filtering abilities, does not drop group privileges before executing commands given on the commandline, allowing an attacker to execute arbitrary commands with privileges of the group mail.
The old stable distribution (woody) is not affected by this problem.
For the stable distribution (sarge) this problem has been fixed in version 1.5.3-1.1sarge1.
For the unstable distribution (sid) this problem has been fixed in version 1.5.3-2.
We recommend that you upgrade your maildrop package.