A buffer overflow exists in exim, which is the standard mail transport agent in Debian. By supplying a specially crafted HELO or EHLO command, an attacker could cause a constant string to be written past the end of a buffer allocated on the heap. This vulnerability is not believed at this time to be exploitable to execute arbitrary code.
For the stable distribution (woody) this problem has been fixed in exim version 3.35-1woody2 and exim-tls version 3.35-3woody1.
For the unstable distribution (sid) this problem has been fixed in exim version 3.36-8. The unstable distribution does not contain an exim-tls package.
We recommend that you update your exim or exim-tls package.