In the Debian bugtracking system: Bug 708164.
In Mitre's CVE dictionary: CVE-2013-2070.
A buffer overflow has been identified in nginx, a small, powerful, scalable web/proxy server, when processing certain chunked transfer encoding requests if proxy_pass to untrusted upstream HTTP servers is used. An attacker may use this flaw to perform denial of service attacks, disclose worker process memory, or possibly execute arbitrary code.
For the stable distribution (wheezy), this problem has been fixed in version 1.2.1-2.2+wheezy1.
For the unstable distribution (sid), this problem has been fixed in version 1.4.1-1.