Last updated 14 January 2026

An old inffast.c optimization turns out to not be optimal anymore with modern compilers, and furthermore was not compliant with the C standard, for which decrementing a pointer before its allocated memory is undefined.

External References:

https://wiki.mozilla.org/images/0/09/Zlib-report.pdf https://docs.google.com/document/d/10i1KZS5so8xDqH2rplRa2xet0tyTvvJlLbQQmZIUIKE/edit#heading=h.t13tvnx4loq7

Upstream patch:

https://github.com/madler/zlib/commit/9aaec95e82117c1cb0f9624264c3618fc380cecb

CVE assignment:

http://seclists.org/oss-sec/2016/q4/602

zlib 1.2 and later versions allows remote attackers to cause a denial of service (crash) via a crafted compressed stream with an incomplete code description of a length greater than 1, which leads to a buffer overflow, as demonstrated using a crafted PNG file.