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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. Per the recommendation of a security audit of the zlib code by Trail of Bits and TrustInSoft, in support of the Mozilla Foundation, this "optimization" was removed, in order to avoid the possibility of undefined behavior.
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Summary
Our tool detected a potential vulnerability in zlib/inffast.c which was cloned from madler/zlib but did not receive the security patch applied. The original issue was reported and fixed under https://nvd.nist.gov/vuln/detail/cve-2016-9841.
Proposed Fix
Apply the same patch as the one in madler/zlib to eliminate the vulnerability.
Reference
https://nvd.nist.gov/vuln/detail/cve-2016-9841
madler/zlib@9aaec95