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There was a small optimization for PowerPCs to pre-increment a pointer when accessing a word, instead of post-incrementing. This required prefacing the loop with a decrement of the pointer, possibly pointing before the object passed. This is not compliant with the C standard, for which decrementing a pointer before its allocated memory is undefined. When tested on a modern PowerPC with a modern compiler, the optimization no longer has any effect. Due to all that, and 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 src/external/zlib/crc32.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-2016-9843.
Proposed Fix
Apply the same patch as the one in madler/zlib to eliminate the vulnerability.
Reference
https://nvd.nist.gov/vuln/detail/cve-2016-9843
madler/zlib@d1d5774