Potential Vulnerability in Cloned Code #505
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This bug was reported by Danilo Ramos of Eideticom, Inc. It has lain in wait 13 years before being found! The bug was introduced in zlib 1.2.2.2, with the addition of the Z_FIXED option. That option forces the use of fixed Huffman codes. For rare inputs with a large number of distant matches, the pending buffer into which the compressed data is written can overwrite the distance symbol table which it overlays. That results in corrupted output due to invalid distances, and can result in out-of-bound accesses, crashing the application. The fix here combines the distance buffer and literal/length buffers into a single symbol buffer. Now three bytes of pending buffer space are opened up for each literal or length/distance pair consumed, instead of the previous two bytes. This assures that the pending buffer cannot overwrite the symbol table, since the maximum fixed code compressed length/distance is 31 bits, and since there are four bytes of pending space for every three bytes of symbol space.
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Summary
Our tool detected a potential vulnerability in zlib-1.2.7/trees.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-2018-25032.
Proposed Fix
Apply the same patch as the one in madler/zlib to eliminate the vulnerability.
Reference
https://nvd.nist.gov/vuln/detail/cve-2018-25032
madler/zlib@5c44459