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Vulnerability record · CVE-2026-85091 · published 3 September 2026

CVE-2026-85091: Out-of-bounds write vulnerability

zlib versions 1.3.1.2 through 1.3.2 contain a heap buffer overflow vulnerability in the gz_vacate() function when processing non-blocking gzwrite() operations with stale external buffer pointers. Attackers can trigger the overflow by calling gzprintf() or gzvprintf() after a write stall, causing an unchecked memmove() to write beyond the internal input buffer boundary.

8.3 CVSS 4.0 High EPSS 0.59% · top 53.9% CWE-787 · Out-of-bounds write Awaiting Analysis
8.3CVSS 4.0 base score
0.59%EPSS exploitation probability, 30 days
NoNot in CISA KEV
0Affected product versions listed by NVD
4References
9 Sep 2026Last modified by NVD

Description

zlib versions 1.3.1.2 through 1.3.2 contain a heap buffer overflow vulnerability in the gz_vacate() function when processing non-blocking gzwrite() operations with stale external buffer pointers. Attackers can trigger the overflow by calling gzprintf() or gzvprintf() after a write stall, causing an unchecked memmove() to write beyond the internal input buffer boundary.

CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:N/VC:N/VI:H/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:

References

LinkTags
https://gist.github.com/thesmartshadow/e0b9481792afb7c31e86fee1ff084490
https://github.com/madler/zlib
https://github.com/madler/zlib/blob/v1.3.2/gzwrite.c#L393
https://www.vulncheck.com/advisories/zlib-1.3.1.2-through-1.3.2-heap-buffer-overflow-via-gz-vacate

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Source: NIST National Vulnerability Database (record CVE-2026-85091), CISA KEV, FIRST EPSS (scores of 2026-09-26). This page is refreshed as NVD updates the record.