Buffer overflow in Zlib
CVE-2026-85091
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.
Vulnerability class: Buffer Overflow
Published · last modified .
CVSS v3 metric
CVSS v3 base score 7.4 (High). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:H.
EPSS exploit prediction
EPSS: 0.004 (27.7th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 28th percentile — 27.7% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.004 · newest: 0.004 · change: -0.001
Affected products
Weakness classification (CWE)
References
- gist.github.com/thesmartshadow/e0b9481792afb7c31e86fee1ff084490 (technical-description, exploit)
- github.com/madler/zlib (product)
- github.com/madler/zlib/blob/v1.3.2/gzwrite.c (technical-description)
- www.vulncheck.com/advisories/zlib-1.3.1.2-through-1.3.2-heap-buffer-overflow-vi… (third-party-advisory)
Frequently asked questions
- What is CVE-2026-85091?
- CVE-2026-85091 is a high-severity vulnerability in Zlib, classified under Out-of-bounds Write. CVSS score: 7.4/10. Published 2026-09-03.
- How severe is CVE-2026-85091?
- High severity. CVSS v3 base score is 7.4 out of 10.