What’s Vulnerable › Vulnerabilities

CVE-2026-85091: zlib 1.3.1.2 through 1.3.2 Heap Buffer Overflow via gz_vacate

• Severity: high (CVSS 7.4 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.5% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-09-03 · updated 2026-09-09
• Weakness: CWE-787

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.

Affected products

• zlib: from 1.3.1.2 through 1.3.2

References

• 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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