---
id: CVE-2026-85091
title: >-
  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
summary: >-
  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 cal…
severity: high
cvss: 7.4
cvssVector: 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:H'
cwe:
  - CWE-787
published: '2026-09-03'
updated: '2026-09-09'
sourceUpdated: '2026-09-09T20:41:07.123'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-85091'
references:
  - url: 'https://gist.github.com/thesmartshadow/e0b9481792afb7c31e86fee1ff084490'
    label: disclosure@vulncheck.com
  - url: 'https://github.com/madler/zlib'
    label: disclosure@vulncheck.com
  - url: 'https://github.com/madler/zlib/blob/v1.3.2/gzwrite.c#L393'
    label: disclosure@vulncheck.com
  - url: >-
      https://www.vulncheck.com/advisories/zlib-1.3.1.2-through-1.3.2-heap-buffer-overflow-via-gz-vacate
    label: disclosure@vulncheck.com
tags:
  - nvd
epss: 0.00592
epssPercentile: 0.45957
ingestedAt: '2026-09-09T21:22:45.564Z'
---

## Overview

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.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.