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

CVE-2026-85091 is fixed in 7 products on 1 platform, first by lib32z1 1:1.3.dfsg-3.1ubuntu2.3, released September 3, 2026. It is not reported as exploited. Patch Radar priority: High.

- Web page: https://patchblox.com/security/cve/CVE-2026-85091
- Priority: High
- CVSS: 7.4 (Important)
- EPSS: 0.0047
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Sep 3, 2026
- Weakness: Out-of-bounds Write
- Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-85091

## 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.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | lib32z1 Ubuntu 24.04.5 LTS (noble) | 1:1.3.dfsg-3.1ubuntu2.3 | [lib32z1 1:1.3.dfsg-3.1ubuntu2.3](https://patchblox.com/security/data/fix/linux/ubuntu-noble-lib32z1-1-1.3.dfsg-3.1ubuntu2.3.md) | Sep 3, 2026 |
| Linux | lib32z1 Ubuntu 26.04.1 LTS (resolute) | 1:1.3.dfsg+really1.3.1-1ubuntu3.2 | [lib32z1 1:1.3.dfsg+really1.3.1-1ubuntu3.2](https://patchblox.com/security/data/fix/linux/ubuntu-resolute-lib32z1-1-1.3.dfsg-really1.3.1-1ubuntu3.2.md) | Sep 3, 2026 |
| Linux | lib64z1 Ubuntu 24.04.5 LTS (noble) | 1:1.3.dfsg-3.1ubuntu2.3 | [lib64z1 1:1.3.dfsg-3.1ubuntu2.3](https://patchblox.com/security/data/fix/linux/ubuntu-noble-lib64z1-1-1.3.dfsg-3.1ubuntu2.3.md) | Sep 3, 2026 |
| Linux | lib64z1 Ubuntu 26.04.1 LTS (resolute) | 1:1.3.dfsg+really1.3.1-1ubuntu3.2 | [lib64z1 1:1.3.dfsg+really1.3.1-1ubuntu3.2](https://patchblox.com/security/data/fix/linux/ubuntu-resolute-lib64z1-1-1.3.dfsg-really1.3.1-1ubuntu3.2.md) | Sep 3, 2026 |
| Linux | libminizip1t64 Ubuntu 24.04.5 LTS (noble) | 1:1.3.dfsg-3.1ubuntu2.3 | [libminizip1t64 1:1.3.dfsg-3.1ubuntu2.3](https://patchblox.com/security/data/fix/linux/ubuntu-noble-libminizip1t64-1-1.3.dfsg-3.1ubuntu2.3.md) | Sep 3, 2026 |
| Linux | libminizip1t64 Ubuntu 26.04.1 LTS (resolute) | 1:1.3.dfsg+really1.3.1-1ubuntu3.2 | [libminizip1t64 1:1.3.dfsg+really1.3.1-1ubuntu3.2](https://patchblox.com/security/data/fix/linux/ubuntu-resolute-libminizip1t64-1-1.3.dfsg-really1.3.1-1ubuntu3.2.md) | Sep 3, 2026 |
| Linux | libx32z1 Ubuntu 24.04.5 LTS (noble) | 1:1.3.dfsg-3.1ubuntu2.3 | [libx32z1 1:1.3.dfsg-3.1ubuntu2.3](https://patchblox.com/security/data/fix/linux/ubuntu-noble-libx32z1-1-1.3.dfsg-3.1ubuntu2.3.md) | Sep 3, 2026 |
| Linux | libx32z1 Ubuntu 26.04.1 LTS (resolute) | 1:1.3.dfsg+really1.3.1-1ubuntu3.2 | [libx32z1 1:1.3.dfsg+really1.3.1-1ubuntu3.2](https://patchblox.com/security/data/fix/linux/ubuntu-resolute-libx32z1-1-1.3.dfsg-really1.3.1-1ubuntu3.2.md) | Sep 3, 2026 |
| Linux | minizip Ubuntu 24.04.5 LTS (noble) | 1:1.3.dfsg-3.1ubuntu2.3 | [minizip 1:1.3.dfsg-3.1ubuntu2.3](https://patchblox.com/security/data/fix/linux/ubuntu-noble-minizip-1-1.3.dfsg-3.1ubuntu2.3.md) | Sep 3, 2026 |
| Linux | minizip Ubuntu 26.04.1 LTS (resolute) | 1:1.3.dfsg+really1.3.1-1ubuntu3.2 | [minizip 1:1.3.dfsg+really1.3.1-1ubuntu3.2](https://patchblox.com/security/data/fix/linux/ubuntu-resolute-minizip-1-1.3.dfsg-really1.3.1-1ubuntu3.2.md) | Sep 3, 2026 |
| Linux | zlib Debian 13.5 (trixie) | 1:1.3.dfsg+really1.3.1-1+deb13u1 | [DEBIAN-CVE-2026-85091](https://patchblox.com/security/data/fix/linux/debian-cve-2026-85091.md) | Sep 3, 2026 |
| Linux | zlib Debian 14 | 1:1.3.dfsg+really1.3.2-4 | [DEBIAN-CVE-2026-85091](https://patchblox.com/security/data/fix/linux/debian-cve-2026-85091.md) | Sep 3, 2026 |
| Linux | zlib1g Ubuntu 24.04.5 LTS (noble) | 1:1.3.dfsg-3.1ubuntu2.3 | [zlib1g 1:1.3.dfsg-3.1ubuntu2.3](https://patchblox.com/security/data/fix/linux/ubuntu-noble-zlib1g-1-1.3.dfsg-3.1ubuntu2.3.md) | Sep 3, 2026 |
| Linux | zlib1g Ubuntu 26.04.1 LTS (resolute) | 1:1.3.dfsg+really1.3.1-1ubuntu3.2 | [zlib1g 1:1.3.dfsg+really1.3.1-1ubuntu3.2](https://patchblox.com/security/data/fix/linux/ubuntu-resolute-zlib1g-1-1.3.dfsg-really1.3.1-1ubuntu3.2.md) | Sep 3, 2026 |

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Data as of 2026-11-09T01:18:40Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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