Vulnerability · Published Sep 3, 2026 · Assigned by VulnCheck
CVE-2026-85091zlib 1.3.1.2 through 1.3.2 Heap Buffer Overflow via gz_vacate
High 7.4
In plain English
A high-severity out-of-bounds write in zlib from zlib. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.5% (higher than 41% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official 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 packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Alpine:v3.21 | zlib | 1.3.2-r1 | ALPINE-CVE-2026-85091 |
| Alpine:v3.22 | zlib | 1.3.2-r1 | ALPINE-CVE-2026-85091 |
| Alpine:v3.23 | zlib | 1.3.2-r1 | ALPINE-CVE-2026-85091 |
| Alpine:v3.24 | zlib | 1.3.2-r1 | ALPINE-CVE-2026-85091 |
| Debian:12 | zlib | no fix listed | DEBIAN-CVE-2026-85091 |
| Debian:13 | zlib | no fix listed | DEBIAN-CVE-2026-85091 |
| Debian:14 | zlib | 1:1.3.dfsg+really1.3.2-4 | DEBIAN-CVE-2026-85091 |
| Ubuntu:22.04:LTS | klibc | no fix listed | UBUNTU-CVE-2026-85091 |
| Ubuntu:24.04:LTS | klibc | no fix listed | UBUNTU-CVE-2026-85091 |
| Ubuntu:26.04:LTS | klibc | no fix listed | UBUNTU-CVE-2026-85091 |
| Ubuntu:26.10 | klibc | no fix listed | UBUNTU-CVE-2026-85091 |
| Ubuntu:22.04:LTS | zlib | no fix listed | UBUNTU-CVE-2026-85091 |
| Ubuntu:24.04:LTS | zlib | 1:1.3.dfsg-3.1ubuntu2.3 | USN-8954-1 |
| Ubuntu:24.04:LTS | zlib | 1:1.3.dfsg-3.1ubuntu2.3 | UBUNTU-CVE-2026-85091 |
| Ubuntu:26.04:LTS | zlib | 1:1.3.dfsg+really1.3.1-1ubuntu3.2 | UBUNTU-CVE-2026-85091 |
| Ubuntu:26.04:LTS | zlib | 1:1.3.dfsg+really1.3.1-1ubuntu3.2 | USN-8954-1 |
| Ubuntu:26.10 | zlib | 1:1.3.dfsg+really1.3.2-3ubuntu1.1 | USN-8954-1 |
| Ubuntu:26.10 | zlib | 1:1.3.dfsg+really1.3.2-3ubuntu1.1 | UBUNTU-CVE-2026-85091 |
| Ubuntu:22.04:LTS | zsync | no fix listed | UBUNTU-CVE-2026-85091 |
| Ubuntu:24.04:LTS | zsync | no fix listed | UBUNTU-CVE-2026-85091 |
| Ubuntu:26.04:LTS | zsync | no fix listed | UBUNTU-CVE-2026-85091 |
| Ubuntu:26.10 | zsync | no fix listed | UBUNTU-CVE-2026-85091 |
Affected products (as reported)
Weakness: CWE-787 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:H
References
- Technical report and proof of concept technical-description, exploit
- https://github.com/madler/zlib/blob/v1.3.2/gzwrite.c#L393 technical-description
- https://github.com/madler/zlib product
- VulnCheck Advisory: zlib 1.3.1.2 through 1.3.2 Heap Buffer Overflow via gz_vacate third-party-advisory