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.

Read the notes from the security team

Status

Show unmaintained releases

Package	Ubuntu Release	Status
zlib	26.04 LTS resolute	Vulnerable, fix deferred
	24.04 LTS noble	Vulnerable, fix deferred
	22.04 LTS jammy	Vulnerable, fix deferred
	20.04 LTS focal	Vulnerable, fix deferred
	18.04 LTS bionic	Vulnerable, fix deferred
	16.04 LTS xenial	Vulnerable, fix deferred
	14.04 LTS trusty	Vulnerable, fix deferred
rsync	26.04 LTS resolute	Not affected
	24.04 LTS noble	Not affected
	22.04 LTS jammy	Not affected
	20.04 LTS focal	Vulnerable, fix deferred
	18.04 LTS bionic	Vulnerable, fix deferred
	16.04 LTS xenial	Vulnerable, fix deferred
	14.04 LTS trusty	Not affected
klibc	26.04 LTS resolute	Vulnerable, fix deferred
	24.04 LTS noble	Vulnerable, fix deferred
	22.04 LTS jammy	Vulnerable, fix deferred
	20.04 LTS focal	Vulnerable, fix deferred
	18.04 LTS bionic	Vulnerable, fix deferred
	16.04 LTS xenial	Vulnerable, fix deferred
	14.04 LTS trusty	Vulnerable, fix deferred
zsync	26.04 LTS resolute	Vulnerable, fix deferred
	24.04 LTS noble	Vulnerable, fix deferred
	22.04 LTS jammy	Vulnerable, fix deferred
	20.04 LTS focal	Vulnerable, fix deferred
	18.04 LTS bionic	Vulnerable, fix deferred
	16.04 LTS xenial	Vulnerable, fix deferred

Notes

mdeslaur

since 3.2.0-1, rsync builds with the system zlib It is unclear by this CVE, and the advisory that goes along with it, what versions of zlib are affected by this issue. Per the advisory, versions below 1.3.1 are not affected, but the reproducer crashes 1.3.0 in the same way it does 1.3.1. The more severe vulnerability was introduced in 1.3.1.2 by the following commit: https://github.com/madler/zlib/commit/81cc0be The patch below has been tagged as possibly fixing the issue, and was made after 1.3.2, but it doesn't fix the issue when tested with the reproducer. The suggested patch in the upstream bug is very different. As of 2026-09-11, there is no patch available from zlib developers to fix this issue. Marking as deferred.

Patch details

For informational purposes only. We recommend not to cherry-pick updates. How can I get the fixes?

Package	Patch details
zlib	Upstream: e3dc0a8

Severity score breakdown

CVSS version:

Base score 8.3 · High

Base metrics

Parameter	Value
Attack vector	Network
Attack complexity	High
Attack requirements	None
Privileges required	None
User interaction	None
Vulnerable system - Confidentiality impact	None
Vulnerable system - Integrity impact	High
Vulnerable system - Availability impact	High
Subsequent system - Confidentiality impact	None
Subsequent system - Integrity impact	None
Subsequent system - Availability impact	None

Scores

Parameter	Value
Base score	8.3 · High
Base + Threat score	-
Base + Environmental score	-
Base + Threat + Environmental score	-

Vector: CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:N/VC:N/VI:H/VA:H/SC:N/SI:N/SA:N

Base score 7.4 · High

Base metrics

Parameter	Value
Attack vector	Network
Attack complexity	High
Privileges required	None
User interaction	None
Scope	Unchanged
Confidentiality impact	None
Integrity impact	High
Availability impact	High

Scores

Parameter	Value
Base score	7.4 · High
Exploitability score	-
Impact score	-

Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:H

CVE-2026-85091

Cvss 3 Severity Score