CVE-2026-42533

Description

A vulnerability exists in NGINX Plus and NGINX Open Source when a map directive uses regex matching and a string expression references the map's regex capture variables before referencing the map output variable. Alternatively, the same result could be achieved by using a non-cacheable variable in a string expression under certain conditions. An unauthenticated attacker along with conditions beyond their control can exploit this vulnerability by sending crafted HTTP requests. This may cause a heap buffer overflow in the NGINX worker process leading to a restart. Additionally, attackers can execute code on systems with Address Space Layout Randomization (ASLR) disabled or when the attacker can bypass ASLR. Impact: This vulnerability may allow remote attackers to cause a denial-of-service (DoS) on the NGINX system or to possibly trigger a code execution. There is no control plane exposure; this is a data plane issue only. Note: Software versions which have reached End of Technical Support (EoTS) are not evaluated.

Read the notes from the security team

Status

Show unmaintained releases

Package	Ubuntu Release	Status
nginx	26.10 stonking	Not affected
	26.04 LTS resolute	Fixed in 1.28.3-2ubuntu1.11
	24.04 LTS noble	Fixed in 1.24.0-2ubuntu7.18
	22.04 LTS jammy	Fixed in 1.18.0-6ubuntu14.21
	20.04 LTS focal	Fixed in 1.18.0-0ubuntu1.7+esm4
	18.04 LTS bionic	Fixed in 1.14.0-0ubuntu1.11+esm5
	16.04 LTS xenial	Fixed in 1.10.3-0ubuntu0.16.04.5+esm10
	14.04 LTS trusty	Fixed in 1.4.6-1ubuntu3.9+esm9

Get expanded security coverage with Ubuntu Pro

Reduce your average CVE exposure time from 98 days to 1 day with expanded CVE patching, ten-years security maintenance and optional support for the full stack of open-source applications. Free for personal use.

Get Ubuntu Pro 30-day free trial

Notes

mdeslaur

the 78950bdc commit applies a similar fix to access log While this CVE was originally fixed in USN-8563-1, it was later discovered that the patches below change ABI and could break third party modules. The fix for this CVE was then reverted in USN-8563-2 pending a better fix. A better fix was released in USN-8563-3, but was reverted again in USN-8563-4 as it introduced a regression in certain environments. This CVE was fixed again in USN-8563-5.

Patch details

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

Package	Patch details
nginx	Upstream: ea47fab Upstream: 7ac6789 Upstream: 326b17b Upstream: 97e40e5 Upstream: 78950bd

Severity score breakdown

CVSS version:

Base score 9.2 · Critical

Base metrics

Parameter	Value
Attack vector	Network
Attack complexity	High
Attack requirements	None
Privileges required	None
User interaction	None
Vulnerable system - Confidentiality impact	High
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	9.2 · Critical
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:H/VI:H/VA:H/SC:N/SI:N/SA:N

Base score 8.1 · High

Base metrics

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

Scores

Parameter	Value
Base score	8.1 · High
Exploitability score	-
Impact score	-

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

References

Related Ubuntu Security Notices (USN)

USN-8563-1
nginx vulnerabilities
20 July 2026

USN-8563-3
nginx vulnerability
19 August 2026

USN-8563-5
nginx vulnerability
14 September 2026

USN-8955-2
nginx vulnerabilities
5 November 2026