F5 Releases Security Advisory for Critical Vulnerability in NGINX and NGINX Plus

Successful exploitation of CVE-2026-42533 could allow an unauthenticated remote attacker to cause a denial-of-service condition or potentially achieve remote code execution on a targeted system

Threat ID:: CC-4817
Threat Severity:: Medium
Published:: 20 July 2026 2:06 PM

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Summary

Successful exploitation of CVE-2026-42533 could allow an unauthenticated remote attacker to cause a denial-of-service condition or potentially achieve remote code execution on a targeted system

Affected platforms

The following platforms are known to be affected:

F5 NGINX

NGINX Open Source
- Branch 1.x version 1.31.2
- Branch 1.x versions 1.30.0 to 1.30.3
NGINX Plus
- Branch 37.x versions 37.0.0.1 to 37.0.2.1
- Branch Rx versions R33 to R36
NGINX Ingress Controller
- Branch 5.x versions 2026-lts-r1 to 2026-lts-r3
- Branch 5.x versions 5.0.0 to 5.5.2
- Branch 4.x versions 4.0.0 to 4.0.1
- Branch 3.x versions 3.5.0 to 3.7.2
NGINX Gateway Fabric
- Branch 2.x versions 2.0.0 through 2.6.6
- Branch 1.x versions 1.3.0 through 1.6.2
NGINX App Protect WAF
- Branch 5.x versions 5.2.0 through 5.8.0
- Branch 4.x versions 4.11.0 through 4.16.0
NGINX Instance Manager
- Branch 2.x versions 2.17.0 through 2.22.1
F5 WAF for NGINX
- Branch 5.x versions 5.9.0 through 5.13.3

Threat details

Proof-of-concept exploit in testing environments

Security researchers have published detailed technical analysis describing exploitation techniques and reported reliable proof-of-concept exploitation in testing environments. F5 advises that successful exploitation may create a denial-of-service condition and, in some circumstances, remote code execution.

NHS England National CSOC assess that future exploitation is likely.

Introduction

F5 has released a security advisory to address a critical vulnerability in NGINX Open Source and NGINX Plus. Successful exploitation could allow an unauthenticated remote attacker to trigger a denial-of-service (DoS) on the NGINX system or to possibly trigger a code execution.

CVE-2026-42533 - Heap-based Buffer Overflow vulnerability - CVSSv4 Base Score: 9.2

Note: Attackers could execute code on systems with Address Space Layout Randomization (ASLR) disabled or when the attacker can bypass ASLR.

Remediation advice

Affected organisations are encouraged to review F5 advisory NGINX map directive and regex matching vulnerability CVE-2026-42533 (K000162097) and apply the relevant update as soon as possible. Organisations should additionally identify NGINX deployments using regex-based map directives and ensure use of named captures only when using the map directive.

Definitive source of threat updates

https://my.f5.com/manage/s/article/K000162097

CVE Vulnerabilities

Status Published

CVE-2026-42533

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.

Last edited: 20 July 2026 2:06 pm