F5 NGINX Plus/NGINX Open Source Map Directive heap-based overflow

Summaryinfo

A vulnerability labeled as critical has been found in F5 NGINX Plus and NGINX Open Source. Affected by this issue is some unknown functionality of the component Map Directive. Executing a manipulation can lead to heap-based overflow. This vulnerability is registered as CVE-2026-42533. It is possible to launch the attack remotely. No exploit is available.

Detailsinfo

A vulnerability classified as critical was found in F5 NGINX Plus and NGINX Open Source (the affected version unknown). This vulnerability affects an unknown part of the component Map Directive. The manipulation with an unknown input leads to a heap-based overflow vulnerability. The CWE definition for the vulnerability is CWE-122. A heap overflow condition is a buffer overflow, where the buffer that can be overwritten is allocated in the heap portion of memory, generally meaning that the buffer was allocated using a routine such as malloc(). As an impact it is known to affect integrity, and availability. CVE summarizes:

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.

The advisory is available at my.f5.com. This vulnerability was named CVE-2026-42533 since 05/05/2026. The exploitation appears to be easy. The attack can be initiated remotely. No form of authentication is required for a successful exploitation. The technical details are unknown and an exploit is not available. The structure of the vulnerability defines a possible price range of USD $5k-$25k at the moment (estimation calculated on 07/15/2026).

There is no information about possible countermeasures known. It may be suggested to replace the affected object with an alternative product.

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Productinfo

Type

• Web Server

Vendor

• F5

Name

• NGINX Open Source
• NGINX Plus

License

• commercial

Website

• Vendor: https://f5.com/

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