F5 NGINX Plus/NGINX Open Source SSI Module heap-based overflow

Summaryinfo

A vulnerability marked as critical has been reported in F5 NGINX Plus and NGINX Open Source. This affects an unknown part of the component SSI Module. The manipulation leads to heap-based overflow. This vulnerability is documented as CVE-2026-56434. The attack can be initiated remotely. There is not any exploit available.

Detailsinfo

A vulnerability, which was classified as critical, has been found in F5 NGINX Plus and NGINX Open Source (the affected version is unknown). This issue affects an unknown code of the component SSI Module. The manipulation with an unknown input leads to a heap-based overflow vulnerability. Using CWE to declare the problem leads to 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(). Impacted is confidentiality, integrity, and availability. The summary by CVE is:

NGINX Plus and NGINX Open Source have a vulnerability in the ngx_http_ssi_module module. This vulnerability may exist when the Server-Side Includes (SSI), proxy_pass, and proxy_buffering off directives are configured. With this configuration, an unauthenticated attacker with man-in-the-middle (MITM) ability to control responses from an upstream server may be able to cause a heap buffer over-read in the NGINX worker process. This issue may lead to limited modification of memory or a restart of the NGINX worker process. Impact: This vulnerability may allow remote attackers to have limited control to modify memory contents or restart the NGINX worker process. 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.

It is possible to read the advisory at my.f5.com. The identification of this vulnerability is CVE-2026-56434 since 07/08/2026. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. The technical details are unknown and an exploit is not publicly available. The pricing for an exploit might be around 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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CPE 2.2info

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