F5 fixes critical nginx flaw CVE-2026-42533 that can crash servers and, in some cases, allow remote code execution through crafted HTTP requests.
F5 released patches for a critical nginx vulnerability, tracked as CVE-2026-42533 (CVSS score of 9.2), that can allow an unauthenticated attacker to trigger a heap buffer overflow using specially crafted HTTP requests.
“heap buffer overflow might occur in a worker process when using the map directive with regex matching if the map variable was included in a string expression after a capture affected by this map; a similar issue might happen when using a non-cacheable variable in a string expression (CVE-2026-42533).” reads the advisory.
CVE-2026-42533 affects NGINX Plus and Open Source when specific regex-based map configurations are used. An unauthenticated attacker can send crafted HTTP requests to trigger a heap buffer overflow, causing crashes or denial of service. In certain conditions, especially if ASLR is disabled or bypassed, the flaw may also allow remote code execution. The issue affects only the data plane, not the control plane.
The vulnerability affects NGINX versions from 0.9.6 through 1.31.2. F5 fixed the issue in NGINX 1.30.4, 1.31.3 and NGINX Plus 37.0.3.1. The flaw impacts only the data plane and does not expose the NGINX control plane.
As a temporary workaround, affected regex-based map configurations can be modified to use named captures instead of numbered captures, but this does not provide complete protection.
The researchers Mufeed VH of Winfunc Research and Maxim Dounin discovered the vulnerability.
Security researcher Stan Shaw (cyberstan) reported that CVE-2026-42533 may have a higher impact than initially described by F5. While F5 highlights denial-of-service risks, Shaw believes the flaw could also help attackers bypass ASLR protections and achieve remote code execution under certain conditions. He has not released exploit details or a proof-of-concept yet.
Shaw released a static scanner that identifies vulnerable NGINX configurations without exploiting the flaw. The tool checks include files, analyzes directive relationships, and detects exploitable patterns. Shaw is delaying publication of the proof-of-concept and technical exploitation details to give organizations time to patch, noting that similar NGINX flaws were exploited soon after PoCs became public.
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