A newly disclosed security flaw, CVE-2026-42533, has revealed a critical Pre-Auth nginx vulnerability that could allow attackers to achieve reliable RCE (remote code execution) without authentication. The issue affects nginx versions 0.9.6 through 1.30.3 (stable) and 1.31.2 (mainline), while patched releases include 1.30.4 and 1.31.3. Affected NGINX Plus versions include R33-R36 (fixed in R36 P7) and 37.0.0.1-37.0.2.1 (fixed in 37.0.3.1).
According to the disclosure, the vulnerability stems from a missing save-and-restore mechanism for PCRE capture state within nginx’s two-pass script evaluation engine. The flaw enables attackers to trigger a heap buffer overflow with attacker-controlled content and length, while also exposing heap pointers through an information leak that can defeat Address Space Layout Randomization (ASLR). Chaining both primitives enables reliable Pre-Auth nginx RCE.
CVE-2026-42533 Impacts Multiple Configurations
The advisory warns that deployments using map directives with regex patterns alongside regex capture sources, including location, server_name, rewrite, or if blocks, may be vulnerable. The issue depends on evaluation order, where regex capture references, such as $1 or named groups, are processed before a regex map variable.
Affected directives include proxy_set_header, proxy_method, proxy_pass, fastcgi_param, uwsgi_param, scgi_param, grpc_set_header, return, add_header, rewrite, set, root, alias, and access_log, among others. Both HTTP and stream modules are affected, and the vulnerable capture and map variables do not need to exist within the same directive.
Technical Root Cause
The researcher explained that nginx evaluates expressions in two stages: a length calculation (LEN) pass followed by a value (VALUE) pass. During execution, regex map evaluation overwrites shared capture data stored in the request object. As a result, the LEN pass and VALUE pass can calculate different capture sizes, causing either a heap overflow or an information leak depending on the relative capture lengths.
The disclosure states that attackers can control both the overflow size and leaked data using ordinary HTTP requests, including request URIs, headers, and bodies. No credentials, client certificates, or unusual configuration beyond the vulnerable pattern are required.
Testing reportedly achieved 10 out of 10 successful exploitations on Ubuntu 24.04 using glibc 2.39 with ASLR enabled.
Mitigation and Disclosure
The researcher said recent fixes for CVE-2026-42945, CVE-2026-9256, CVE-2026-42055, and CVE-2026-48142 do not address CVE-2026-42533. Administrators are advised to upgrade immediately to nginx 1.30.4, 1.31.3, or the corresponding patched NGINX Plus releases.
Until systems are updated, defenders should audit configurations that combine regex captures with regex map variables in the same evaluation path. The researcher also released a static configuration scanner that identifies vulnerable configurations without exploiting them.
The initial report was submitted to F5 SIRT on May 17, 2026, with follow-up analyses covering additional variants, including cross-directive triggering and named capture clobbering. While a proof-of-concept exploit exists, the researcher said it will be withheld until users have sufficient time to apply patches, citing concerns over rapid exploitation following previous nginx vulnerability disclosures.
