CVE-2026-42945: NGINX ngx_http_rewrite_module Heap Buffer Overflow

NGINX Plus and NGINX Open Source are susceptible to a heap buffer overflow vulnerability (CVE-2026-42945) within the ngx_http_rewrite_module. This flaw arises when the rewrite directive is used in conjunction with a subsequent rewrite, if, or set directive, and an unnamed Perl-Compatible Regular Expression (PCRE) capture (e.g., $1, $2) includes a question mark (?) within its replacement string. An unauthenticated attacker, by sending specially crafted HTTP requests, can exploit this condition. Successful exploitation can lead to a heap buffer overflow in the NGINX worker process, resulting in a restart and potential denial of service. On systems where Address Space Layout Randomization (ASLR) is disabled, successful exploitation may lead to arbitrary code execution.

Attack Chain

1. The attacker crafts a malicious HTTP request targeting a vulnerable NGINX server. This request is designed to trigger the flawed rewrite logic.
2. The request contains a specific URI or header that will be processed by the ngx_http_rewrite_module.
3. The NGINX configuration utilizes the rewrite directive followed by either rewrite, if, or set.
4. The rewrite directive uses an unnamed PCRE capture (e.g., $1, $2) with a replacement string.
5. The replacement string within the PCRE capture includes a question mark (?). This is a crucial component of the exploit.
6. When the NGINX worker processes the crafted request and applies the rewrite rules, the question mark within the PCRE capture’s replacement string causes a heap buffer overflow.
7. The heap buffer overflow corrupts memory within the NGINX worker process.
8. The corruption leads to a crash of the NGINX worker process, causing a restart and potential denial of service. On systems with ASLR disabled, the attacker might achieve code execution.

Impact

Successful exploitation of CVE-2026-42945 can lead to a denial-of-service condition, as the NGINX worker process crashes and restarts. This can disrupt web services and applications served by the affected NGINX instance. In scenarios where ASLR is disabled, the attacker could potentially achieve arbitrary code execution on the server, leading to complete system compromise. The number of affected systems depends on the prevalence of vulnerable NGINX configurations.

Recommendation

• Apply the official patch or upgrade to a version of NGINX Plus or NGINX Open Source that addresses CVE-2026-42945.
• Deploy the Sigma rules provided to detect exploitation attempts targeting CVE-2026-42945 in your NGINX webserver logs.
• Enable Address Space Layout Randomization (ASLR) on systems running NGINX to mitigate potential code execution following a heap overflow.
• Review NGINX configurations for instances of the rewrite directive used in conjunction with rewrite, if, or set and unnamed PCRE captures containing question marks (?).