RCE Proof of concept for CVE-2026-42945, a critical heap buffer overflow in NGINX's ngx_http_rewrite_module introduced in 2008. The bug enables unauthenticated remote code execution against servers using rewrite and set directives.

This vulnerability — along with three other memory corruption issues (CVE-2026-42946, CVE-2026-40701, CVE-2026-42934) — was autonomously discovered by depthfirst's security analysis system after a single click of onboarding the NGINX source.

Want to find issues like this in your own code? Try the same system at https://depthfirst.com/open-defense.

NGINX's script engine uses a two-pass process: first compute the required buffer size, then copy data in. The is_args flag is set on the main engine when a rewrite replacement contains ?, but the length-calculation pass runs on a freshly zeroed sub-engine. So:

• Length pass sees is_args = 0 → returns raw capture length.
• Copy pass sees is_args = 1 → calls ngx_escape_uri with NGX_ESCAPE_ARGS, expanding each escapable byte to 3 bytes.

The copy overflows the undersized heap buffer with attacker-controlled URI data. Exploitation uses cross-request heap feng shui to corrupt an adjacent ngx_pool_t's cleanup pointer (sprayed via POST bodies, since URI bytes can't contain null bytes), redirecting it to a fake ngx_pool_cleanup_s invoking system() on pool destruction.

Read more about this bug in our technical write-up.

Full vendor advisory: https://my.f5.com/manage/s/article/K000160932

Tested on Ubuntu 24.04.3 LTS.

1. ./setup.sh — build the container.
2. docker compose -f env/docker-compose.yml up — start the vulnerable NGINX server.
3. python3 poc.py --shell — pop a shell.