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BusyBox httpd reverse proxy (CONFIG_FEATURE_HTTPD_PROXY, default enabled). When a P: rule matches, handle_incoming_and_exit() in networking/httpd.c rewrites only the first request line to the backend path (lines 2458-2462), sets G.POST_len = INT_MAX (line 2467), and enters cgi_io_loop_and_exit() (line 2468), which relays further client bytes to the backend without HTTP parsing. A pipelined second request on the same TCP connection therefore reaches the backend at an attacker-chosen path and method, bypassing the P: prefix boundary. This is distinct from Basic auth on proxied URLs: upstream documents that proxying is done without password checks (comment near line 76); maintainer Denys Vlasenko (22 Sep 2026) confirmed that framing as an auth bypass is intended. The pipelining/path-boundary issue was not addressed upstream. Introduced with verbatim proxy relay (commit c69f64845, 2019); affects 1.31.0 through master. No fix. Fedora enables the proxy feature; Debian does not. Independent verification: Kaiden Opus 2026-10-14 (reject-dispute run) confirms smuggling; withdraws auth-bypass as CVE scope. Reported by Shubham Raj, Causal Security (https://causalsecurity.com).