Summary

AVideo's _session_start() function accepts arbitrary session IDs via the PHPSESSID GET parameter and sets them as the active PHP session. A session regeneration bypass exists for specific blacklisted endpoints when the request originates from the same domain. Combined with the explicitly disabled session regeneration in User::login(), this allows a classic session fixation attack where an attacker can fix a victim's session ID before authentication and then hijack the authenticated session.

Details

The vulnerability is a chain of three weaknesses that together enable session fixation:

1. Attacker-controlled session ID acceptance (objects/functionsPHP.php:344-367)

function _session_start(array $options = [])
{
    // ...
    if (isset($_GET['PHPSESSID']) && !_empty($_GET['PHPSESSID'])) {
        $PHPSESSID = $_GET['PHPSESSID'];
        // ...
        if (!User::isLogged()) {
            if ($PHPSESSID !== session_id()) {
                _session_write_close();
                session_id($PHPSESSID);   // <-- sets session to attacker's ID
            }
            $session = @session_start($options);  // <-- starts with attacker's ID

The code reads $_GET['PHPSESSID'] and programmatically calls session_id($PHPSESSID), which bypasses both session.use_only_cookies and session.use_strict_mode PHP settings since the session ID is set via the PHP API, not via cookie/URL handling.

2. Session regeneration bypass for blacklisted endpoints (objects/functionsPHP.php:375-378, objects/functions.php:3100-3116)

// functionsPHP.php:375-378
if (!blackListRegenerateSession()) {
    _session_regenerate_id();  // <-- SKIPPED when blacklisted + same-domain
}

// functions.php:3100-3116
function blackListRegenerateSession()
{
    if (!requestComesFromSafePlace()) {
        return false;
    }
    $list = [
        'objects/getCaptcha.php',
        'objects/userCreate.json.php',
        'objects/videoAddViewCount.json.php',
    ];
    foreach ($list as $needle) {
        if (str_ends_with($_SERVER['SCRIPT_NAME'], $needle)) {
            return true;  // <-- regeneration skipped for these endpoints
        }
    }
    return false;
}

The requestComesFromSafePlace() check at objects/functionsSecurity.php:182 only verifies that HTTP_REFERER matches the AVideo domain. When a victim clicks a link from within the AVideo platform (e.g., in a comment or video description), the browser naturally sets the Referer to the AVideo domain, satisfying this check.

3. Disabled session regeneration on login (objects/user.php:1315-1317)

// Call custom session regenerate logic
// this was regenerating the session all the time, making harder to save info in the session
//_session_regenerate_id();  // <-- COMMENTED OUT

The session regeneration after authentication is explicitly disabled. This means the session ID persists unchanged through the login transition, which is the fundamental requirement for session fixation to succeed.

Amplifying factors

• objects/phpsessionid.json.php exposes session IDs to any same-origin JavaScript without authentication (line 12: $obj->phpsessid = session_id())
• view/js/session.js stores the session ID in a global window.PHPSESSID variable and logs it to console (line 15)
• No session-to-IP or session-to-user-agent binding exists (verified via codebase search)

PoC

Step 1: Attacker obtains a session ID

# Attacker visits the site to get a valid session ID
curl -v https://target.example.com/ 2>&1 | grep 'set-cookie.*PHPSESSID'
# Response: Set-Cookie: PHPSESSID=attacker_known_session_id; ...

Step 2: Attacker injects a link on the platform

The attacker posts a comment on a video or creates content containing a link:

https://target.example.com/objects/getCaptcha.php?PHPSESSID=attacker_known_session_id

This can be placed in a video comment, video description, user bio, or forum post — anywhere AVideo renders user-provided links.

Step 3: Victim clicks the link while browsing AVideo

When the victim clicks the link from within the AVideo platform:

1. Browser sets Referer: https://target.example.com/... (same-domain)
2. _session_start() processes $_GET['PHPSESSID'], victim is not logged in, so session_id('attacker_known_session_id') is called
3. blackListRegenerateSession() returns true (script is getCaptcha.php + same-domain Referer)
4. _session_regenerate_id() is skipped
5. Victim's session is now fixed to attacker_known_session_id

Step 4: Victim logs in

The victim navigates to the login page and authenticates. User::login() populates $_SESSION['user'] but does NOT regenerate the session ID (line 1317 is commented out).

Step 5: Attacker hijacks the authenticated session

# Attacker uses the known session ID to access victim's account
curl -b "PHPSESSID=attacker_known_session_id" https://target.example.com/objects/user.php?userAPI=1
# Response: victim's user data, confirming session hijack

Impact

• Full account takeover: An attacker can hijack any user's authenticated session, including administrator accounts
• Data access: Full access to the victim's videos, private content, messages, and personal information
• Privilege escalation: If the victim is an admin, the attacker gains full administrative control over the AVideo instance
• Lateral actions: The attacker can perform any action as the victim — upload/delete content, modify settings, access admin panel

Recommended Fix

Fix 1: Re-enable session regeneration on login (objects/user.php:1317)

// Replace the commented-out line:
//_session_regenerate_id();

// With:
_session_regenerate_id();

This is the most critical fix. Session regeneration on authentication transition is a fundamental defense against session fixation (OWASP recommendation).

Fix 2: Remove GET-based session ID acceptance (objects/functionsPHP.php:344-383)

Remove or restrict the $_GET['PHPSESSID'] handling entirely. If it is needed for specific use cases (e.g., CAPTCHA), validate the session ID against a server-side token rather than blindly accepting arbitrary values:

// Instead of accepting any GET PHPSESSID, remove this block entirely.
// If CAPTCHA requires session continuity, pass a CSRF token instead.
if (isset($_GET['PHPSESSID']) && !_empty($_GET['PHPSESSID'])) {
    // REMOVED: Do not accept session IDs from URL parameters
}

Fix 3: Remove session ID exposure (objects/phpsessionid.json.php, view/js/session.js)

The phpsessionid.json.php endpoint and the session.js global variable negate the httponly cookie flag. If JavaScript needs to reference the session for AJAX requests, the browser automatically includes session cookies — there is no need to expose the session ID value to JavaScript.

References

• GHSA-x3pr-vrhq-vq43