Executive Summary

CVE-2025-35939 is a medium-severity vulnerability affecting Craft CMS versions prior to 4.15.3 and between 5.0.0 and 5.7.5. The vulnerability arises from improper handling of session files, allowing unauthenticated users to inject arbitrary content, potentially leading to remote code execution (RCE) if exploited in conjunction with other vulnerabilities. Given the low complexity of exploitation and the lack of authentication requirements, this vulnerability poses a significant risk to organizations using affected versions of Craft CMS. Immediate patching and robust detection strategies are essential to mitigate potential exploitation.

Vulnerability Deep Dive

Root Cause Analysis

The vulnerability is rooted in the way Craft CMS handles session data. Specifically, the application stores arbitrary content provided by unauthenticated users in session files located at /var/lib/php/sessions. The session files are named sess_[session_value], where [session_value] is derived from the client's Set-Cookie response header. The core issue lies in the failure to sanitize the return URL parameters, allowing attackers to inject malicious content into session files.

Historically, this type of vulnerability often stems from:
- Inadequate Input Validation: The application does not properly validate or sanitize user inputs before storing them.
- Session Management Flaws: Improper handling of session data can lead to unauthorized access or manipulation.

Technical Mechanism

The vulnerability allows an attacker to craft a request that includes malicious PHP code in the return URL parameter. When this value is stored in the session file, it can potentially be executed if the application later processes this session data without adequate checks. The exploitation mechanism can be summarized as follows:
1. An unauthenticated user sends a request with a crafted return URL.
2. Craft CMS stores this URL in a session file without sanitization.
3. If the session data is later executed or included in a PHP context, the malicious code can be executed.

Attack Prerequisites

For exploitation to occur, the following conditions must be met:
- The attacker must be able to send HTTP requests to the Craft CMS server.
- The attacker must craft a request that includes a malicious return URL.
- The application must process the session data in a way that allows for the execution of arbitrary code.

Threat Intelligence

Known Exploitation

As of the publication date of this CVE, there are no confirmed reports of active exploitation in the wild. However, given the nature of the vulnerability and its low complexity, it is reasonable to anticipate that threat actors may begin to exploit it, especially in environments where Craft CMS is widely used.

Threat Actor Activity

Potential threat actors include:
- Script Kiddies: Due to the low technical barrier for exploitation.
- Ransomware Gangs: Could leverage this vulnerability to gain initial access to systems for further exploitation.

Attack Patterns

The exploitation pattern is likely to follow a straightforward approach:
1. Scanning for vulnerable Craft CMS installations.
2. Sending crafted requests to inject malicious payloads.
3. Executing the payloads to gain further access or control over the server.

Technical Analysis

Proof of Concept

A basic proof-of-concept (PoC) for exploiting this vulnerability can be constructed as follows:

import requests

# Target URL of the vulnerable Craft CMS instance
target_url = "http://example.com/path/to/craft"

# Malicious return URL that contains PHP code
malicious_return_url = "http://example.com/path/to/malicious_code.php"

# Crafting the request
payload = {
    "returnUrl": malicious_return_url
}

# Sending the request
response = requests.get(target_url, params=payload)

print("Response Code:", response.status_code)

Exploitation Techniques

Attackers may use various techniques to exploit this vulnerability, including:
- Direct Injection: As demonstrated in the PoC, injecting a malicious URL directly.
- Chaining Vulnerabilities: Combining this vulnerability with other known vulnerabilities in the application or server configuration to escalate privileges or execute arbitrary code.

Bypass Methods

Common mitigations that may be bypassed include:
- Basic Input Validation: If the application performs minimal checks on the return URL, attackers can craft inputs that evade these checks.
- Session Management Controls: Weak session management practices can allow attackers to manipulate session files easily.

Detection & Response

Behavioral Indicators

Organizations should monitor for unusual session file creations or modifications in /var/lib/php/sessions. Indicators of compromise may include:
- Session files with unexpected names or contents.
- Increased traffic to the Craft CMS application from unknown IP addresses.

Forensic Artifacts

Forensic analysis should focus on:
- Reviewing session files for unauthorized modifications.
- Analyzing web server logs for suspicious request patterns.

Hunting Queries

Security teams can employ the following hunting queries to detect potential exploitation attempts:

SELECT * FROM logs WHERE request LIKE '%returnUrl=%' AND response_code != 200;

Mitigation Engineering

Immediate Actions

  • Patch: Upgrade to Craft CMS versions 4.15.3 or 5.7.5 immediately.
  • Monitor: Implement logging and monitoring for session file access.

Long-term Hardening

  • Input Validation: Enforce strict input validation and sanitization for all user inputs.
  • Session Management: Review and strengthen session management practices to prevent unauthorized access.

Architectural Improvements

  • Web Application Firewalls (WAF): Deploy a WAF to filter out malicious requests before they reach the application.
  • Code Reviews: Conduct regular code reviews focusing on security vulnerabilities, particularly around input handling and session management.

Real-World Impact

Case Studies

While specific case studies related to CVE-2025-35939 are not available due to its recent disclosure, similar vulnerabilities in web applications have historically led to significant breaches, including unauthorized access to sensitive data and system control.

Business Risk

Organizations using vulnerable versions of Craft CMS face risks including:
- Data breaches resulting from unauthorized access.
- Financial losses due to operational downtime or remediation costs.
- Reputational damage from public disclosures of security incidents.

Industry Analysis

The risk associated with this vulnerability is particularly acute in industries that rely heavily on web applications for customer interaction, such as e-commerce and content management.

Intelligence Outlook

Threat Evolution

As exploitation techniques evolve, attackers may develop more sophisticated methods to exploit similar vulnerabilities, particularly in widely used platforms like Craft CMS.

Organizations should remain vigilant for related vulnerabilities in session management and input validation across other web applications.

Future Considerations

The emergence of vulnerabilities like CVE-2025-35939 underscores the need for continuous security assessments and proactive measures to safeguard web applications against evolving threats.

In conclusion, CVE-2025-35939 presents a significant risk to Craft CMS users, necessitating immediate action and ongoing vigilance to mitigate potential exploitation.