CVE-2025-41074: LimeSurvey DoS Vulnerability

CVE-2025-41074 Overview

CVE-2025-41074 is a Denial of Service (DoS) vulnerability affecting LimeSurvey version 6.13.0. The vulnerability exists in the /optout endpoint, which triggers an infinite HTTP redirect loop when accessed directly. This behavior can be exploited by attackers to exhaust server or client resources, leading to service degradation or browser instability. The system lacks proper redirect loop detection and prevention mechanisms, allowing the infinite loop to persist.

Critical Impact

Attackers can exploit this vulnerability to cause service disruption by overwhelming server resources or causing client-side browser instability through infinite HTTP redirects.

Affected Products

• LimeSurvey 6.13.0

Discovery Timeline

• 2025-11-20 - CVE-2025-41074 published to NVD
• 2025-11-21 - Last updated in NVD database

Technical Details for CVE-2025-41074

Vulnerability Analysis

This vulnerability is classified under CWE-835 (Loop with Unreachable Exit Condition), commonly known as an Infinite Loop vulnerability. The /optout endpoint in LimeSurvey 6.13.0 contains flawed redirect logic that causes the server to continuously issue HTTP redirect responses when the endpoint is accessed directly without proper parameters or context.

When a user or automated tool sends a request to the vulnerable /optout endpoint, the application responds with a redirect that points back to the same endpoint or creates a circular redirect chain. Since there is no mechanism to detect and break this redirect loop, the process continues indefinitely. This consumes server resources (connection handlers, memory, CPU cycles) for each request and can also cause client browsers to become unresponsive or crash.

The vulnerability can be triggered remotely without authentication, making it accessible to any network attacker who can reach the LimeSurvey installation.

Root Cause

The root cause is improper handling of redirect logic in the /optout endpoint. The application fails to implement proper exit conditions or redirect depth limiting, resulting in an infinite loop condition (CWE-835). The code path responsible for generating redirect responses does not include validation to ensure the target URL differs from the source or that a maximum redirect count has not been exceeded.

Attack Vector

The vulnerability is exploited via network access with no authentication required. An attacker can send direct HTTP requests to the /optout endpoint on a vulnerable LimeSurvey installation. This triggers the infinite redirect loop, which can be weaponized in several ways:

1. Direct DoS: Sending multiple concurrent requests to the endpoint exhausts server connection pools and resources
2. Client-side attack: Directing victims to the vulnerable endpoint causes their browsers to become unresponsive
3. Amplification: The redirect loop amplifies a single request into continuous server processing

The attack is straightforward to execute and requires no special tools or authentication credentials. Any HTTP client capable of sending GET requests can trigger the vulnerability.

Detection Methods for CVE-2025-41074

Indicators of Compromise

• Unusual spike in HTTP requests targeting the /optout endpoint
• Server logs showing repeated redirect responses (HTTP 301/302/307) for the same client session
• Increased server resource utilization (CPU, memory, connection handlers) without corresponding legitimate traffic
• Client reports of browser hanging or crashing when accessing survey opt-out links

Detection Strategies

• Monitor web server access logs for patterns of repeated requests to /optout from the same source IP
• Implement rate limiting rules that trigger alerts on excessive redirect responses
• Configure web application firewalls (WAF) to detect and block redirect loop patterns
• Review LimeSurvey application logs for error conditions related to the opt-out functionality

Monitoring Recommendations

• Set up alerting for abnormal traffic patterns to the /optout endpoint
• Monitor server resource metrics (CPU, memory, active connections) for sudden spikes
• Track HTTP response codes and alert on unusual volumes of redirect responses
• Implement logging for redirect chain depth to identify potential loop conditions

How to Mitigate CVE-2025-41074

Immediate Actions Required

• Review the INCIBE Security Notice for vendor-specific guidance
• Implement rate limiting on the /optout endpoint at the web server or WAF level
• Consider temporarily restricting access to the /optout endpoint if not critical to operations
• Monitor for exploitation attempts while awaiting an official patch

Patch Information

Consult the INCIBE Security Notice for the latest patch information and upgrade recommendations from the LimeSurvey vendor. Organizations should upgrade to a patched version of LimeSurvey as soon as one becomes available.

Workarounds

• Configure web server or reverse proxy to limit redirect depth (e.g., Nginx proxy_redirect directives)
• Implement rate limiting on the /optout endpoint to prevent resource exhaustion
• Add WAF rules to detect and block requests that trigger excessive redirects
• Temporarily disable or restrict access to the /optout endpoint if the functionality is not required

# Example Nginx rate limiting configuration for the vulnerable endpoint
# Add to nginx.conf or site configuration

limit_req_zone $binary_remote_addr zone=optout_limit:10m rate=5r/s;

location /optout {
    limit_req zone=optout_limit burst=10 nodelay;
    # Additional redirect depth limiting may require application-level fixes
}