LimeSurvey DoS Alert: CVE-2025-41075 Exploits /optin Endpoint

Overview

CVE-2025-41075 identifies a critical vulnerability in LimeSurvey version 6.13.0. This vulnerability resides within the /optin endpoint and results in an infinite HTTP redirect loop when accessed directly. This behavior can be exploited by malicious actors to launch a Denial of Service (DoS) attack, potentially exhausting server or client resources and rendering the application unavailable. The system’s inability to break the redirect loop leads to service degradation and possible browser instability for users attempting to access the affected endpoint.

Technical Details

The vulnerability stems from improper handling of direct requests to the /optin endpoint in LimeSurvey 6.13.0. When a user, or an automated script, accesses this endpoint directly, the application initiates a redirect. However, due to a logical flaw in the redirect implementation, the application continuously redirects to the same or similar URL, creating an endless loop. This loop consumes server resources (CPU, memory, network bandwidth) as it processes each redirect. Clients, such as web browsers, also experience resource exhaustion as they attempt to follow the infinite redirects, potentially leading to browser crashes or freezes.

CVSS Analysis

Currently, the CVSS score for CVE-2025-41075 is listed as N/A. However, given the potential for a Denial of Service attack, it is likely to be assigned a Medium to High severity score once officially evaluated. A successful exploit requires no authentication and can be triggered remotely, increasing the attack surface.

Possible Impact

• Denial of Service (DoS): The primary impact is the exhaustion of server resources, leading to the LimeSurvey application becoming unresponsive or unavailable to legitimate users.
• Browser Instability: Users attempting to access the /optin endpoint directly may experience browser crashes or freezes due to the infinite redirect loop.
• Service Degradation: Even if the server doesn’t completely crash, the high resource consumption caused by the attack can lead to overall performance degradation of the LimeSurvey application.

Mitigation or Patch Steps

The most effective mitigation is to upgrade to a patched version of LimeSurvey that addresses this vulnerability. Contact LimeSurvey support or check their official website for the latest information on available patches. Until a patch is available, consider the following temporary mitigation steps:

• Restrict Direct Access: Implement web server rules (e.g., using .htaccess in Apache, or similar configurations in Nginx) to prevent direct access to the /optin endpoint. Redirect unauthorized requests to a safe page or return a 403 Forbidden error.
• Rate Limiting: Implement rate limiting on requests to the LimeSurvey application to mitigate the impact of automated attacks attempting to exploit this vulnerability.
• Web Application Firewall (WAF): Deploy a Web Application Firewall (WAF) with rules specifically designed to detect and block malicious requests targeting the /optin endpoint. Configure your WAF to recognize and block suspicious redirect patterns.

References

INCIBE-CERT Advisory