Executive Summary

This vulnerability is an open redirect issue (CWE-601) in the _safe_redirect function of the click-tracking endpoint (/c/<token>/) in Mailerup versions before 1.0.0. It allows remote unauthenticated attackers to redirect users to arbitrary external websites by manipulating a crafted query parameter. Although the URL scheme is validated to block dangerous schemes like javascript: and data:, the destination host is not restricted to an allowlist. Additionally, a signing.BadSignature exception is silently caught, so attackers do not need a valid signed token to exploit this.

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Impact Analysis

This vulnerability can be exploited by attackers to redirect victims to malicious external sites, enabling phishing attacks. Users may be tricked into visiting harmful websites that could steal sensitive information or install malware. Because the redirect does not require authentication or a valid token, it can be used broadly against any user of the affected Mailerup service.

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Detection Guidance

This vulnerability involves an open redirect in the click-tracking endpoint (/c/<token>/) of Mailerup versions prior to 1.0.0. Detection can focus on monitoring HTTP requests to this endpoint with crafted query parameters that redirect to external sites.

You can detect attempts to exploit this vulnerability by inspecting web server logs or using network monitoring tools to identify requests to the /c/<token>/ endpoint containing suspicious or unexpected external URLs in the query parameters.

Example commands to detect such activity might include:

• Using grep on web server logs to find requests to the vulnerable endpoint with suspicious query parameters: `grep "/c/" /var/log/nginx/access.log | grep "u=http"`
• Using tcpdump or tshark to capture HTTP traffic and filter for requests to the /c/ path: `tshark -Y 'http.request.uri contains "/c/"' -T fields -e http.request.full_uri`

Further analysis can be done by reviewing the URLs in the query parameters to check if they redirect to untrusted external domains.

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Mitigation Strategies

To mitigate this open redirect vulnerability in Mailerup versions prior to 1.0.0, immediate steps include:

• Upgrade Mailerup to version 1.0.0 or later where the vulnerability has been fixed.
• Apply the security fixes that validate redirect URLs to only allow http and https schemes, blocking dangerous schemes like javascript: or data:.
• Restrict or monitor access to the /c/<token>/ endpoint to detect and prevent unauthorized or suspicious redirect attempts.
• Review and implement administrative controls for user account creation to prevent anonymous registrations, as done in the fix.
• Ensure API keys are not exposed in responses or admin overlays to reduce attack surface.

These steps collectively reduce the risk of phishing attacks leveraging this vulnerability.

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Compliance Impact

The vulnerability is an open redirect that allows remote unauthenticated attackers to redirect victims to arbitrary external sites and conduct phishing attacks. This can lead to potential security risks such as phishing, which may indirectly impact compliance with regulations like GDPR and HIPAA that require protection of user data and prevention of unauthorized access or attacks.

However, the provided information does not explicitly describe how this vulnerability affects compliance with specific standards or regulations such as GDPR or HIPAA.

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