Executive Summary

CVE-2026-21881 is a critical authentication bypass vulnerability in Kanboard versions up to 1.2.48 when the REVERSE_PROXY_AUTH feature is enabled. The vulnerability occurs because Kanboard blindly trusts HTTP headers (like X-Remote-User) for user authentication without verifying that the request comes from a trusted reverse proxy. An attacker can exploit this by sending a spoofed HTTP header with any username, including administrators, to impersonate any user and gain unauthorized access. This allows full administrative control, including modifying settings and creating backdoor accounts. The issue is fixed in version 1.2.49. [1]

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Impact Analysis

This vulnerability can have severe impacts as it allows an attacker to bypass authentication without any credentials or user interaction. An attacker can impersonate any user, including administrators, gaining full access to the Kanboard dashboard and administrative settings. This can lead to unauthorized data access, modification of system settings, creation of persistent backdoor admin accounts, and complete compromise of the Kanboard instance. The attack vector is network-based with low complexity and no privileges required. [1]

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Detection Guidance

You can detect this vulnerability by attempting to access the Kanboard instance with and without spoofed HTTP headers that are used for reverse proxy authentication, such as 'X-Remote-User' or 'REMOTE_USER'. For example, using curl commands: 1. Without spoofed header (should be denied): curl -v https://your-kanboard-instance/ 2. With spoofed header (should gain access if vulnerable): curl -v -H 'X-Remote-User: admin' https://your-kanboard-instance/ If the request with the spoofed header returns HTTP 200 and grants access to the dashboard or admin pages, the system is vulnerable. Additionally, monitoring HTTP requests for unexpected or unauthorized use of these headers can help detect exploitation attempts. [1]

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Mitigation Strategies

Immediate mitigation steps include: 1. Disable the REVERSE_PROXY_AUTH feature if you are not using a reverse proxy by setting `define('REVERSE_PROXY_AUTH', false);` in the configuration. 2. Implement IP-based allowlisting in the ReverseProxyAuth authentication logic to only trust HTTP headers from known and trusted reverse proxy IP addresses or networks. 3. Configure your web server (Nginx or Apache) to strip or override incoming authentication headers from clients and only set them from trusted authentication modules or proxies. 4. Upgrade Kanboard to version 1.2.49 or later, which includes enhanced validation of trusted proxy networks via the `TRUSTED_PROXY_NETWORKS` configuration option, preventing header spoofing from untrusted sources. [1, 2]

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Compliance Impact

This vulnerability allows an attacker to bypass authentication and gain full administrative access to Kanboard, potentially exposing or modifying sensitive project management data. Such unauthorized access can lead to violations of data protection regulations like GDPR and HIPAA, which require strict access controls and protection of personal and sensitive information. Therefore, the vulnerability negatively impacts compliance by undermining confidentiality and integrity requirements mandated by these standards. [1]

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