Platform
php
Component
kanboard
Opgelost in
1.2.50
CVE-2026-21881 is a critical authentication bypass vulnerability affecting Kanboard project management software versions 1.2.48 and earlier. An attacker can exploit this flaw to impersonate any user, including administrators, gaining unauthorized access to sensitive project data and control over the system. The vulnerability is caused by the application's failure to properly validate HTTP headers when REVERSEPROXYAUTH is enabled. A fix is available in version 1.2.49.
This authentication bypass vulnerability poses a significant risk to Kanboard deployments. An attacker can leverage it to gain complete control over the system by impersonating an administrator. This could lead to unauthorized modification or deletion of project data, creation of fake users, and potentially even the installation of malicious code. The blast radius extends to all data managed within the Kanboard instance, and the attacker could use this access to pivot to other systems on the network if Kanboard is integrated with other services. The ease of exploitation, requiring only the ability to craft HTTP headers, makes this a high-priority concern.
CVE-2026-21881 was publicly disclosed on 2026-01-08. No public proof-of-concept (PoC) code has been released at the time of writing, but the vulnerability's simplicity suggests that a PoC is likely to emerge quickly. The CVSS score of 9.1 (CRITICAL) reflects the high likelihood of exploitation and the significant impact. It is not currently listed on CISA KEV, but its severity warrants monitoring. Active exploitation is not yet confirmed.
Organizations using Kanboard for project management, particularly those with REVERSEPROXYAUTH enabled, are at risk. This includes teams relying on Kanboard for task tracking, bug reporting, and collaboration. Shared hosting environments where Kanboard is installed alongside other applications are also at increased risk due to the potential for cross-site contamination.
• php: Examine Kanboard configuration files for the presence and value of REVERSEPROXYAUTH. Check access logs for unusual authentication attempts with spoofed HTTP headers.
• generic web: Use curl to test authentication with crafted HTTP headers. Example:
curl -H "X-Kanboard-User: attacker" http://kanboard.example.com/login• generic web: Monitor web server access logs for requests containing suspicious or unexpected values in HTTP headers related to authentication (e.g., X-Kanboard-User).
disclosure
Exploit Status
EPSS
0.32% (55% percentiel)
CISA SSVC
CVSS-vector
The primary mitigation for CVE-2026-21881 is to immediately upgrade Kanboard to version 1.2.49 or later. If upgrading is not immediately feasible, disabling the REVERSEPROXYAUTH feature can reduce the attack surface, but this may impact legitimate users relying on reverse proxy authentication. Consider implementing a Web Application Firewall (WAF) with rules to filter suspicious HTTP headers, specifically those related to user authentication. Regularly review Kanboard's access logs for any unusual activity. After upgrading, confirm the fix by attempting to authenticate with a spoofed HTTP header; authentication should fail.
Actualice Kanboard a la versión 1.2.49 o superior. Esta versión corrige la vulnerabilidad de omisión de autenticación al validar correctamente el origen de las cabeceras HTTP cuando REVERSE_PROXY_AUTH está habilitado. La actualización previene la suplantación de usuarios, incluyendo administradores.
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CVE-2026-21881 is a critical vulnerability in Kanboard versions 1.2.48 and below that allows attackers to bypass authentication by spoofing HTTP headers, potentially impersonating administrators.
You are affected if you are using Kanboard version 1.2.48 or earlier and have REVERSEPROXYAUTH enabled. Upgrade to version 1.2.49 to mitigate the risk.
The recommended fix is to upgrade Kanboard to version 1.2.49 or later. If upgrading is not possible, disable REVERSEPROXYAUTH as a temporary workaround.
Active exploitation has not been confirmed at this time, but the vulnerability's simplicity suggests it may be exploited soon.
Refer to the Kanboard security advisory for detailed information and updates: [https://kanboard.org/security/advisories](https://kanboard.org/security/advisories)
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