CVE-2026-90913 in Joomla Extension
Summary
by MITRE • 09/29/2026
Joomla! Core - [20260903] - Core - Improper ACL checks for access level webservice endpoints in Joomla 4.0.0-5.4.8, 6.0.0-6.1.3 - An improper access check allows unauthorized users to perform mutation actions in access level endpoints.
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Analysis
by VulDB Data Team • 09/29/2026
The vulnerability identified as CVE-20260903 represents a critical failure in the Access Control List (ACL) implementation within the Joomlas role-based access control model.
From a technical perspective, this vulnerability is classified under CWE-284, which denotes Improper Access Control. The root cause lies in the API layer where web service requests are processed without adequate verification of the user's permission set relative to the specific action being attempted. When an attacker sends a mutation request to these endpoints, the system accepts and executes the command regardless of whether the requester has been granted explicit authorization for such operations. This lack of server-side validation allows any authenticated user, including those with minimal privileges such as registered users or guests depending on configuration, to alter critical access level settings. The flaw is particularly dangerous because it exploits the trust placed in API endpoints that are often assumed to be secure due to their programmatic nature rather than direct browser interaction.
The operational impact of this vulnerability is severe and multifaceted. By exploiting improper ACL checks for access level webservice endpoints, an attacker can escalate privileges within the Joomla! installation. This could involve creating new administrator accounts, modifying existing user groups to grant elevated permissions, or disabling security features that rely on strict access controls. Such actions compromise the confidentiality, integrity, and availability of the web application. An adversary with this capability can take full control of the website, inject malicious content, steal sensitive data from users, or use the compromised server as a pivot point for further attacks against internal networks. The ability to mutate access levels essentially nullifies the security model designed to protect administrative functions from unauthorized interference.
This vulnerability aligns with several tactics in the MITRE ATT&CK framework, particularly those related to Privilege Escalation and Defense Evasion. Attackers may leverage this flaw as part of a broader campaign to gain persistent access or maintain control over compromised systems by altering permission structures to ensure future entry even if initial credentials are revoked. The exploitation typically involves crafting specific HTTP requests targeting the affected API endpoints with payloads designed to modify user group assignments or permissions. Since the vulnerability exists in core code, it affects all installations running the specified versions unless patched, making widespread remediation essential for maintaining security posture across the Joomla! ecosystem.
Mitigation strategies must prioritize immediate patching of the underlying software. Administrators should upgrade their Joomla! instances to a version that includes the fix for this specific ACL validation failure. Until patches are applied or deployed via custom overrides, organizations can implement additional layers of defense such as Web Application Firewalls configured to detect and block anomalous API requests targeting access level endpoints. It is also advisable to review server logs for unusual patterns in web service calls, particularly those involving mutations on user management APIs. Furthermore, enforcing strict network segmentation and limiting direct exposure of administrative interfaces can reduce the attack surface available to potential exploiters. Regular audits of user permissions and adherence to the principle of least privilege remain critical complementary measures to mitigate risks associated with such core vulnerabilities.