CVE-2025-9072 in Mattermost
Summary
by MITRE • 09/15/2025
Mattermost versions 10.10.x <= 10.10.1, 10.5.x <= 10.5.9, 10.9.x <= 10.9.4 fail to validate the redirect_to parameter, allowing an attacker to craft a malicious link that, once a user authenticates with their SAML provider, could post the user’s cookies to an attacker-controlled URL.
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Analysis
by VulDB Data Team • 10/02/2025
This vulnerability exists in Mattermost server versions within the 10.5.x, 10.9.x, and 10.10.x release branches up to and including 10.5.9, 10.9.4, and 10.10.1 respectively. The flaw resides in the SAML authentication implementation where the redirect_to parameter validation is insufficient, creating a critical security gap that enables open redirect attacks. The vulnerability manifests when users authenticate through SAML providers, as the application fails to properly validate the destination URL parameter that determines where users are redirected after successful authentication.
The technical implementation flaw stems from improper input validation of the redirect_to parameter within the SAML authentication flow. When a user initiates SAML authentication, they are typically redirected to an identity provider for verification before being returned to the Mattermost application. The application should validate that the redirect_to URL belongs to the legitimate Mattermost domain or explicitly authorized external domains. However, the current implementation accepts any URL provided in the redirect_to parameter without sufficient validation, allowing attackers to craft malicious links that redirect users to attacker-controlled domains.
This vulnerability directly maps to CWE-601, which describes open redirect vulnerabilities where applications redirect users to unvalidated external URLs. The operational impact is severe as it enables attackers to perform credential theft through session hijacking attacks. When a victim authenticates through a maliciously crafted SAML link, their browser cookies containing authentication tokens are automatically sent to the attacker-controlled endpoint, effectively compromising the user's session. This creates a vector for privilege escalation attacks, as attackers can gain access to sensitive information and perform actions on behalf of authenticated users.
The attack scenario begins with an attacker crafting a malicious SAML authentication link with a crafted redirect_to parameter pointing to a server they control. When a victim clicks this link and authenticates through their SAML provider, they are redirected to the attacker's domain where their cookies are captured. This technique leverages the trust relationship between the Mattermost application and its SAML identity providers, exploiting the legitimate authentication flow to steal session tokens. The attack requires no special privileges or complex exploitation techniques beyond crafting the malicious link and setting up the malicious endpoint to receive the cookies.
Organizations using affected Mattermost versions should immediately upgrade to the latest stable releases that contain the patched redirect validation logic. The mitigation strategy involves implementing proper URL validation that ensures redirect_to parameters only point to trusted domains or using absolute URL validation with domain whitelisting. Additionally, organizations should consider implementing additional security measures such as secure cookie flags, HttpOnly attributes, and session management controls. The vulnerability demonstrates the importance of validating all user-supplied input parameters, particularly those used in authentication flows, and aligns with ATT&CK technique T1566.002 for credential access through SAML redirection attacks. Regular security audits should verify that all authentication flows properly validate redirect parameters and that no open redirect vulnerabilities exist in the application's session management mechanisms.