CVE-2026-19600 in Mattermost
Summary
by MITRE • 10/16/2026
Mattermost versions 11.9.x <= 11.9.1, 11.8.x <= 11.8.5, 11.7.x <= 11.7.10, 11.10.x <= 11.10.1 fail to sanitize authentication tokens before writing them to server log entries, which allows an attacker with access to server logs to obtain valid session tokens or OAuth access tokens and impersonate authenticated users via the debug-level Mattermost server log.. Mattermost Advisory ID: MMSA-2026-00763
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Analysis
by VulDB Data Team • 10/16/2026
The vulnerability identified in Mattermost versions 11.9.x through 11.9.1, 11.8.x through 11.8.5, 11.7.x through 11.7.10, and 11.10.x through 11.10.1 represents a critical information disclosure flaw rooted in improper input sanitization within the application's logging subsystem. This issue specifically affects the handling of authentication tokens when they are written to server log entries during debug-level operations. The core technical failure lies in the absence of adequate filtering or masking mechanisms for sensitive credential data before it is persisted to disk-based logs. Consequently, plaintext session identifiers and OAuth access tokens are recorded verbatim alongside routine operational messages, creating a direct pathway for unauthorized actors who have gained read access to these log files to extract valid credentials.
From an architectural perspective, this flaw aligns with CWE-532, which classifies the insertion of sensitive information into log files as a significant security weakness. The vulnerability is exacerbated by the common operational practice of retaining detailed debug logs for troubleshooting purposes, often without strict access controls that would prevent unauthorized personnel or compromised accounts from viewing them. When an attacker gains access to these server logs, whether through direct file system access, misconfigured logging aggregation services like ELK Stack or Splunk, or via a separate vulnerability in the log management infrastructure, they can retrieve active session tokens and OAuth access tokens. These credentials are typically long-lived enough to allow sustained unauthorized access until expiration, but often persist for hours or days depending on configuration settings.
The operational impact of this vulnerability is severe, as it enables full account takeover without requiring any additional exploitation steps beyond log file retrieval. An attacker can impersonate authenticated users by replaying the stolen session tokens in HTTP requests directed at the Mattermost API endpoints. This capability allows the adversary to read private messages, exfiltrate sensitive data shared within channels and direct messages, modify user profiles, or potentially escalate privileges if they target administrative accounts. Furthermore, OAuth access tokens may grant access to integrated third-party services linked to the compromised Mattermost account, thereby expanding the blast radius of the initial breach beyond the immediate application environment. This scenario is consistent with ATT&CK technique T1078, Valid Accounts, where adversaries leverage legitimate credentials obtained through information disclosure to maintain persistent access and move laterally within a networked environment.
Mitigation strategies must address both the immediate remediation of the software defect and broader logging hygiene practices. The primary defense is to upgrade Mattermost to versions 12.0.0 or later, which include patches for this specific sanitization issue as outlined in Advisory MMSA-2026-00763. For environments where upgrading is not immediately feasible, administrators should strictly limit the logging level on production servers to INFO or WARNING, thereby preventing debug-level statements that contain sensitive data from being written to logs. Additionally, access controls on log directories and centralized logging platforms must be rigorously audited to ensure that only authorized security operations personnel can view raw log content. Implementing automated token rotation policies can also reduce the window of opportunity for attackers who manage to exfiltrate these credentials. Regular monitoring of authentication endpoints for anomalous activity originating from stolen tokens remains a critical compensating control to detect potential exploitation attempts in real time.