CVE-2026-2457 in Mattermost
Summary
by MITRE • 03/16/2026
Mattermost versions 11.3.x <= 11.3.0, 11.2.x <= 11.2.2, 10.11.x <= 10.11.10 fail to sanitize client-supplied post metadata which allows an authenticated attacker to spoof permalink embeds impersonating other users via crafted PUT requests to the post update API endpoint.. Mattermost Advisory ID: MMSA-2025-00569
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Analysis
by VulDB Data Team • 03/20/2026
This vulnerability resides in the Mattermost collaboration platform where improper input sanitization creates a critical security flaw in the post metadata handling mechanism. The issue affects specific versions including 11.3.0 and below, 11.2.2 and below, and 10.11.10 and below, making it a widespread concern across multiple release branches. The core problem manifests when the system processes client-supplied post metadata through the post update API endpoint, failing to adequately sanitize or validate the incoming data before processing.
The technical exploitation occurs through crafted PUT requests that manipulate the metadata fields associated with posts, allowing authenticated attackers to inject malicious content that can be interpreted by the application as legitimate user-generated data. This vulnerability specifically targets the permalink embed functionality where the system should verify the authenticity of user identifiers but instead accepts potentially forged metadata values. The flaw enables attackers to create posts that appear to originate from other users, effectively impersonating legitimate participants within the communication platform.
The operational impact of this vulnerability extends beyond simple impersonation as it undermines the trust model of the Mattermost platform and compromises the integrity of user communications. Attackers can craft deceptive posts that reference other users' accounts, potentially leading to social engineering attacks, misinformation campaigns, or unauthorized access to user-specific content and conversations. The vulnerability particularly affects collaborative environments where users rely on accurate attribution of posts and where the system's ability to distinguish between genuine and forged content is critical for maintaining secure communication channels.
From a cybersecurity perspective, this vulnerability aligns with CWE-116 - Improper Encoding or Escaping of Output and CWE-20 - Improper Input Validation, representing a combination of output encoding failures and input validation weaknesses that together create an exploitable condition. The attack pattern follows ATT&CK technique T1078.004 - Valid Accounts: SSH Keys, where the attacker leverages legitimate authentication to manipulate system behavior rather than bypassing authentication entirely. The vulnerability also maps to ATT&CK technique T1566.001 - Phishing: Spearphishing Attachment, as the spoofed content could be used to craft convincing phishing attempts that appear to originate from trusted users. Organizations should immediately implement the vendor-recommended patches and consider additional monitoring for suspicious metadata modifications in their Mattermost environments. The security implications require comprehensive review of all user-facing metadata handling mechanisms and implementation of proper input sanitization procedures to prevent similar vulnerabilities from emerging in other components of the system.
This vulnerability represents a significant concern for organizations relying on Mattermost for secure communications, as it enables attackers to compromise the authenticity guarantees that are fundamental to collaborative platforms. The combination of authenticated access requirements with the ability to manipulate metadata creates a dangerous attack vector where insider threats or compromised accounts could be leveraged to create widespread confusion and potential security incidents.