CVE-2026-5132 in Mattermost
Summary
by MITRE • 09/14/2026
Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7, 10.11.x <= 10.11.22 fail to limit size of unpacked SDP messages compressed with zlib, which allows attacker to deny service or crash server via sending many SDP messages that unpack to large size.. Mattermost Advisory ID: MMSA-2026-00643
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Analysis
by VulDB Data Team • 09/14/2026
The vulnerability identified in Mattermost versions 11.9.x through 11.7.7, as well as version 10.11.22 and earlier, represents a critical resource exhaustion flaw within the platform's handling of Session Description Protocol messages. Specifically, the software fails to enforce adequate limits on the size of unpacked SDP payloads when these messages are compressed using the zlib algorithm. This deficiency allows an attacker to craft maliciously sized inputs that appear small in their compressed form but expand significantly upon decompression by the server. By sending a high volume of such optimized packets, an adversary can rapidly consume available system memory and CPU resources, leading to a denial of service condition or complete application crash.
From a technical perspective, this issue stems from insufficient validation logic during the decompression phase of message processing. While many applications correctly limit the size of compressed input data, they often neglect to verify the resulting uncompressed output against predefined thresholds. In the context of Mattermost's real-time communication infrastructure, SDP messages are integral for establishing media sessions between users. The lack of a strict upper bound on the decompressed payload size means that malicious actors can exploit this gap to trigger excessive memory allocation. This type of flaw is commonly categorized under CWE-400, which describes uncontrolled resource consumption, and aligns with ATT&CK technique T1499, Endpoint Denial of Service, as it directly impacts the availability of the service for legitimate users.
The operational impact of this vulnerability is severe, particularly for organizations relying on Mattermost for critical communications or those hosting public-facing instances. An attacker does not necessarily need high privileges to exploit this flaw; a standard authenticated user can initiate the attack vector by transmitting the malformed SDP packets. The resulting denial of service disrupts ongoing conversations, prevents new connections from being established, and may require manual intervention or server restarts to restore functionality. In environments where Mattermost is integrated with other enterprise systems, such outages can cascade into broader operational disruptions, affecting productivity and potentially violating service level agreements related to uptime and availability.
Mitigation strategies should prioritize immediate patching to the latest stable release of Mattermost that addresses this advisory, specifically version MMSA-2026-00643. For organizations unable to upgrade immediately due to compatibility constraints or operational risks, implementing network-level controls is essential. Deploying a Web Application Firewall with rules designed to detect and block abnormally large decompressed payloads can provide an effective layer of defense. Additionally, configuring rate limiting on SDP-related endpoints can help mitigate the volume of requests processed by the server, thereby reducing the likelihood of resource exhaustion even if malicious inputs are received. Regular security audits focusing on input validation and resource management practices should also be conducted to prevent similar vulnerabilities in future updates or custom integrations.