CVE-2026-3114: 
vulnerability analysis and mitigation

CVE-2026-3114 is a denial-of-service vulnerability in Mattermost Server caused by improper handling of zip bomb archives during file extraction. Affected versions include 11.4.x ≤ 11.4.0, 11.3.x ≤ 11.3.1, 11.2.x ≤ 11.2.3, and 10.11.x ≤ 10.11.11. The vulnerability was disclosed on March 26, 2026, under Mattermost Advisory ID MMSA-2026-00598. It carries a CVSS v3.1 base score of 6.5 (Medium) (Red Hat Advisory, Red Hat Bugzilla).

The root cause is classified under CWE-409 (Improper Handling of Highly Compressed Data / Data Amplification) and CWE-770 (Allocation of Resources Without Limits or Throttling). Mattermost fails to validate the decompressed size of archive entries before or during extraction, allowing a small, highly compressed zip file (a "zip bomb") to expand to an extremely large size in memory. An authenticated attacker with file upload permissions can exploit this by uploading a crafted zip archive via the standard file upload interface, triggering uncontrolled memory allocation on the server. No special privileges beyond standard file upload access are required, and no user interaction is needed beyond the upload action itself (Red Hat Bugzilla, Red Hat Advisory).

Successful exploitation results in server memory exhaustion, causing a denial of service that renders the Mattermost instance unavailable to all legitimate users. The impact is limited to availability (rated High); there is no confidentiality or integrity impact, and the scope is unchanged (limited to the affected server). In environments where Mattermost is used for critical team communications or DevOps workflows, an outage could disrupt operations significantly (Red Hat Advisory).

1. Obtain authenticated access: Log in to a vulnerable Mattermost instance (versions 10.11.0–10.11.11, 11.2.0–11.2.3, 11.3.0–11.3.1, or 11.4.0) with an account that has file upload permissions in at least one channel.
2. Craft a zip bomb: Create a highly compressed zip archive (e.g., using nested or single-layer compression) where the compressed payload is small (e.g., a few kilobytes) but decompresses to gigabytes of data. Tools such as zip with maximum compression or purpose-built zip bomb generators can be used.
3. Upload the malicious archive: Navigate to any channel or direct message where file uploads are permitted and upload the crafted zip archive via the Mattermost file upload interface.
4. Trigger extraction: The Mattermost server processes the uploaded zip archive without validating decompressed entry sizes, causing uncontrolled memory allocation as the archive is extracted.
5. Achieve denial of service: Server memory is exhausted, causing the Mattermost process to crash or become unresponsive, denying service to all users (Red Hat Bugzilla, Red Hat Advisory).

• Network: Unusual file upload requests (HTTP POST to Mattermost file upload endpoints) involving small zip files from authenticated user accounts.
• Logs: Mattermost server logs showing file extraction operations followed by out-of-memory errors or process crashes; entries referencing zip archive processing at the time of the incident.
• Process/System: Sudden spike in server memory consumption coinciding with a file upload event; Mattermost server process termination or restart due to memory exhaustion (OOM killer events in system logs on Linux).
• File System: Presence of unusually small zip files in the Mattermost file storage directory that, when inspected, have extremely high compression ratios.

Upgrade Mattermost Server to the patched versions: 10.11.12, 11.2.4, 11.3.2, or 11.4.1 (or later). As a workaround, restrict file upload permissions to trusted users only and consider disabling zip archive uploads if they are not required for your deployment. Implementing OS-level memory limits (e.g., cgroups) on the Mattermost process can help contain the impact of a successful attack. Monitor file extraction operations for anomalous memory usage (Mattermost Security Updates, Red Hat Bugzilla).