Description

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 the amount of memory allocated when decoding uploaded image files which allows an authenticated user to cause excessive server memory consumption and potential denial of service via uploading a specially crafted image as a profile picture, channel file attachment, team icon, or custom brand image. Mattermost Advisory ID: MMSA-2026-00719

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "6.5",
  "pubDate": "2026-09-14T11:17:03.697Z",
  "pubdate": "2026-09-14T11:17:03.697Z",
  "executiveSummary": "Mattermost is susceptible to a denial-of-service (DoS) vulnerability caused by improper memory management during image processing.\nThe vulnerability originates from a failure to enforce memory allocation limits when decoding uploaded image files.\nAffected versions include Mattermost 11.9.0 and earlier, 11.8.4 and earlier, 11.7.7 and earlier, and 10.11.22 and earlier.\nAn authenticated attacker can trigger excessive memory consumption, leading to service instability or crash, by uploading a maliciously crafted image file.\nExploitation requires the attacker to have authenticated access to the application, allowing them to upload images through various vectors including profile pictures, channel attachments, team icons, or custom brand images.\nThis vulnerability poses a significant risk to availability, as a single malicious payload can deplete server resources, potentially impacting all users of the affected Mattermost instance.",
  "technicalDetails": "The vulnerability resides in the server-side image processing pipeline of the Mattermost application. Specifically, the component responsible for decoding uploaded image files fails to implement adequate constraints or bounds checking on memory allocation during the deserialization or decoding process.\nWhen a user uploads an image file, the server attempts to parse the file structure to validate and process the image (e.g., resizing or generating thumbnails). A specially crafted image file can be designed to exploit this logic by declaring dimensions or header information that require the underlying image decoding libraries to allocate disproportionately large amounts of memory to represent the pixel data in RAM.\nThe attack flow begins with an authenticated user initiating an upload request through the Mattermost interface. This can occur via any endpoint that accepts image data, such as changing a profile picture, attaching a file to a channel message, updating a team icon, or modifying a custom brand image.\nThe server receives the malicious image file and triggers the vulnerable decoding function. Because the application lacks memory allocation limits for this specific operation, the decoding process proceeds to attempt memory reservation based on the malicious metadata. If the requested memory exceeds available heap space or system limits, it leads to resource exhaustion.\nThe direct impact of this memory saturation is a denial of service (DoS). The excessive allocation can trigger an Out-Of-Memory (OOM) killer event, crashing the server process, or cause the application to become unresponsive due to constant garbage collection pressure or memory swapping, effectively disrupting service availability for all users.\nThe vulnerability affects multiple versions of Mattermost, specifically 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7, and 10.11.x <= 10.11.22. Successful exploitation is contingent upon the attacker having an authenticated session, but it does not require administrative privileges, as standard user-level upload actions are sufficient to trigger the parsing routines. The issue is persistent as long as the malicious image remains accessible or until the processing pipeline is addressed."
}