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 properly enforce the limit of concurrent files being processed and handled failed files, which allows a user with permission to upload files to spawn more goroutines than intended and block the indexing of other files via uploading heavy files constantly to the server.. Mattermost Advisory ID: MMSA-2026-00696

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "4.3",
  "pubDate": "2026-09-14T11:17:02.800Z",
  "pubdate": "2026-09-14T11:17:02.800Z",
  "executiveSummary": "Mattermost is susceptible to a Denial-of-Service (DoS) vulnerability due to improper enforcement of concurrent file processing limits and inadequate handling of failed file operations. The vulnerability exists within the file handling subsystem, allowing an authenticated user with file upload permissions to exhaust system resources.\nBy continuously uploading heavy files, an attacker can trigger the creation of excessive goroutines, overwhelming the server's scheduling capacity. This resource exhaustion effectively blocks the indexing of legitimate files, disrupting critical system services.\nThe vulnerability affects multiple long-term support and stable releases, specifically Mattermost versions 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. This issue represents a significant availability risk as it allows a malicious or compromised user to impede system performance without elevated privileges beyond standard upload capabilities.\nSuccessful exploitation requires the attacker to hold valid user credentials that grant permission to upload files. Once authenticated, the attacker can systematically inject high-resource payloads to induce a state of service degradation for other users.",
  "technicalDetails": "The vulnerability resides in the concurrency management logic of the Mattermost file processing engine. The system fails to implement an effective semaphore or resource-limiting mechanism to govern the number of concurrent goroutines spawned during file ingestion and subsequent indexing tasks. Furthermore, the error-handling routines for failed file operations are insufficient, failing to terminate or clean up associated processes effectively when an exception occurs.\nThe attack flow begins when an authenticated user initiates multiple concurrent requests to upload resource-intensive files. Because the application lacks a strictly enforced cap on concurrent operations, every file upload request spawns new goroutines to handle processing and indexing concurrently. By flooding the server with these 'heavy' payloads, the attacker forces the system to allocate a large number of goroutines simultaneously.\nAs the goroutine count climbs, the Go runtime scheduler faces contention, leading to increased latency and potential exhaustion of available memory or CPU scheduling cycles. Because the system also fails to manage failed file operations gracefully, corrupted or intentionally stalled file processing tasks remain in a 'zombie' or pending state, consuming system resources indefinitely rather than being purged.\nThe primary impact of this resource exhaustion is the blocking of the file indexing pipeline. Legitimate files submitted by other users are queued behind the malicious payloads or fail to process entirely due to the saturated worker pool. This effectively results in a persistent Denial-of-Service condition where file search, metadata extraction, and storage operations are halted.\nAffected versions include 11.9.0, 11.8.4, 11.7.7, and 10.11.22 and their respective prior versions within those branches. The exploitation does not require administrative privileges, merely the ability to upload files, which is a standard functional requirement for most collaborative workspaces. Consequently, any compromised account with standard access can leverage this vector to degrade the performance of the entire server instance.\nThe root cause is a failure in backpressure management and concurrent task synchronization within the application's file processing architecture. By failing to properly limit concurrent operations, the server becomes vulnerable to resource starvation attacks initiated by legitimate, albeit malicious, user input."
}