CVE-2025-14822: 
vulnerability analysis and mitigation

CVE-2025-14822 is a denial-of-service vulnerability in Mattermost Server caused by insufficient input size validation during hashtag processing. An authenticated attacker can exhaust CPU resources by sending a single HTTP request containing a post with thousands of space-separated tokens. Affected versions include Mattermost Server 10.11.0 through 10.11.8 and 11.0.0 through 11.1.x (prior to 11.2.0). The vulnerability was published on January 16, 2026. The NVD assigns a CVSS v3.1 base score of 6.5 (Medium), while the GitHub Advisory Database scores it 3.1 (Low) using a higher attack complexity (Github Advisory, Mattermost Security).

The root cause lies in the ParseHashtags function in server/public/model/utils.go, which uses inefficient string concatenation (CWE-407: Inefficient Algorithmic Complexity) and lacks any limit on the number of tokens processed before building the hashtag string (CWE-770: Allocation of Resources Without Limits or Throttling). When a post containing thousands of space-separated tokens is submitted, the function iterates over every token, performing regex matching and string operations without any early exit or size guard, causing quadratic-like CPU consumption. The fix, applied in commits 4d86263 and b3d6c0c, replaces naive string concatenation with strings.Builder for efficiency and adds improved test coverage for large inputs (Github Advisory, Patch Commit).

Successful exploitation results in excessive CPU consumption on the Mattermost server, leading to denial-of-service conditions where legitimate users may experience service degradation or complete unavailability. There is no impact on confidentiality or data integrity — the vulnerability is purely an availability issue. Because only a single authenticated HTTP request is required to trigger the condition, even low-privileged users (e.g., regular channel members) can disrupt service for all users on the instance (Github Advisory, Feedly).

1. Obtain authenticated access: Acquire valid credentials for any user account on the target Mattermost instance (e.g., a standard channel member account).
2. Craft a malicious post payload: Construct an HTTP POST request to the Mattermost post creation API endpoint (e.g., POST /api/v4/posts) with a JSON body containing a message field populated with thousands of space-separated tokens — ideally hashtag-like strings (e.g., #a #b #c ... repeated thousands of times) to maximize processing time in ParseHashtags.
3. Submit the request: Send the crafted request to the target server. A single request is sufficient to trigger significant CPU exhaustion due to the inefficient token iteration and regex matching in the ParseHashtags function.
4. Observe denial of service: Monitor the server for elevated CPU usage and degraded response times for other users, indicating successful resource exhaustion (Github Advisory, Patch Commit).

• Network: Unusual HTTP POST requests to /api/v4/posts with abnormally large message payloads (e.g., tens of kilobytes of space-separated tokens); single requests with message bodies containing thousands of whitespace-delimited tokens.
• Logs: Mattermost application logs showing unusually long processing times for post creation requests; server access logs with large Content-Length values on POST requests to post endpoints.
• Process: Sustained high CPU utilization by the Mattermost server process (mattermost) without a corresponding spike in concurrent connections; Go runtime goroutine stalls or timeouts associated with hashtag parsing.

Mattermost has released patched versions 10.11.9 and 11.2.0 that address this vulnerability by replacing inefficient string concatenation with strings.Builder in the ParseHashtags function (Github Advisory, Mattermost Security). Administrators should upgrade immediately to 10.11.9 (for the 10.11.x branch) or 11.2.0 (for the 11.x branch). As interim mitigations, consider implementing network-level rate limiting on post creation API endpoints and monitoring for abnormally large POST request payloads.