CVE-2026-33555

import socket import ssl # This is a conceptual PoC for HTTP/3 Request Smuggling in HAProxy. # Actual exploitation requires a QUIC library implementation (e.g., aioquic). def send_smuggling_request(target_host, target_port): # In a real scenario, construct a QUIC stream with: # 1. A HEADERS frame containing "Content-Length: 100" # 2. A DATA frame with 0 bytes payload but the FIN stream flag set. # 3. This causes HAProxy to expect 100 bytes but receive 0, triggering desync. print(f"[*] Targeting {target_host}:{target_port}") print("[*] Sending malicious HTTP/3 stream with CL mismatch...") # Pseudo-code for the packet logic headers = { ":method": "POST", ":path": "/", "content-length": "100" # Announce length } body_payload = b"" # Empty payload fin_flag = True # Close stream immediately # The vulnerability is that HAProxy does not check: len(body_payload) == headers['content-length'] # when fin_flag is True. print("[+] Request sent. Backend desynchronization possible.") if __name__ == "__main__": send_smuggling_request("example.com", 443)

{"cve": {"id": "CVE-2026-33555", "sourceIdentifier": "[email protected]", "published": "2026-04-13T17:16:28.237", "lastModified": "2026-04-22T19:17:02.273", "vulnStatus": "Awaiting Analysis", "cveTags": [], "descriptions": [{"lang": "en", "value": "An issue was discovered in HAProxy before 3.3.6. The HTTP/3 parser does not check that the received body length matches a previously announced content-length when the stream is closed via a frame with an empty payload. This can cause desynchronization issues with the backend server and could be used for request smuggling. The earliest affected version is 2.6."}], "metrics": {"cvssMetricV31": [{"source": "[email protected]", "type": "Secondary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:L/A:N", "baseScore": 4.0, "baseSeverity": "MEDIUM", "attackVector": "NETWORK", "attackComplexity": "HIGH", "privilegesRequired": "NONE", "userInteraction": "NONE", "scope": "CHANGED", "confidentialityImpact": "NONE", "integrityImpact": "LOW", "availabilityImpact": "NONE"}, "exploitabilityScore": 2.2, "impactScore": 1.4}]}, "weaknesses": [{"source": "[email protected]", "type": "Secondary", "description": [{"lang": "en", "value": "CWE-130"}]}], "references": [{"url": "https://github.com/haproxy/haproxy/commit/05a295441c621089ffa4318daf0dbca2dd756a84", "source": "[email protected]"}, {"url": "https://r3verii.github.io/cve/2026/04/14/haproxy-h3-standalone-fin-smuggling.html", "source": "[email protected]"}, {"url": "https://www.haproxy.com/documentation/haproxy-aloha/changelog/", "source": "[email protected]"}, {"url": "https://www.haproxy.org", "source": "[email protected]"}, {"url": "https://www.mail-archive.com/[email protected]/msg46752.html", "source": "[email protected]"}, {"url": "https://r3verii.github.io/cve/2026/04/14/haproxy-h3-standalone-fin-smuggling.html", "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0"}]}}