# HTTP Connection Request Smuggling {{#include ../banners/hacktricks-training.md}} **This page summarizes, extends and updates** the seminal PortSwigger research on [Browser-Powered Desync Attacks](https://portswigger.net/research/browser-powered-desync-attacks) and subsequent work on HTTP/2 connection-state abuse. It focuses on vulnerabilities where **an origin is determined only once per TCP/TLS connection**, enabling an attacker to “smuggle” requests to a different internal host once the channel is established. ## Connection-State Attacks ### First-request Validation When routing requests, reverse proxies might depend on the **Host** (or **:authority** in HTTP/2) header to determine the destination back-end server, often relying on a whitelist of hosts that are permitted access. However, a vulnerability exists in a number of proxies where the whitelist is **only enforced on the very first request in a connection**. Consequently, attackers can access internal virtual hosts by first sending an allowed request and then re-using the same underlying connection: ```http GET / HTTP/1.1 Host: allowed-external-host.example GET /admin HTTP/1.1 Host: internal-only.example ``` ### First-request Routing Many HTTP/1.1 reverse proxies map an outbound connection to a back-end pool **based exclusively on the first request they forward**. All subsequent requests sent through the same front-end socket are silently re-used, regardless of their Host header. This can be combined with classic [Host header attacks](https://portswigger.net/web-security/host-header) such as password-reset poisoning or [web cache poisoning](https://portswigger.net/web-security/web-cache-poisoning) to obtain SSRF-like access to other virtual hosts: ```http GET / HTTP/1.1 Host: public.example POST /pwreset HTTP/1.1 Host: private.internal ``` > [!TIP] > In Burp Suite Professional ≥2022.10 you can enable **HTTP Request Smuggler → Connection-state probe** to automatically detect these weaknesses. --- ## NEW in 2023-2025 – HTTP/2/3 Connection Coalescing Abuse Modern browsers routinely **coalesce** HTTP/2 and HTTP/3 requests onto a single TLS connection when the certificate, ALPN protocol and IP address match. If a front-end only authorizes the first request, every subsequent coalesced request inherits that authorisation – **even if the Host/:authority changes**. ### Exploitation scenario 1. The attacker controls `evil.com` which resolves to the same CDN edge node as the target `internal.company`. 2. The victim’s browser already has an open HTTP/2 connection to `evil.com`. 3. The attacker embeds a hidden `` in their page. 4. Because the connection parameters match, the browser re-uses the **existing** TLS connection and multiplexes the request for `internal.company`. 5. If the CDN/router only validated the first request, the internal host is exposed. PoCs for Chrome/Edge/Firefox are available in James Kettle’s talk *“HTTP/2: The Sequel is Always Worse”* (Black Hat USA 2023). ### Tooling * **Burp Suite 2023.12** introduced an experimental **HTTP/2 Smuggler** insertion point that automatically attempts coalescing and TE/CL techniques. * **smuggleFuzz** (https://github.com/microsoft/smugglefuzz) – A Python framework released in 2024 to brute-force front-end/back-end desync vectors over HTTP/2 and HTTP/3, including connection-state permutations. ### Mitigations * Always **re-validate Host/:authority on every request**, not only on connection creation. * Disable or strictly scope **origin coalescing** on CDN/load-balancer layers (e.g. `http2_origin_cn` off in NGINX). * Deploy separate certificates or IP addresses for internal and external hostnames so the browser cannot legally coalesce them. * Prefer **connection: close** or `proxy_next_upstream` after each request where practical. --- ## Real-World Cases (2022-2025) | Year | Component | CVE | Notes | |------|-----------|-----|-------| | 2022 | AWS Application Load Balancer | – | Host header only validated on first request; fixed by patching rules engine (disclosed by SecurityLabs). | | 2023 | Apache Traffic Server < 9.2.2 | CVE-2023-39852 | Allowed request smuggling via HTTP/2 connection reuse when `CONFIG proxy.config.http.parent_proxy_routing_enable` was set. | | 2024 | Envoy Proxy < 1.29.0 | CVE-2024-2470 | Improper validation of :authority after first stream enabled cross-tenant request smuggling in shared meshes. | --- ## Detection Cheat-Sheet 1. Send two requests in the **same** TCP/TLS connection with different Host or :authority headers. 2. Observe whether the second response originates from the first host (safe) or the second host (vulnerable). 3. In Burp: `Repeat → keep-alive → Send → Follow`. 4. When testing HTTP/2, open a **dedicated** stream (ID 1) for a benign host, then multiplex a second stream (ID 3) to an internal host and look for a reply. --- ## References * PortSwigger Research – *HTTP/2: The Sequel is Always Worse* (Black Hat USA 2023) * Envoy Security Advisory CVE-2024-2470 – Improper authority validation {{#include ../banners/hacktricks-training.md}}