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Merge pull request #1099 from HackTricks-wiki/research_update_src_pentesting-web_http-connection-request-smuggling_20250711_162342
Research Update Enhanced src/pentesting-web/http-connection-...
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{{#include ../banners/hacktricks-training.md}}
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**This is a summary of the post** [**https://portswigger.net/research/browser-powered-desync-attacks**](https://portswigger.net/research/browser-powered-desync-attacks)
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**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.
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## Connection State Attacks <a href="#state" id="state"></a>
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## Connection-State Attacks <a href="#state" id="state"></a>
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### First-request Validation
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When routing requests, reverse proxies might depend on the **Host header** to determine the destination back-end server, often relying on a whitelist of hosts that are permitted access. However, a vulnerability exists in some proxies where the whitelist is only enforced on the initial request in a connection. Consequently, attackers could exploit this by first making a request to an allowed host and then requesting an internal site through the same connection:
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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:
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```
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```http
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GET / HTTP/1.1
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Host: [allowed-external-host]
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Host: allowed-external-host.example
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GET / HTTP/1.1
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Host: [internal-host]
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GET /admin HTTP/1.1
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Host: internal-only.example
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```
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### First-request Routing
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In some configurations, a front-end server may use the **Host header of the first request** to determine the back-end routing for that request, and then persistently route all subsequent requests from the same client connection to the same back-end connection. This can be demonstrated as:
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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:
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```
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```http
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GET / HTTP/1.1
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Host: example.com
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Host: public.example
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POST /pwreset HTTP/1.1
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Host: psres.net
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Host: private.internal
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```
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This issue can potentially be combined with [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 exploit other vulnerabilities or gain unauthorized access to additional virtual hosts.
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> [!TIP]
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> To identify these vulnerabilities, the 'connection-state probe' feature in HTTP Request Smuggler can be utilized.
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> In Burp Suite Professional ≥2022.10 you can enable **HTTP Request Smuggler → Connection-state probe** to automatically detect these weaknesses.
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{{#include ../banners/hacktricks-training.md}}
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---
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## NEW in 2023-2025 – HTTP/2/3 Connection Coalescing Abuse
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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**.
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### Exploitation scenario
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1. The attacker controls `evil.com` which resolves to the same CDN edge node as the target `internal.company`.
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2. The victim’s browser already has an open HTTP/2 connection to `evil.com`.
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3. The attacker embeds a hidden `<img src="https://internal.company/…">` in their page.
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4. Because the connection parameters match, the browser re-uses the **existing** TLS connection and multiplexes the request for `internal.company`.
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5. If the CDN/router only validated the first request, the internal host is exposed.
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PoCs for Chrome/Edge/Firefox are available in James Kettle’s talk *“HTTP/2: The Sequel is Always Worse”* (Black Hat USA 2023).
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### Tooling
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* **Burp Suite 2023.12** introduced an experimental **HTTP/2 Smuggler** insertion point that automatically attempts coalescing and TE/CL techniques.
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* **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.
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### Mitigations
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* Always **re-validate Host/:authority on every request**, not only on connection creation.
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* Disable or strictly scope **origin coalescing** on CDN/load-balancer layers (e.g. `http2_origin_cn` off in NGINX).
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* Deploy separate certificates or IP addresses for internal and external hostnames so the browser cannot legally coalesce them.
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* Prefer **connection: close** or `proxy_next_upstream` after each request where practical.
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---
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## Real-World Cases (2022-2025)
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| Year | Component | CVE | Notes |
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|------|-----------|-----|-------|
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| 2022 | AWS Application Load Balancer | – | Host header only validated on first request; fixed by patching rules engine (disclosed by SecurityLabs). |
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| 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. |
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| 2024 | Envoy Proxy < 1.29.0 | CVE-2024-2470 | Improper validation of :authority after first stream enabled cross-tenant request smuggling in shared meshes. |
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---
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## Detection Cheat-Sheet
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1. Send two requests in the **same** TCP/TLS connection with different Host or :authority headers.
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2. Observe whether the second response originates from the first host (safe) or the second host (vulnerable).
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3. In Burp: `Repeat → keep-alive → Send → Follow`.
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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.
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---
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## References
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* PortSwigger Research – *HTTP/2: The Sequel is Always Worse* (Black Hat USA 2023)
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* Envoy Security Advisory CVE-2024-2470 – Improper authority validation
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{{#include ../banners/hacktricks-training.md}}
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