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A normal HTTP connection typically lasts only for the duration of a single request. However, H2C or “**http2 over cleartext”** is where a normal transient http **connection is upgraded to a persistent connection that uses the http2 binary protocol** to communicate continuously instead of for one request using the plaintext http protocol.
The second part of the smuggling occurs when a **reverse proxy is used**. Normally, when http requests are made to a reverse proxy, the proxy will handle the request, process a series of routing rules, then forward the request onto the backend and then return the response. When a http request includes a `Connection: Upgrade` header, such as for a websocket connection, the reverse **proxy will maintain the persistent connection** between the client and server, **allowing for the continuous communication needed for these procotols**. For a H2C Connection, the RFC requires 3 headers to be present:
So where is the bug? **When upgrading a connection, the reverse proxy will often stop handling individual requests**, assuming that once the connection has been established, its routing job is done. Using H2C Smuggling, we can bypass rules a reverse proxy uses when processing requests such as path based routing, authentication, or the WAF processing provided we can establish a H2C connection first.
Note from the explanation of the vulnerability that the proxy server needs to **forward the Upgrade header**, and sometimes the **Connection header** also needs to be successfully forwarded.
By default, the following services **do** forward **Upgrade** and **Connection headers** during proxy-pass, thereby enabling h2c smuggling out-of-the-box.:
* HAProxy
* Traefik
* Nuster
By default, these services **do not** forward both Upgrade and Connection headers during proxy-pass, but **can be configured in an insecure manner** (by passing unfiltered Upgrade and Connection headers):
The original blog post points out that not all servers will forward the required headers for a compliant H2C connection upgrade. This means load balancers like AWS ALB/CLB, NGINX, and Apache Traffic Server amongst others will **prevent a H2C connection by default**. However, at the end of the blog post, he does mention that “not all backends were compliant, and we could **test with the non-compliant `Connection: Upgrade` variant, where the `HTTP2-Settings` value is omitted** from the `Connection` header.”
Note that even if the `proxy_pass` URL (the endpoint the proxy forwards the connection) was pointing to a specific **path** such as `http://backend:9999/socket.io` the connection will be stablished with `http://backend:9999` so you can **contact any other path inside that internal endpoint abusing this technique. So it doesn't matter if a path is specified in the URL of proxy\_pass.**
Using the tools [**https://github.com/BishopFox/h2csmuggler**](https://github.com/BishopFox/h2csmuggler) **and** [**https://github.com/assetnote/h2csmuggler**](https://github.com/assetnote/h2csmuggler) you can try to **bypass the protections imposed** by the proxy establishing a H2C connection and access proxy protected resources.
Follow this link for[ **more info about this vulnerability in Nginx**](../network-services-pentesting/pentesting-web/nginx.md#proxy\_set\_header-upgrade-and-connection).
Similar to previous technique, this one **instead** of creating a **HTTP2 tunnel** to an endpoint accessible via a proxy, it will create a **Websocket tunnel** for the same purpose, **bypass potential proxies limitations** and talk directly to the endpoint:
We have backend that exposes public **WebSocket API** and also has **internal REST API not available** from outside. Malicious client wants to access internal REST API.
On the **first** step client sends **Upgrade request** to reverse proxy but with **wrong protocol version** inside header `Sec-WebSocket-Version`. **Proxy** doesn't validate `Sec-WebSocket-Version` header and thinks that **Upgrade request is correct**. Further it translates request to the backend.
On the second step backend sends **response with status code `426` because protocol version is incorrect** inside header `Sec-WebSocket-Version`. However, **reverse proxy doesn't check** enough response from backend (including status code) and **thinks that backend is ready for WebSocket communication**. Further it translates request to the client.
Finally, reverse **proxy thinks** that **WebSocket connection is established between client and backend**. In reality there is no WebSocket connection - backend refused Upgrade request. At the same time, proxy keeps TCP or TLS connection between client and backend in open state. **Client can easily access private REST API by sending HTTP request over the connection.**
It was found that following reverse proxies are affected:
* Varnish - team refused to fix described issue.
* Envoy proxy 1.8.0 (or older) - in newer versions upgrade mechanism has been changed.
* Others - TBA.
### Scenario 2
The majority of reverse proxies (e.g. NGINX) **check status code from backend** during handshake part. This makes attack harder but not impossible.
Let's observe second scenario. We have backend that exposes public WebSocket API and public REST API for health checking and also has **internal REST API not available from outside**. Malicious client wants to access internal REST API. NGINX is used as reverse proxy. WebSocket API is available on path `/api/socket.io/` and healthcheck API on path `/api/health`.
Healthcheck API is invoked by sending POST request, parameter with name `u` controls URL. Backend reaches external resource and returns status code back to the client.
On the **first** step client sends POST request to invoke **healthcheck API but with additional HTTP header `Upgrade: websocket`**. NGINX thinks that it's a **normal Upgrade request**, it looks only for `Upgrade` header skipping other parts of the request. Further proxy translates request to the backend.
On the **second** step backend invokes healtcheck API. It reaches external resource controlled by malicious users that returns HTTP **response with status code `101`**. Backend translates that response to the reverse proxy. Since NGINX validates only status code **it will think that backend is ready for WebSocket communication**. Further it translates request to the client.
Note how this scenario is much more complex to exploit as you need to be able to contact some endpoint that **returns status code 101**.
{% endhint %}
Finally, **NGINX thinks that WebSocket connection is established between client and backend**. In reality there is no WebSocket connection - healthcheck REST API was invoked on backend. At the same time, reverse proxy keeps TCP or TLS connection between client and backend in open state. **Client can easily access private REST API by sending HTTP request over the connection.**
The majority of reverse proxies should be affected by that scenario. However, exploitation requires existence of external SSRF vulnerability (usually considered low-severity issue).
### Labs
Check the labs to test both scenarios in [https://github.com/0ang3el/websocket-smuggle.git](https://github.com/0ang3el/websocket-smuggle.git)
- Do you work in a **cybersecurity company**? Do you want to see your **company advertised in HackTricks**? or do you want to have access to the **latest version of the PEASS or download HackTricks in PDF**? Check the [**SUBSCRIPTION PLANS**](https://github.com/sponsors/carlospolop)!
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