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A typical activity in advanced net purposes is to route requests to completely different back-end servers for load balancing. More often than not, a reverse proxy is used for this. Such reverse proxies work on the utility degree (over HTTP) and requests are routed primarily based on the worth of the Host header (:authority for HTTP/2) or components of the route.

A typical misconfiguration is when the reverse proxy immediately makes use of this data because the backend tackle. This could result in Server-Facet Request Forgery (SSRF) vulnerabilities that enable attackers to entry servers behind the reverse proxy and, for instance, steal data from AWS metadata. I made a decision to research related assaults on proxy setups that function at different ranges/protocols, particularly SNI proxies.

What’s TLS SNI?

Server Identify Indication (SNI) is an extension of the TLS protocol that gives the muse of HTTPS. When a browser needs to ascertain a safe connection to a server, it initiates a TLS handshake by sending a ClientHello message. This message might comprise an SNI extension area that features the area identify of the server. In its ServerHello message, the server might return an acceptable certificates for the desired server identify. The everyday use case for that is when there are a number of digital hosts behind one IP tackle.

What’s an SNI proxy?

When a reverse proxy (extra accurately, a load balancer) makes use of a price of the SNI area to pick out a selected backend server, now we have an SNI proxy. With the widespread use of TLS and HTTPS particularly, this strategy is rising in popularity. (Notice that one other that means of SNI proxy refers to the usage of such proxy servers to bypass censorship in some nations.)

There are two essential choices for operating an SNI proxy: with or with out SSL termination. In each circumstances, the SNI proxy makes use of the worth of the SNI area to pick out an acceptable backend. When operating with SSL termination, the TLS connection is established with the SNI proxy, after which the proxy forwards the decrypted visitors to the backend. Within the second case, the SNI proxy forwards your complete information stream, and really works extra like a TCP proxy.

A typical SNI proxy configuration

Many reverse proxies/load balancers help SNI proxy configurations, together with Nginx, Haproxy, Envoy, ATS, and others. It seems to be like you possibly can even use an SNI proxy on Kubernetes.

To offer an Nginx instance, the best setup can be as follows (word that this requires the Nginx modules ngx_stream_core_module Y ngx_stream_ssl_preread_module to work):

stream 
    map $ssl_preread_server_name $targetBackend 
        test1.instance.com backend1:443;
        test2.instance.com backend2:9999;
    

    server 
        pay attention 443; 
        resolver 127.0.0.11;
        proxy_pass $targetBackend:443;       
        ssl_preread on;
    

Right here we configure a server (TCP proxy) referred to as stream and allow SNI entry utilizing ssl_preread on. Relying on the worth of the SNI area (in $ssl_preread_server_name), Nginx will route your complete TLS connection to backend1 both backend2.

Incorrect SNI proxy configurations resulting in SSRF

The only misconfiguration that may let you hook up with an arbitrary backend would seem like this:

stream 
    server 
        pay attention 443; 
        resolver 127.0.0.11;
        proxy_pass $ssl_preread_server_name:443;       
        ssl_preread on;
    

Right here, the worth of the SNI area is used immediately because the backend tackle.

With this insecure configuration, we are able to exploit the SSRF vulnerability just by specifying the specified IP or area identify within the SNI area. For instance, the next command would pressure Nginx to connect with inner.host.com:

openssl s_client -connect goal.com:443 -servername "inner.host.com" -crlf

Usually, in keeping with RFC 6066, IP addresses should not used on SNI values, however in apply, we are able to nonetheless use them. What’s extra, we are able to even ship arbitrary symbols on this area, together with null bytes, which might be helpful for exploitation. As you possibly can see under, the server identify might be modified to an arbitrary string. Though for this particular Nginx setup, sadly, I did not discover a option to change the backend port:

One other class of weak configurations is much like typical HTTP reverse proxy misconfigurations and includes common expression (regex) errors. On this instance, the visitors is forwarded to the backend if the identify offered by way of SNI matches the common expression:

stream 
    map $ssl_preread_server_name $targetBackend 
        ~^www.instance.com    $ssl_preread_server_name;
      

    server 
        pay attention 443; 
        resolver 127.0.0.11;
        proxy_pass $targetBackend:443;       
        ssl_preread on;
    

This common expression is inaccurate as a result of the primary interval character in www.instance.com doesn’t escape, and the expression lacks the $ terminator on the finish. The ensuing common expression matches not solely www.instance.com but additionally URL like www.instance.com.attacker.com both wwwAexample.com. In consequence, we are able to carry out SSRF and hook up with an arbitrary backend. Whereas we won’t use the IP tackle immediately right here, we are able to bypass this restriction just by telling our DNS server to www.instance.com.attacker.com ought to resolve to 127.0.0.1.

Potential Instructions for SNI Proxy Abuse and Investigation

In a 2016 paper on scanning IPv4 for open SNI proxies, the researchers managed to seek out round 2,500 servers with a reasonably fundamental testing strategy. Whereas this quantity could appear low, SNI proxy configurations have turn into extra standard since 2016 and are broadly supported, as even a fast GitHub search reveals.

As steerage for future analysis, I can counsel a few issues to consider for configurations with out TLS termination. An SNI proxy checks solely the primary ClientHello message after which processes all subsequent visitors, even when they don’t seem to be right TLS messages. Additionally, though the RFC specifies that you may solely have one SNI area, in apply, we are able to ship a number of completely different names (TLS-Attacker is a useful gizmo right here). As a result of Nginx solely checks the primary worth, there might (theoretically) be a option to achieve further entry if a backend accepts such. ClientHello message however then makes use of the second SNI worth.

Keep away from SNI Proxy Vulnerabilities

Everytime you configure a reverse proxy, you ought to be conscious that any incorrect configuration can result in SSRF vulnerabilities that expose back-end programs to assaults. The identical is true of SNI proxies, particularly as they’re gaining recognition in large-scale manufacturing programs. Usually, to keep away from vulnerabilities when configuring a reverse proxy, you must perceive what information an attacker might management and keep away from utilizing it immediately in insecure methods.

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SSRF vulnerabilities caused by SNI proxy misconfigurations

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