SSRF vulnerabilities brought on by SNI proxy misconfigurations | Tech Ops

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A typical activity in complicated internet functions 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 software stage (over HTTP) and requests are routed based mostly on the worth of the Host header (:authority for HTTP/2) or elements of the route.

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

What’s TLS SNI?

Server Title Indication (SNI) is an extension of the TLS protocol that gives the inspiration of HTTPS. When a browser needs to determine a safe connection to a server, it initiates a TLS handshake by sending a ClientHello message. This message could include an SNI extension subject that features the area identify of the server. In its ServerHello message, the server could return an applicable certificates for the required server identify. The everyday use case for that is when there are a number of digital hosts behind one IP handle.

What’s an SNI proxy?

When a reverse proxy (extra accurately, a load balancer) makes use of a worth of the SNI subject to pick out a selected backend server, we’ve got an SNI proxy. With the widespread use of TLS and HTTPS specifically, this strategy is rising in popularity. (Observe that one other that means of SNI proxy refers to using such proxy servers to bypass censorship in some international locations.)

There are two fundamental choices for working an SNI proxy: with or with out SSL termination. In each circumstances, the SNI proxy makes use of the worth of the SNI subject to pick out an applicable backend. When working with SSL termination, the TLS connection is established with the SNI proxy, after which the proxy forwards the decrypted site visitors to the backend. Within the second case, the SNI proxy forwards all the information stream, and truly works extra like a TCP proxy.

A typical SNI proxy configuration

Many reverse proxies/load balancers assist SNI proxy configurations, together with Nginx, Haproxy, Envoy, ATS, and others. It appears to be like like you’ll be able to even use an SNI proxy on Kubernetes.

To provide an Nginx instance, the best setup can be as follows (notice 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) known as stream and allow SNI entry utilizing ssl_preread on. Relying on the worth of the SNI subject (in $ssl_preread_server_name), Nginx will route all the TLS connection to backend1 both backend2.

Incorrect SNI proxy configurations resulting in SSRF

The best misconfiguration that will will let you connect with an arbitrary backend would appear to be 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 subject is used instantly because the backend handle.

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

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

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

One other class of susceptible configurations is just like typical HTTP reverse proxy misconfigurations and entails common expression (regex) errors. On this instance, the site 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 in addition URL like www.instance.com.attacker.com both wwwAexample.com. Consequently, we are able to carry out SSRF and connect with an arbitrary backend. Whereas we won’t use the IP handle instantly 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 primary testing strategy. Whereas this quantity could appear low, SNI proxy configurations have change into extra well-liked since 2016 and are extensively supported, as even a fast GitHub search reveals.

As steering 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 site visitors, even when they aren’t right TLS messages. Additionally, though the RFC specifies you can solely have one SNI subject, in follow, we are able to ship a number of completely different names (TLS-Attacker is a great tool right here). As a result of Nginx solely checks the primary worth, there might (theoretically) be a approach to acquire extra 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 need 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 reputation in large-scale manufacturing programs. Generally, to keep away from vulnerabilities when configuring a reverse proxy, you could perceive what information an attacker might management and keep away from utilizing it instantly in insecure methods.

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SSRF vulnerabilities caused by SNI proxy misconfigurations