Add qhelp for Ruby SSRF

ruby/ql/src/queries/security/cwe-918/ServerSideRequestForgery.qhelp

@@ -0,0 +1,41 @@
+<!DOCTYPE qhelp PUBLIC
+  "-//Semmle//qhelp//EN"
+  "qhelp.dtd">
+<qhelp>
+
+
+<overview>
+<p>Directly incorporating user input into an HTTP request without validating the input
+can facilitate server-side request forgery (SSRF) attacks. In these attacks, the
+request may be changed, directed at a different server, or via a different
+protocol. This can allow the attacker to obtain sensitive information or perform
+actions with escalated privilege.
+</p>
+
+</overview>
+<recommendation>
+
+<p>To guard against SSRF attacks you should avoid putting user-provided input
+directly into a request URL. Instead, maintain a list of authorized
+URLs on the server; then choose from that list based on the input provided.
+Alternatively, ensure requests constructed from user input are limited to
+a particular host or more restrictive URL prefix.</p>
+
+</recommendation>
+<example>
+
+<p>The following example shows an HTTP request parameter being used directly to form a
+new request without validating the input, which facilitates SSRF attacks.
+It also shows how to remedy the problem by validating the user input against a known fixed string.
+</p>
+
+<sample src="ServerSideRequestForgery.rb" />
+
+</example>
+<references>
+  <li>
+    <a href="https://owasp.org/www-community/attacks/Server_Side_Request_Forgery">OWASP SSRF</a>
+  </li>
+
+</references>
+</qhelp>

ruby/ql/src/queries/security/cwe-918/ServerSideRequestForgery.rb

@@ -0,0 +1,25 @@
+require "excon"
+require "json"
+
+class PostsController < ActionController::Base
+  def create
+    user = params[:user_id]
+
+    # BAD - user can control the entire URL of the request
+    users_service_domain = params[:users_service_domain]
+    response = Excon.post("#{users_service_domain}/logins", body: {user_id: user}).body
+    token = JSON.parse(response)["token"]
+
+    # GOOD - path is validated against a known fixed string
+    path = if params[:users_service_path] == "v1/users"
+             "v1/users"
+           else 
+             "v2/users"
+           end
+    response = Excon.post("users-service/#{path}", body: {user_id: user}).body
+    token = JSON.parse(response)["token"]
+
+    @post = Post.create(params[:post].merge(user_token: token))
+    render @post
+  end
+end