Server-side request forgery¶

ID: rb/request-forgery
Kind: path-problem
Security severity: 9.1
Severity: error
Precision: high
Tags:
   - security
   - external/cwe/cwe-918
Query suites:
   - ruby-code-scanning.qls
   - ruby-security-extended.qls
   - ruby-security-and-quality.qls

Click to see the query in the CodeQL repository

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.

Recommendation¶

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.

Example¶

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.

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

References¶

OWASP SSRF
Common Weakness Enumeration: CWE-918.

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