Stack Overflow in Sinatra

Stack Overflow vulnerabilities in Sinatra applications typically emerge through recursive route handling, deeply nested template rendering, and unbounded recursion in custom middleware. Unlike Rails applications with their complex middleware stack, Sinatra's minimalist design can make these issues more apparent and sometimes more severe.

One common manifestation occurs in route handlers that recursively call themselves through redirects or chained requests. Consider a Sinatra application handling nested comments where a comment can reply to another comment:

post '/comments' do
  comment = Comment.create(text: params[:text], parent_id: params[:parent_id])
  if comment.parent
    redirect to "/comments/#{comment.parent.id}"
  else
    comment.to_json
  end
end

This creates a recursion chain where each comment redirect triggers another request, potentially exhausting the stack if comment threads are deep enough. The Sinatra runtime has a default stack limit of around 1000 frames, and with each redirect adding to the call stack, a comment thread with 100+ levels could trigger a crash.

Template rendering presents another attack vector. Sinatra's ERB and Haml templates execute Ruby code, and recursive template includes can create stack overflows:

# In views/comments.erb
<% if @comment.parent %>
  <%= erb :comments, locals: { comment: @comment.parent } %>
<% end %>

This template recursively includes itself for each parent comment, creating a stack overflow when comment threads exceed the recursion limit. The issue becomes more severe when combined with user-controlled input that determines recursion depth.

Custom middleware that processes requests can also introduce stack overflow risks. A middleware that recursively processes nested JSON structures:

class RecursiveProcessor
  def initialize(app)
    @app = app
  end
  
  def call(env)
    request = Rack::Request.new(env)
    if request.content_type == 'application/json'
      process_json(JSON.parse(request.body.read))
    end
    @app.call(env)
  end
  
  def process_json(data)
    data.each do |key, value|
      if value.is_a?(Hash)
        process_json(value) # Recursive call without depth limit
      end
    end
  end
end

This middleware processes JSON data recursively without any depth limiting, making it vulnerable to stack overflow when processing maliciously crafted nested JSON structures.

Detecting stack overflow vulnerabilities in Sinatra applications requires both static analysis of route patterns and dynamic testing of recursive paths. The Sinatra framework's DSL makes certain patterns easy to identify through code review.

Static analysis should focus on route handlers that contain recursive patterns. Look for routes that redirect to themselves or to related routes that could create call chains. Use grep or similar tools to find patterns like:

grep -r "redirect to" . --include="*.rb" | grep -E "(self|related|parent|child)"

Template files should be scanned for recursive includes. In ERB templates, search for patterns where templates include themselves:

grep -r "erb :" . --include="*.erb" | grep -E "(self|parent|child)"

Dynamic testing involves creating test cases that exercise deep recursion paths. For the comment system example, create a script that generates a deeply nested comment thread:

require 'net/http'
require 'json'

def create_nested_comment(text, depth)
  uri = URI('http://localhost:4567/comments')
  http = Net::HTTP.new(uri.host, uri.port)
  req = Net::HTTP::Post.new(uri.path)
  req.set_form_data(text: text, parent_id: depth > 0 ? depth : nil)
  http.request(req)
end

# Create 100 nested comments
100.times do |i|
  create_nested_comment("Comment #{i}", i - 1)
end

middleBrick's black-box scanning can automatically detect stack overflow vulnerabilities by testing API endpoints with varying input depths. The scanner tests recursive endpoints by sending progressively deeper requests and monitoring for stack overflow errors or timeout responses. For Sinatra applications, middleBrick specifically checks:

• Recursive route patterns that could lead to stack exhaustion
• Template rendering paths with unbounded recursion
• Middleware that processes nested data structures without depth limits
• API endpoints that accept nested JSON and process it recursively

The scanner's LLM security module also checks for stack-related vulnerabilities in AI endpoints, testing for prompt injection patterns that could trigger recursive processing in LLM-based Sinatra applications.

Remediating stack overflow vulnerabilities in Sinatra requires implementing depth limits, iterative processing, and safe recursion patterns. Sinatra's lightweight nature means you'll need to implement these protections manually rather than relying on framework-level safeguards.

For recursive route handlers, replace redirects with iterative processing or implement depth limits:

MAX_RECURSION_DEPTH = 50

post '/comments' do
  comment = Comment.create(text: params[:text], parent_id: params[:parent_id])
  if comment.parent && comment.parent.depth < MAX_RECURSION_DEPTH
    redirect to "/comments/#{comment.parent.id}"
    comment.to_json
  end
end

This prevents stack overflow by limiting the recursion depth and returning results instead of continuing the redirect chain when the limit is reached.

Template recursion should be replaced with iterative approaches or limited recursion. For comment trees, use a stack-based approach instead of recursive includes:

# In views/comments.erb
<% stack = [@comment] %>
<% while !stack.empty? %>
  <% current = stack.pop %>
  <div class="comment">
    <%= current.text %>
    <% if current.children %>
      <% current.children.each { |child| stack.push(child) } %>
    <% end %>
  </div>
<% end %>

This iterative approach using a stack prevents stack overflow while maintaining the same visual output as the recursive version.

For middleware that processes nested data, implement depth tracking and limits:

class SafeRecursiveProcessor
  MAX_DEPTH = 20
  
  def initialize(app)
    @app = app
  end
  
  def call(env)
    request = Rack::Request.new(env)
    if request.content_type == 'application/json'
      process_json(JSON.parse(request.body.read), 0)
    end
    @app.call(env)
  end
  
  def process_json(data, depth)
    raise "Max depth exceeded" if depth >= MAX_DEPTH
    data.each do |key, value|
      if value.is_a?(Hash)
        process_json(value, depth + 1)
      end
    end
  end
end

This version tracks recursion depth and raises an error when the maximum depth is exceeded, preventing stack overflow attacks.

For Sinatra applications using ActiveRecord or similar ORMs, be cautious with eager loading of deeply nested associations. Use includes with limits or implement pagination for nested data:

get '/comments/:id' do
  comment = Comment.find(params[:id])
  comment.children.limit(10) # Limit nested loading
  comment.to_json(include: { children: { limit: 10 } })
end

middleBrick's scanning can verify that these protections are in place by testing endpoints with deep nesting and verifying that stack overflow vulnerabilities are properly mitigated. The scanner checks that depth limits are enforced and that recursive processing doesn't exceed safe bounds.