CVE-2025-46727: Rack Ruby Web Server DoS Vulnerability

CVE-2025-46727 Overview

CVE-2025-46727 is a Denial of Service (DoS) vulnerability in Rack, the modular Ruby web server interface. The vulnerability exists in Rack::QueryParser, which parses query strings and application/x-www-form-urlencoded bodies into Ruby data structures without imposing any limit on the number of parameters. This allows attackers to send requests with extremely large numbers of parameters, consuming excessive memory and CPU resources during parsing.

Critical Impact

An attacker can trigger full service disruption by sending specifically crafted HTTP requests with hundreds of thousands of parameters, causing memory exhaustion or CPU resource starvation that stalls or crashes the Rack server until affected workers are restarted.

Affected Products

• Rack versions prior to 2.2.14
• Rack versions prior to 3.0.16
• Rack versions prior to 3.1.14

Discovery Timeline

• 2025-05-07 - CVE CVE-2025-46727 published to NVD
• 2025-06-17 - Last updated in NVD database

Technical Details for CVE-2025-46727

Vulnerability Analysis

The vulnerability is classified as CWE-400 (Uncontrolled Resource Consumption). The flaw stems from the Rack::QueryParser component, which is responsible for parsing incoming HTTP query strings and form-encoded request bodies. When processing these inputs, the parser iterates over each &-separated key-value pair and adds it to a Ruby Hash data structure. The critical issue is that no upper bound is enforced on the total number of parameters that can be processed.

This architectural oversight allows an attacker to construct a single HTTP request containing an extremely large number of parameters—potentially hundreds of thousands or more. As the parser attempts to process each parameter, it allocates memory for every key-value pair and consumes CPU cycles during the parsing operation. The cumulative effect of processing such a massive parameter set leads to resource exhaustion.

The attack can be executed remotely over the network without any authentication or user interaction required. While the vulnerability does not compromise data confidentiality or integrity, it has a complete impact on system availability, making it a significant concern for any Ruby web application built on the Rack interface.

Root Cause

The root cause is the absence of input validation controls in Rack::QueryParser to limit the number of parameters accepted from incoming requests. The parser was designed to handle arbitrary parameter counts without considering the security implications of unbounded iteration over attacker-controlled input data. This represents a classic resource exhaustion vulnerability where algorithmic complexity can be weaponized through malicious input.

Attack Vector

The attack is network-based and requires no privileges or user interaction. An attacker can exploit this vulnerability by:

1. Crafting an HTTP request with an extremely large query string containing numerous &-separated parameters (e.g., param1=value1¶m2=value2&... repeated hundreds of thousands of times)
2. Alternatively, sending a POST request with a large application/x-www-form-urlencoded body containing similar parameter counts
3. Directing this request to any endpoint on a vulnerable Rack-based application

When the malicious request reaches the Rack server, the QueryParser begins iterating over each parameter. The parsing process consumes increasing amounts of memory as each parameter is added to the Hash structure, while CPU resources are consumed by the parsing logic. This can result in worker processes becoming unresponsive, exhausting available memory, or causing the entire Rack server to crash.

Detection Methods for CVE-2025-46727

Indicators of Compromise

• HTTP requests with abnormally large query strings (significantly exceeding typical application usage patterns)
• POST requests with unusually large application/x-www-form-urlencoded bodies containing thousands of parameters
• Sudden spikes in memory consumption on Rack server processes
• Web worker processes becoming unresponsive or timing out
• Increased CPU utilization during request parsing phases

Detection Strategies

• Monitor web application logs for requests with query strings exceeding normal length thresholds
• Implement alerting on Rack worker process memory consumption anomalies
• Configure web application firewall (WAF) rules to flag or block requests with excessive parameter counts
• Use application performance monitoring (APM) tools to track parsing latency and resource utilization
• Analyze reverse proxy logs for requests that exceed size thresholds before reaching the application

Monitoring Recommendations

• Set up alerts for Rack worker processes exceeding memory thresholds
• Monitor request queue depths for signs of worker starvation
• Track HTTP 503 error rates that may indicate service degradation
• Implement logging at the reverse proxy layer to capture oversized requests
• Monitor CPU utilization patterns on application servers for parsing-related spikes

How to Mitigate CVE-2025-46727

Immediate Actions Required

• Upgrade Rack to patched versions: 2.2.14, 3.0.16, or 3.1.14 immediately
• If immediate patching is not possible, implement request size limits at the reverse proxy or CDN layer
• Deploy middleware to enforce maximum query string size or parameter count limits
• Review and configure Nginx, Apache, or other reverse proxy settings to reject oversized requests
• Monitor application servers for signs of exploitation attempts

Patch Information

The Rack maintainers have released patched versions that address this vulnerability by implementing parameter count limits in the QueryParser component. The following versions contain the fix:

• Rack 2.2.14 for the 2.x branch
• Rack 3.0.16 for the 3.0.x branch
• Rack 3.1.14 for the 3.1.x branch

Relevant commits and security advisory:

• GitHub Commit Update
• GitHub Commit Fix
• GitHub Commit Improvement
• GitHub Security Advisory GHSA-gjh7-p2fx-99vx

Workarounds

• Deploy middleware that enforces a maximum query string size or parameter count before requests reach Rack::QueryParser
• Use a reverse proxy such as Nginx to limit request sizes and reject oversized query strings or request bodies
• Configure CDN-level protections to limit request body sizes and query string lengths
• Implement rate limiting to reduce the impact of repeated malicious requests
• Consider deploying a web application firewall with rules to block requests exceeding parameter thresholds

# Nginx configuration to limit request sizes
# Add to server or http block in nginx.conf

# Limit the maximum size of the client request body
client_max_body_size 1m;

# Limit the buffer size for reading client request body
client_body_buffer_size 16k;

# Limit URI length (includes query string)
large_client_header_buffers 4 8k;