CVE-2026-34826: Rack Web Server Interface DoS Vulnerability

CVE-2026-34826 Overview

A denial of service vulnerability exists in Rack, the modular Ruby web server interface. The Rack::Utils.get_byte_ranges function parses the HTTP Range header without limiting the number of individual byte ranges that can be specified. While a previous fix for CVE-2024-26141 addressed ranges whose total byte coverage exceeds the file size, it failed to restrict the count of ranges in a single request. This oversight allows attackers to supply numerous small overlapping ranges (such as 0-0,0-0,0-0,...) to trigger disproportionate CPU, memory, I/O, and bandwidth consumption per request, resulting in a denial of service condition affecting Rack file-serving paths that process multipart byte range responses.

Critical Impact

Attackers can exploit this vulnerability remotely without authentication to cause resource exhaustion and denial of service in Ruby web applications using Rack's file-serving capabilities.

Affected Products

• Rack versions prior to 2.2.23
• Rack versions prior to 3.1.21
• Rack versions prior to 3.2.6

Discovery Timeline

• 2026-04-02 - CVE CVE-2026-34826 published to NVD
• 2026-04-02 - Last updated in NVD database

Technical Details for CVE-2026-34826

Vulnerability Analysis

This vulnerability is classified as CWE-400 (Uncontrolled Resource Consumption). The flaw exists in how Rack handles HTTP Range header parsing within the Rack::Utils.get_byte_ranges function. When a client requests specific byte ranges of a file, the server typically processes each range and constructs a multipart response. The vulnerability arises because there is no validation on the number of ranges that can be specified in a single request.

An attacker can craft a malicious HTTP Range header containing thousands of overlapping or identical range specifications. Each range, even if requesting the same bytes repeatedly, triggers processing overhead including memory allocation, I/O operations, and response generation. The cumulative effect of processing numerous ranges in a single request leads to significant resource exhaustion on the server.

This is a bypass of the incomplete fix for CVE-2024-26141, which only addressed scenarios where the total byte coverage exceeded file size but neglected to limit the range count itself.

Root Cause

The root cause is missing input validation in Rack::Utils.get_byte_ranges that fails to enforce a maximum limit on the number of byte ranges parsed from the HTTP Range header. While the function validates individual range boundaries and total byte coverage, it does not restrict how many ranges can be specified, allowing attackers to submit arbitrarily large numbers of range specifications.

Attack Vector

The attack is conducted remotely over the network without requiring authentication. An attacker sends HTTP requests to file-serving endpoints with a specially crafted Range header containing many small, overlapping byte ranges.

The exploitation follows this pattern:

1. Attacker identifies a Rack-based application serving static files
2. Attacker crafts an HTTP request with a Range header containing thousands of range specifications (e.g., Range: bytes=0-0,0-0,0-0,... repeated extensively)
3. The server parses each range specification, allocating resources for multipart response generation
4. Server resources (CPU, memory, I/O bandwidth) become exhausted processing the excessive ranges
5. Legitimate users experience degraded service or complete denial of access

For detailed technical information about the vulnerability mechanism, refer to the GitHub Security Advisory.

Detection Methods for CVE-2026-34826

Indicators of Compromise

• HTTP requests containing Range headers with an unusually high number of comma-separated byte range specifications
• Elevated CPU and memory utilization on web servers serving static files through Rack
• Abnormal I/O patterns associated with file-serving operations
• Multiple requests from the same source with malformed or excessive Range headers

Detection Strategies

• Implement web application firewall (WAF) rules to detect and block HTTP requests with Range headers containing excessive range specifications
• Monitor application logs for requests with abnormally long Range header values
• Configure intrusion detection systems (IDS) to alert on HTTP requests with more than a reasonable number of byte ranges (e.g., >100 ranges)
• Deploy rate limiting on file-serving endpoints to mitigate automated exploitation attempts

Monitoring Recommendations

• Set up alerts for sudden spikes in CPU or memory usage on Ruby application servers
• Monitor network traffic for patterns of repeated requests to static file endpoints with large Range headers
• Track response times for file-serving endpoints to detect degradation indicating ongoing attacks
• Log and analyze HTTP Range header patterns to establish baselines and identify anomalies

How to Mitigate CVE-2026-34826

Immediate Actions Required

• Upgrade Rack to patched versions: 2.2.23, 3.1.21, or 3.2.6 immediately
• Review and identify all Ruby applications using Rack for file serving functionality
• Implement temporary WAF rules to limit the number of byte ranges accepted in HTTP Range headers
• Consider temporarily disabling byte range support on critical file-serving endpoints if upgrading is not immediately possible

Patch Information

The vulnerability has been addressed in Rack versions 2.2.23, 3.1.21, and 3.2.6. Organizations should update their Rack gem dependencies to one of these patched versions based on their current major version line.

For Bundler-managed applications, update the Gemfile to specify the minimum patched version:

gem 'rack', '>= 3.2.6'  # For Rack 3.2.x users

Then run bundle update rack to apply the update.

For additional details on the security fix, see the GitHub Security Advisory.

Workarounds

• Deploy a reverse proxy (such as nginx or Apache) in front of the application configured to limit the number of byte ranges in Range headers
• Implement application-level middleware to validate and reject requests with excessive byte ranges before they reach Rack's file-serving logic
• Disable multipart byte range responses by configuring the web server to ignore Range headers for static file requests
• Apply network-level rate limiting to reduce the impact of exploitation attempts

# Example nginx configuration to limit Range header abuse
# Add to server or location block for static file serving
if ($http_range ~* "bytes=([0-9]+-[0-9]+,){50,}") {
    return 416;
}