CVE-2025-61921: Sinatrarb Sinatra DoS Vulnerability

CVE-2025-61921 Overview

CVE-2025-61921 is a denial of service vulnerability affecting Sinatra, a popular domain-specific language for creating web applications in Ruby. The vulnerability exists in the If-Match and If-None-Match header parsing component of Sinatra when the etag method is used during response construction. Carefully crafted input can cause these header parsing operations to consume an unexpected amount of time, potentially resulting in a denial of service condition.

Critical Impact

Any Sinatra application using the etag method when generating responses is vulnerable to denial of service attacks through malicious ETag-related HTTP headers.

Affected Products

• Sinatrarb Sinatra versions prior to 4.2.0

Discovery Timeline

• 2025-10-10 - CVE-2025-61921 published to NVD
• 2025-10-31 - Last updated in NVD database

Technical Details for CVE-2025-61921

Vulnerability Analysis

This vulnerability is classified as CWE-1333: Inefficient Regular Expression Complexity. The flaw resides in how Sinatra parses the If-Match and If-None-Match HTTP headers, which are integral to ETag-based cache validation mechanisms. When an application leverages the etag method to generate response headers, the incoming request headers are processed through parsing logic that can be exploited with specially crafted input patterns.

The attack targets the ETag header value parsing mechanism, causing the parser to enter a computationally expensive processing state. This algorithmic complexity attack allows remote attackers to degrade application performance or cause complete service unavailability without requiring authentication.

Root Cause

The root cause is inefficient regular expression processing in the header parsing component. When parsing If-Match and If-None-Match headers, the implementation uses patterns susceptible to catastrophic backtracking or excessive iteration when given maliciously constructed input strings. This is a classic Regular Expression Denial of Service (ReDoS) pattern where the computational complexity grows non-linearly with input length. Related discussions can be found in Ruby Issue #19104 and GitHub Issue #2120.

Attack Vector

The attack is network-based and can be executed remotely without any user interaction or prior authentication. An attacker sends HTTP requests with malformed If-Match or If-None-Match headers to any endpoint that uses the etag method in its response generation. The malicious header values are designed to trigger worst-case parsing behavior, consuming server resources and blocking request processing.

The vulnerability manifests in the ETag header parsing functions when processing conditional request headers. An attacker can craft header values that exploit inefficient parsing patterns, causing significant CPU consumption during header validation. For technical implementation details, see the GitHub Security Advisory GHSA-mr3q-g2mv-mr4q.

Detection Methods for CVE-2025-61921

Indicators of Compromise

• Unusual spike in CPU utilization on application servers hosting Sinatra applications
• HTTP requests containing abnormally long or malformed If-Match or If-None-Match header values
• Increased request latency or timeouts for endpoints utilizing ETag-based caching
• Log entries showing slow request processing times for specific endpoints

Detection Strategies

• Implement request rate limiting and monitor for patterns of requests with large or unusual ETag-related headers
• Configure web application firewalls (WAF) to inspect and limit the size of If-Match and If-None-Match headers
• Set up application performance monitoring (APM) to detect sudden increases in request processing time
• Review application logs for requests that correlate with CPU spikes or service degradation

Monitoring Recommendations

• Monitor server CPU utilization and set alerts for sustained high-CPU conditions
• Track request latency percentiles (p95, p99) for endpoints using the etag method
• Implement logging for requests with oversized headers or unusual character patterns
• Establish baseline performance metrics to quickly identify deviations indicative of active exploitation

How to Mitigate CVE-2025-61921

Immediate Actions Required

• Upgrade Sinatra to version 4.2.0 or later, which contains the fix for this vulnerability
• Implement request header size limits at the reverse proxy or load balancer level
• Consider temporarily disabling ETag functionality if immediate patching is not possible
• Deploy rate limiting to reduce the impact of potential exploitation attempts

Patch Information

The vulnerability is fixed in Sinatra version 4.2.0. The fix addresses the inefficient parsing logic in the If-Match and If-None-Match header handling. For details on the implementation, see GitHub Pull Request #2121 and GitHub Pull Request #1823. The official security advisory is available at GitHub Security Advisory GHSA-mr3q-g2mv-mr4q.

Workarounds

• Configure upstream proxies (nginx, HAProxy) to limit maximum header sizes and reject requests with excessively long ETag-related headers
• Implement middleware to validate and sanitize If-Match and If-None-Match headers before they reach the Sinatra application
• Temporarily remove etag method calls from response generation if they are not critical to application functionality
• Deploy additional reverse proxy rules to rate-limit requests containing ETag conditional headers

# Example nginx configuration to limit header size
# Add to http or server block
large_client_header_buffers 4 8k;
client_header_buffer_size 1k;

# Rate limiting for suspicious requests
limit_req_zone $binary_remote_addr zone=etag_limit:10m rate=10r/s;
limit_req zone=etag_limit burst=20 nodelay;