CVE-2023-27539: Rack Header Parsing DOS Vulnerability

CVE-2023-27539 Overview

CVE-2023-27539 is a denial of service vulnerability affecting the header parsing component of Rack, a modular Ruby web server interface. The vulnerability exists in the parse_http_accept_header function, which uses regular expressions susceptible to Regular Expression Denial of Service (ReDoS) attacks. An attacker can exploit this flaw by sending specially crafted HTTP headers that cause catastrophic backtracking in the regex engine, leading to excessive CPU consumption and service unavailability.

Critical Impact

Attackers can remotely trigger denial of service conditions against Ruby web applications using vulnerable Rack versions by sending malicious HTTP Accept headers, potentially causing significant service disruption without authentication.

Affected Products

• Rack (Ruby web server interface)
• Debian Linux 10.0
• Debian Linux 11.0

Discovery Timeline

• 2025-01-09 - CVE CVE-2023-27539 published to NVD
• 2025-10-10 - Last updated in NVD database

Technical Details for CVE-2023-27539

Vulnerability Analysis

This vulnerability is classified as an Algorithmic Complexity Attack, specifically a Regular Expression Denial of Service (ReDoS). The flaw resides in Rack's HTTP Accept header parsing logic within lib/rack/request.rb. The vulnerable code uses regex patterns with quantifiers (\s*,\s* and \s*;\s*) that can cause exponential backtracking when processing specially crafted input strings.

When the parse_http_accept_header method receives a maliciously constructed header value containing patterns that exploit the regex's backtracking behavior, the Ruby regex engine enters a state of excessive computation. This can consume significant CPU resources on the server, effectively blocking the processing of legitimate requests.

The attack is network-accessible and requires no authentication or user interaction, making it trivially exploitable against any exposed Ruby web application using vulnerable Rack versions. While the impact is limited to availability (no data confidentiality or integrity impact), the ease of exploitation makes this a significant concern for production environments.

Root Cause

The root cause is the use of inefficient regular expressions in the header parsing logic. The original implementation used /\s*,\s*/ and /\s*;\s*/ patterns for splitting header values. These patterns, when combined with certain input strings, trigger catastrophic backtracking—a well-known weakness in regex engines where the number of possible match paths grows exponentially with input length.

Attack Vector

The attack vector is network-based, targeting the HTTP Accept header parsing functionality. An attacker can send HTTP requests with specially crafted Accept headers to any endpoint of a vulnerable Ruby web application. The malicious header values are designed to maximize backtracking iterations in the regex engine, causing the server to spend excessive time parsing a single header value. Repeated requests can amplify the effect, potentially rendering the service completely unavailable.

# Security patch from lib/rack/request.rb - Avoid ReDoS problem
# Source: https://github.com/rack/rack/commit/231ef369ad0b542575fb36c74fcfcfabcf6c530c

# Before (vulnerable):
# header.to_s.split(/\s*,\s*/).map do |part|
#   attribute, parameters = part.split(/\s*;\s*/, 2)

# After (fixed):
def parse_http_accept_header(header)
  header.to_s.split(",").each(&:strip!).map do |part|
    attribute, parameters = part.split(";", 2).each(&:strip!)
    quality = 1.0
    if parameters and /\Aq=([\d.]+)/ =~ parameters
      quality = $1.to_f
    end
    # ... rest of implementation
  end
end

Detection Methods for CVE-2023-27539

Indicators of Compromise

• Unusually high CPU utilization on Ruby application servers without corresponding increase in legitimate traffic
• HTTP requests containing abnormally long or malformed Accept headers with repeating whitespace patterns
• Web application response timeouts or increased latency affecting all users
• Application server process hangs or unresponsiveness during header parsing operations

Detection Strategies

• Monitor web server access logs for requests with exceptionally large or unusual Accept header values
• Implement application performance monitoring (APM) to detect anomalous CPU spikes during request processing
• Configure web application firewalls (WAF) to flag or block requests with oversized or suspicious header patterns
• Use SentinelOne's runtime application protection to identify ReDoS attack patterns targeting Ruby applications

Monitoring Recommendations

• Set up alerts for sustained high CPU utilization on application servers running Ruby/Rack applications
• Monitor request processing times and establish baseline thresholds for anomaly detection
• Implement rate limiting on endpoints to reduce the impact of DoS attempts
• Review web server logs regularly for patterns indicating exploitation attempts

How to Mitigate CVE-2023-27539

Immediate Actions Required

• Update Rack to the latest patched version immediately across all environments
• Review deployed Ruby applications for Rack version inventory and prioritize upgrades
• Implement request timeout limits at the reverse proxy or load balancer level as a temporary safeguard
• Consider implementing header size limits at the web server or WAF level

Patch Information

Security patches have been released to address this vulnerability. The fix replaces the vulnerable regex-based string splitting with literal character splitting followed by explicit whitespace trimming, eliminating the ReDoS attack surface. Refer to the GitHub Security Advisory GHSA-c6qg-cjj8-47qp for detailed patching guidance. Debian users should apply updates from DSA-5530 or the appropriate LTS security announcement.

Workarounds

• Deploy a web application firewall (WAF) rule to limit Accept header length and complexity
• Implement request timeouts at the application or reverse proxy level to limit DoS impact
• Use rate limiting to reduce the effectiveness of repeated exploitation attempts
• Consider temporarily blocking requests with suspicious Accept header patterns until patches can be applied

# Example nginx configuration to limit header sizes
# Add to http or server block

large_client_header_buffers 4 8k;
client_header_buffer_size 1k;

# Optional: Add request timeout
proxy_read_timeout 30s;
proxy_connect_timeout 10s;