CVE-2008-3656 in Ruby
Summary
by MITRE
Algorithmic complexity vulnerability in the WEBrick::HTTPUtils.split_header_value function in WEBrick::HTTP::DefaultFileHandler in WEBrick in Ruby 1.8.5 and earlier, 1.8.6 through 1.8.6-p286, 1.8.7 through 1.8.7-p71, and 1.9 through r18423 allows context-dependent attackers to cause a denial of service (CPU consumption) via a crafted HTTP request that is processed by a backtracking regular expression.
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Analysis
by VulDB Data Team • 01/17/2025
The CVE-2008-3656 vulnerability represents a critical algorithmic complexity issue within the WEBrick web server implementation in Ruby versions up to 1.8.7-p71 and 1.9 through r18423. This flaw specifically targets the WEBrick::HTTPUtils.split_header_value function which is responsible for parsing HTTP headers in the DefaultFileHandler component. The vulnerability stems from the use of regular expressions that are susceptible to catastrophic backtracking behavior when processing malformed input. When attackers craft specific HTTP requests containing carefully constructed header values, the regular expression engine enters into exponential time complexity execution patterns, causing significant CPU resource consumption and ultimately leading to system unresponsiveness.
The technical exploitation of this vulnerability occurs through the manipulation of HTTP header parsing logic within WEBrick's HTTP utilities. The WEBrick::HTTPUtils.split_header_value function employs regular expressions that exhibit backtracking behavior when confronted with malicious input patterns. This backtracking creates a combinatorial explosion in the number of possible execution paths the regular expression engine must evaluate, resulting in exponential time complexity growth. The vulnerability is context-dependent because it requires specific HTTP request construction that triggers the problematic parsing path, making it difficult to exploit without proper understanding of the internal parsing mechanisms. This behavior aligns with CWE-1333, which specifically addresses algorithmic complexity vulnerabilities in regular expressions that can lead to denial of service conditions.
From an operational impact perspective, this vulnerability enables attackers to perform effective denial of service attacks against Ruby applications running WEBrick web servers. The CPU consumption spike occurs during the header parsing phase of HTTP request processing, where a single malicious request can cause the server to consume excessive processing resources for extended periods. The vulnerability affects not only standalone WEBrick instances but also any Ruby application that utilizes WEBrick as its HTTP server component, including development environments, testing frameworks, and production applications. The impact extends beyond simple resource exhaustion as it can render entire web applications unavailable to legitimate users, potentially causing business disruption and service unavailability. Attackers can amplify the effect by sending multiple malicious requests or by crafting requests that cause the server to consume resources for extended periods, making this vulnerability particularly dangerous in production environments.
The mitigation strategies for CVE-2008-3656 primarily involve upgrading to patched versions of Ruby where the vulnerable regular expressions have been replaced with more efficient alternatives or rewritten to prevent backtracking behavior. Organizations should also implement request rate limiting and input validation measures at network boundaries to prevent exploitation attempts. Additionally, application-level protections such as setting timeouts on HTTP request processing and monitoring CPU usage patterns can help detect and mitigate exploitation attempts. The vulnerability demonstrates the importance of regular security updates and the dangers of using regular expressions with unbounded backtracking in security-critical applications. This issue relates to ATT&CK technique T1499.004 which involves resource exhaustion attacks, and highlights the need for proper input sanitization and robust parsing mechanisms in web server implementations. Organizations should also consider implementing web application firewalls and intrusion detection systems to monitor for suspicious HTTP header patterns that may indicate exploitation attempts. The vulnerability serves as a reminder of the critical importance of algorithmic complexity analysis in security-critical code paths, particularly in parsing functions that process untrusted input data.