CVE-2021-22902 in actionpack Gem
Summary
by MITRE • 06/11/2021
The actionpack ruby gem (a framework for handling and responding to web requests in Rails) before 6.0.3.7, 6.1.3.2 suffers from a possible denial of service vulnerability in the Mime type parser of Action Dispatch. Carefully crafted Accept headers can cause the mime type parser in Action Dispatch to do catastrophic backtracking in the regular expression engine.
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Analysis
by VulDB Data Team • 06/13/2021
The CVE-2021-22902 vulnerability represents a critical denial of service flaw affecting the Ruby on Rails framework through its actionpack gem component. This vulnerability specifically targets the Mime type parser within Action Dispatch, which is responsible for processing HTTP Accept headers that define the content types a client can accept. The issue affects versions prior to 6.0.3.7 and 6.1.3.2, making it a widespread concern for applications utilizing these framework versions. The vulnerability stems from a flaw in the regular expression pattern used to parse MIME types, creating an opportunity for malicious actors to exploit the system's parsing logic.
The technical implementation of this vulnerability involves catastrophic backtracking within the regular expression engine when processing specifically crafted Accept headers. This occurs because the regex pattern used for parsing MIME types contains constructs that can lead to exponential time complexity when matched against malicious input. The vulnerability manifests when an attacker sends a carefully constructed Accept header that causes the regular expression engine to perform an excessive number of backtracking operations, ultimately consuming CPU resources and leading to system unresponsiveness. This behavior aligns with CWE-1333, which categorizes issues related to regular expression denial of service, and represents a classic example of how regex engine behavior can be exploited for resource exhaustion attacks.
The operational impact of CVE-2021-22902 extends beyond simple service disruption, as it can be leveraged by attackers to perform resource exhaustion attacks against web applications. When exploited, the vulnerability can cause the application server to consume excessive CPU cycles, potentially leading to complete service unavailability for legitimate users. This makes it particularly dangerous in production environments where applications must maintain high availability and responsiveness. The vulnerability can be exploited through simple HTTP requests, making it accessible to attackers with minimal technical expertise and potentially enabling automated exploitation at scale. The attack vector directly relates to ATT&CK technique T1499.004, which covers network denial of service attacks through resource exhaustion.
Mitigation strategies for this vulnerability require immediate application of the security patches released by the Rails team, specifically updating to versions 6.0.3.7 or 6.1.3.2. Organizations should also implement rate limiting and input validation measures at the application level and network perimeter to detect and prevent exploitation attempts. Additionally, monitoring systems should be configured to detect unusual CPU utilization patterns that might indicate exploitation activity. Security teams should conduct comprehensive vulnerability assessments to identify all affected applications and ensure proper patch management processes are in place. The vulnerability highlights the importance of regular security updates and proper input validation in web applications, particularly when dealing with user-provided data that gets processed through regex patterns. Organizations should also consider implementing web application firewalls and intrusion detection systems to provide additional layers of protection against similar exploitation techniques.