CVE-2016-1133 in H2O
Summary
by MITRE
CRLF injection vulnerability in the on_req function in lib/handler/redirect.c in H2O before 1.6.2 and 1.7.x before 1.7.0-beta3 allows remote attackers to inject arbitrary HTTP headers and conduct HTTP response splitting attacks via a crafted URI.
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Analysis
by VulDB Data Team • 07/03/2022
The CVE-2016-1133 vulnerability represents a critical CRLF injection flaw in the H2O web server software that affects versions prior to 1.6.2 and 1.7.0-beta3. This vulnerability resides within the on_req function located in the lib/handler/redirect.c file, making it a server-side processing issue that can be exploited by remote attackers. The flaw specifically enables attackers to inject arbitrary HTTP headers into responses, which creates a pathway for HTTP response splitting attacks. The vulnerability stems from insufficient input validation and sanitization of URI parameters that are processed by the redirect handler functionality. When the web server processes a crafted URI containing CRLF sequences, these sequences are not properly escaped or filtered, allowing malicious input to be interpreted as part of the HTTP response header structure rather than as part of the requested resource identifier.
The technical exploitation of this vulnerability follows established patterns for CRLF injection attacks that are categorized under CWE-113 in the Common Weakness Enumeration system. Attackers can craft malicious URIs containing carriage return and line feed characters encoded as %0d%0a or similar sequences to inject additional headers into HTTP responses. This capability enables several dangerous attack vectors including session hijacking, cross-site scripting attacks, and cache poisoning. The HTTP response splitting occurs because the vulnerable code fails to properly sanitize user input before incorporating it into HTTP headers, allowing attackers to inject malformed response data that can be interpreted by web browsers or intermediary proxies as separate HTTP responses. This vulnerability directly maps to techniques described in the MITRE ATT&CK framework under the T1190 category for exploitation of vulnerabilities in web applications.
The operational impact of CVE-2016-1133 extends beyond simple header injection, as it provides attackers with the ability to manipulate HTTP responses in ways that can compromise user sessions and bypass security controls. When successfully exploited, this vulnerability can lead to session fixation attacks where attackers can establish and maintain control over user sessions, or enable cross-site scripting scenarios where malicious scripts can be injected into web responses. The vulnerability affects the core redirect functionality of H2O, making it particularly dangerous as it can be triggered by any request that involves URL redirection. Organizations using affected versions of H2O are exposed to potential data breaches, user impersonation, and service disruption. The vulnerability's impact is amplified by the fact that it operates at the HTTP protocol level, making it difficult to detect through traditional network monitoring and potentially bypassing security controls that operate at higher protocol layers.
Mitigation strategies for CVE-2016-1133 focus on immediate software updates to versions 1.6.2 or 1.7.0-beta3 and later, which contain the necessary patches to prevent CRLF injection in the redirect handler. Organizations should also implement input validation measures that sanitize all URI parameters before they are processed by the web server, particularly in redirect scenarios. Network-level protections such as web application firewalls can provide additional defense-in-depth by detecting and blocking suspicious CRLF sequences in HTTP requests. Security teams should conduct thorough audits of their H2O configurations to ensure that redirect handlers are properly secured and that no custom code introduces similar vulnerabilities. Regular security assessments and vulnerability scanning should include checks for similar patterns in other web server components that might be susceptible to CRLF injection attacks. The fix implemented in the patched versions typically involves proper encoding or escaping of CRLF characters in URI processing, ensuring that these sequences cannot be interpreted as part of HTTP headers during response generation.