CVE-2016-7835 in H2O
Summary
by MITRE
Use-after-free vulnerability in H2O allows remote attackers to cause a denial-of-service (DoS) or obtain server certificate private keys and possibly other information.
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Analysis
by VulDB Data Team • 12/08/2022
The CVE-2016-7835 vulnerability represents a critical use-after-free flaw in the H2O web server software that exposes systems to significant security risks. This vulnerability specifically affects the HTTP/2 implementation within H2O, creating a scenario where memory that has been freed is still accessed by the application, leading to unpredictable behavior and potential exploitation. The flaw arises from improper memory management during the handling of HTTP/2 connections, particularly when processing certain frame types that trigger the release of memory resources without adequate validation of subsequent access patterns. The vulnerability impacts both the stability and security of affected systems, as it can be leveraged to cause system instability through denial-of-service conditions or to extract sensitive cryptographic material.
The technical nature of this vulnerability aligns with CWE-416, which describes the use of freed memory condition where a program continues to reference memory that has already been deallocated. In the context of H2O, this occurs during the processing of HTTP/2 frames where the application allocates memory for connection state tracking and subsequently frees it without proper safeguards against subsequent access attempts. Attackers can exploit this by crafting malicious HTTP/2 requests that trigger the specific memory management path leading to the use-after-free condition. The exploitation mechanism typically involves sending carefully constructed requests that cause the server to free memory associated with connection handling while simultaneously attempting to access that same memory region, creating a race condition that can lead to memory corruption.
The operational impact of CVE-2016-7835 extends beyond simple denial-of-service scenarios to include potential information disclosure vulnerabilities. Remote attackers who successfully exploit this flaw can potentially extract server certificate private keys, which compromises the confidentiality and integrity of encrypted communications. This information disclosure aspect makes the vulnerability particularly dangerous as it can lead to man-in-the-middle attacks, impersonation of legitimate services, and broader compromise of the TLS infrastructure. The vulnerability also enables attackers to gain access to other sensitive information that may be stored in the freed memory regions, potentially including session data, user credentials, or application-specific secrets. Organizations running affected versions of H2O face significant risk of service disruption and data compromise, particularly in environments where the web server handles sensitive information or serves as a critical infrastructure component.
Mitigation strategies for CVE-2016-7835 primarily focus on immediate patching and system hardening measures. Organizations should prioritize upgrading to H2O versions that contain the fix for this vulnerability, as the official release includes proper memory management safeguards that prevent the use-after-free condition. Additionally, implementing network-level protections such as rate limiting and connection monitoring can help detect and prevent exploitation attempts. Security teams should also consider deploying intrusion detection systems that can identify suspicious HTTP/2 traffic patterns associated with this vulnerability. The mitigation approach should align with ATT&CK framework tactics such as TA0005 (Defense Evasion) and TA0006 (Credential Access) by ensuring that defensive measures prevent both the exploitation of memory corruption vulnerabilities and the subsequent extraction of sensitive information. Organizations should conduct thorough vulnerability assessments to identify all systems running affected H2O versions and implement comprehensive monitoring to detect potential exploitation attempts. The fix typically involves implementing proper reference counting mechanisms and ensuring that memory deallocation occurs only when no further references exist, preventing the conditions that lead to use-after-free scenarios.