CVE-2019-0197 in HTTP Server
Summary
by MITRE
A vulnerability was found in Apache HTTP Server 2.4.34 to 2.4.38. When HTTP/2 was enabled for a http: host or H2Upgrade was enabled for h2 on a https: host, an Upgrade request from http/1.1 to http/2 that was not the first request on a connection could lead to a misconfiguration and crash. Server that never enabled the h2 protocol or that only enabled it for https: and did not set "H2Upgrade on" are unaffected by this issue.
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Analysis
by VulDB Data Team • 10/03/2023
This vulnerability in Apache HTTP Server affects versions between 2.4.34 and 2.4.38 and represents a critical configuration flaw that can lead to denial of service conditions. The issue specifically manifests when HTTP/2 protocol is enabled for HTTP hosts or when H2Upgrade is configured for HTTP/2 on HTTPS hosts. The vulnerability stems from improper handling of Upgrade requests that attempt to transition from HTTP/1.1 to HTTP/2 protocols, particularly when these upgrade requests occur after the initial connection has been established. This misconfiguration creates a scenario where the server's internal state becomes inconsistent, leading to potential crashes and service disruption.
The technical implementation of this vulnerability involves the server's protocol negotiation mechanism failing to properly validate upgrade requests that are not the first transaction on a connection. When an HTTP/1.1 client sends an Upgrade request to HTTP/2 after establishing a connection, the server's handling of this transition becomes problematic. This issue is particularly dangerous because it can be exploited through simple network requests without requiring authentication or special privileges. The vulnerability falls under CWE-20: Improper Input Validation, as the server fails to properly validate the sequence and context of protocol upgrade requests. It also relates to CWE-122: Heap Overflow and CWE-119: Improper Restriction of Operations within a Limited Memory Buffer, as the misconfiguration can lead to memory corruption during protocol transitions.
From an operational impact perspective, this vulnerability creates significant risk for organizations relying on Apache HTTP Server for web services, as it can result in complete service outages. The crash condition affects the server's ability to maintain stable connections and process subsequent requests, potentially causing cascading failures in web applications that depend on the server. Attackers can exploit this vulnerability by sending carefully crafted upgrade requests that trigger the server's inconsistent state handling. This vulnerability directly aligns with ATT&CK technique T1499.004: Endpoint Denial of Service, as it enables an attacker to cause a denial of service condition through protocol manipulation. The vulnerability also maps to ATT&CK technique T1595.001: Network Denial of Service, since the service disruption can affect network availability for legitimate users.
Organizations should immediately implement mitigations by either upgrading to Apache HTTP Server version 2.4.39 or later, which contains the necessary patches for this vulnerability, or by carefully reviewing their server configurations to ensure that HTTP/2 is not enabled for HTTP hosts or that H2Upgrade is not enabled for HTTP/2 on HTTPS hosts without proper safeguards. The recommended configuration approach involves setting H2Upgrade off for HTTP hosts and ensuring that HTTP/2 protocol is only enabled for HTTPS hosts where the protocol transition is properly managed. Additionally, organizations should implement monitoring solutions that can detect unusual upgrade request patterns and alert administrators to potential exploitation attempts. Security teams should also consider implementing rate limiting for upgrade requests and network segmentation to limit the potential impact of exploitation. The vulnerability demonstrates the importance of proper protocol handling in web servers and highlights the need for comprehensive testing of edge cases in protocol transitions, particularly in mixed HTTP/HTTPS environments where protocol upgrades may occur dynamically.