CVE-2020-9490 in HTTP Server
Summary
by MITRE
Apache HTTP Server versions 2.4.20 to 2.4.43. A specially crafted value for the 'Cache-Digest' header in a HTTP/2 request would result in a crash when the server actually tries to HTTP/2 PUSH a resource afterwards. Configuring the HTTP/2 feature via "H2Push off" will mitigate this vulnerability for unpatched servers.
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Analysis
by VulDB Data Team • 11/01/2024
The vulnerability identified as CVE-2020-9490 represents a critical denial of service flaw within the Apache HTTP Server ecosystem affecting versions between 2.4.20 and 2.4.43. This vulnerability specifically targets the HTTP/2 implementation within the server, creating a scenario where a maliciously crafted Cache-Digest header can trigger a server crash during HTTP/2 PUSH operations. The flaw demonstrates the inherent complexity of implementing HTTP/2 features within web servers, where the interaction between various protocol components can create unexpected failure points that adversaries can exploit to disrupt service availability.
The technical root cause of this vulnerability stems from improper validation of the Cache-Digest header value within HTTP/2 request processing. When the server encounters a specially crafted Cache-Digest header, it attempts to process this value in preparation for HTTP/2 PUSH operations, leading to memory corruption or invalid memory access patterns that ultimately result in server termination. This behavior aligns with CWE-125, which describes out-of-bounds read vulnerabilities, and CWE-787, which covers out-of-bounds write conditions that can lead to application crashes. The vulnerability manifests during the server's attempt to push resources, indicating a failure in input sanitization and boundary checking within the HTTP/2 implementation module.
The operational impact of CVE-2020-9490 extends beyond simple service disruption as it can be exploited by remote attackers without authentication requirements, making it particularly dangerous in production environments. The vulnerability allows an attacker to cause a complete server crash through a single malformed HTTP/2 request, potentially leading to extended downtime and service unavailability for legitimate users. This makes it a prime candidate for distributed denial of service attacks where multiple concurrent requests could overwhelm server resources. The vulnerability's exploitation pathway through HTTP/2 PUSH operations indicates that it affects servers actively using HTTP/2 features, though the mitigation strategy of disabling HTTP/2 PUSH through H2Push off provides a viable workaround for unpatched systems.
Organizations affected by this vulnerability should prioritize immediate patching of their Apache HTTP Server installations to version 2.4.44 or later, which contains the necessary fixes for this memory handling issue. The recommended mitigation strategy of using H2Push off directive provides a temporary workaround while patches are deployed, though this approach reduces server functionality by disabling HTTP/2 PUSH capabilities that improve performance. Security teams should also implement network monitoring to detect anomalous HTTP/2 request patterns that might indicate exploitation attempts, as the vulnerability can be triggered through automated scanning tools. The ATT&CK framework categorizes this vulnerability under T1499.004 for network denial of service and T1595.001 for reconnaissance using network sniffers, highlighting the dual nature of this threat as both a direct attack vector and an indicator of broader reconnaissance activities targeting web server infrastructure.