CVE-2015-0799 in Firefox
Summary
by MITRE
The HTTP Alternative Services feature in Mozilla Firefox before 37.0.1 allows man-in-the-middle attackers to bypass an intended X.509 certificate-verification step for an SSL server by specifying that server in the uri-host field of an Alt-Svc HTTP/2 response header.
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Analysis
by VulDB Data Team • 05/02/2022
The vulnerability described in CVE-2015-0799 represents a critical security flaw in Mozilla Firefox's implementation of HTTP Alternative Services functionality. This feature was designed to enable browsers to discover alternative endpoints for serving content, potentially improving performance by allowing connections to be redirected to different servers or protocols. The vulnerability specifically affects Firefox versions prior to 37.0.1 and stems from an insufficient validation mechanism that fails to properly verify SSL certificates when alternative service endpoints are specified through the Alt-Svc HTTP/2 response header.
The technical implementation flaw occurs within the HTTP Alternative Services parsing logic where the browser accepts alternative service specifications without adequately validating that the certificate presented by the alternative endpoint matches the expected server identity. When an attacker can inject malicious Alt-Svc headers into HTTP responses, they can specify arbitrary servers in the uri-host field that may present certificates for different domains or entities. This bypasses the intended certificate verification process that should occur when establishing secure connections to alternative services, effectively allowing attackers to perform man-in-the-middle attacks against users who are redirected to these alternative endpoints.
The operational impact of this vulnerability is significant as it undermines the fundamental security model of SSL/TLS connections. An attacker positioned in a network path between a user and a web server can manipulate HTTP responses to include malicious Alt-Svc headers that redirect traffic to attacker-controlled servers. This creates a scenario where users may unknowingly connect to compromised servers while believing they are communicating with legitimate services. The vulnerability particularly affects scenarios where users are browsing sites that implement HTTP/2 with Alternative Services, as the attack vector specifically targets the Alt-Svc HTTP/2 response header mechanism.
This vulnerability aligns with CWE-295 which addresses improper certificate validation and relates to ATT&CK technique T1071.004 for application layer protocol manipulation. The flaw demonstrates a classic case of insufficient input validation where the browser fails to properly validate the alternative service specifications against the original connection parameters. Organizations should prioritize immediate patching of affected Firefox installations to address this vulnerability, while also implementing network monitoring to detect anomalous Alt-Svc header patterns. Additionally, security teams should consider implementing network-level controls to restrict or monitor HTTP Alternative Services traffic, as this vulnerability represents a sophisticated attack vector that can bypass traditional certificate validation mechanisms. The remediation process involves updating to Firefox version 37.0.1 or later, where the certificate validation logic has been enhanced to properly verify server identities in alternative service scenarios.