CVE-2017-7789 in Firefox
Summary
by MITRE
If a server sends two Strict-Transport-Security (STS) headers for a single connection, they will be rejected as invalid and HTTP Strict Transport Security (HSTS) will not be enabled for the connection. This vulnerability affects Firefox < 55.
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Analysis
by VulDB Data Team • 11/06/2019
The vulnerability described in CVE-2017-7789 represents a critical flaw in Firefox's implementation of HTTP Strict Transport Security (HSTS) protocol handling. This issue specifically manifests when a web server sends duplicate Strict-Transport-Security headers within a single HTTP response, causing Firefox to reject the entire HSTS policy and fail to enforce secure communication for that connection. The vulnerability exists at the protocol parsing layer where Firefox incorrectly processes multiple STS headers, leading to a complete failure of the security mechanism designed to prevent downgrade attacks and enforce encrypted connections.
The technical root cause of this vulnerability stems from Firefox's inadequate header validation logic within its HSTS implementation. According to the Common Weakness Enumeration framework, this flaw can be categorized as a weakness in input validation where the application fails to properly handle malformed or unexpected header structures. When multiple identical STS headers are present, the browser's parser incorrectly treats this as a malformed response rather than a legitimate policy that should be processed. This behavior violates the HTTP specification which allows for multiple headers with the same name, though the specific handling of duplicate headers in the HSTS context creates a security gap. The vulnerability directly impacts the security posture by preventing the enforcement of HSTS policies, leaving users susceptible to man-in-the-middle attacks and protocol downgrade scenarios.
From an operational perspective, this vulnerability creates a significant risk for users connecting to websites that may inadvertently send duplicate headers or where attackers could exploit this behavior to bypass security measures. The impact extends beyond simple connection failures as it undermines the fundamental security promise of HSTS, which is designed to prevent attackers from forcing users into insecure HTTP connections even if they initially attempt to access the site via HTTPS. This vulnerability affects Firefox versions prior to 55, meaning users on older versions would be exposed to potential attacks where an attacker could manipulate the HTTP response to include duplicate headers, causing Firefox to ignore the HSTS policy entirely. The attack surface is particularly concerning in environments where web servers might have misconfigurations or where attackers could leverage this behavior to bypass security controls.
The mitigation strategy for this vulnerability requires immediate upgrading of Firefox to version 55 or later, where the parsing logic has been corrected to properly handle duplicate STS headers. Security administrators should also implement server-side remediation measures to ensure that web servers do not send duplicate Strict-Transport-Security headers in their responses, as this prevents the vulnerability from being exploited. Additionally, organizations should conduct security assessments to identify any web applications that might be inadvertently generating duplicate headers, which could expose users to this vulnerability. The remediation aligns with the ATT&CK framework's mitigation strategies for protocol manipulation attacks, where proper header validation and response handling are critical controls. Organizations should also consider implementing network monitoring to detect unusual header patterns that might indicate attempts to exploit this vulnerability, as the behavior of rejecting duplicate headers could be used as a signal of potential attack activity.