CVE-2016-7152 in Web Browser
Summary
by MITRE
The HTTPS protocol does not consider the role of the TCP congestion window in providing information about content length, which makes it easier for remote attackers to obtain cleartext data by leveraging a web-browser configuration in which third-party cookies are sent, aka a "HEIST" attack.
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Analysis
by VulDB Data Team • 09/08/2025
The CVE-2016-7152 vulnerability represents a sophisticated side-channel attack exploiting the fundamental design assumptions of the HTTPS protocol and its interaction with TCP congestion control mechanisms. This vulnerability specifically targets the way web browsers handle third-party cookies and how TCP congestion windows can inadvertently leak information about content length during secure HTTP communications. The attack leverages the fact that when browsers send third-party cookies, they create predictable patterns in network traffic that can be analyzed by remote attackers to infer sensitive information about the content being transmitted.
The technical flaw underlying this vulnerability stems from the HTTPS protocol's failure to account for TCP congestion window behavior as an information source during secure communications. When a browser sends third-party cookies, it creates specific network patterns that reveal information about the size and nature of the content being transmitted. The TCP congestion window, which is designed to optimize network performance by adjusting data transmission rates based on network conditions, inadvertently provides attackers with statistical information about the content length. This occurs because the size of the congestion window can correlate with the amount of data being transmitted, creating a side-channel that attackers can exploit to reconstruct sensitive information.
The operational impact of CVE-2016-7152 is significant as it allows remote attackers to perform content inference attacks that can reveal sensitive data without directly breaking the encryption. The vulnerability is particularly dangerous in scenarios where browsers are configured to send third-party cookies, which is common in modern web browsing environments. Attackers can use this technique to determine the size of content being transferred, potentially inferring the nature of the data, including personal information, financial details, or confidential communications. The attack is classified as a HEIST (HTTP Eavesdropping using Information from TCP Congestion Windows) attack, which demonstrates how seemingly benign protocol behaviors can create security vulnerabilities.
The vulnerability aligns with CWE-200 (Information Exposure) and represents a classic example of how network-level information leakage can compromise application-level security. From an ATT&CK framework perspective, this vulnerability maps to T1041 (Exfiltration Over C2 Channel) and T1566 (Phishing) as attackers can use the information gained to craft more targeted phishing attacks or to understand the nature of their targets' communications. The attack demonstrates the importance of considering all aspects of network communication as potential attack vectors rather than focusing solely on traditional cryptographic vulnerabilities.
Mitigation strategies for CVE-2016-7152 require a multi-layered approach addressing both browser configuration and network-level protections. Organizations should disable third-party cookie functionality where possible, implement proper cookie policies, and ensure that browsers are configured to minimize information leakage through network traffic patterns. Network administrators should consider implementing traffic analysis tools to detect anomalous patterns that might indicate exploitation attempts. Additionally, browser vendors have addressed this vulnerability through protocol improvements and better handling of cookie transmission behaviors, emphasizing the need for continuous security updates and the importance of understanding how different protocol components interact to create unexpected security risks. The vulnerability serves as a reminder that security considerations must extend beyond encryption strength to include all aspects of network communication and information flow analysis.