A new cybersecurity threat has emerged, dubbed the “cookie sandwich” attack, which exploits vulnerabilities in web server cookie parsing to potentially expose sensitive data and hijack user sessions. This technique bypasses the HttpOnly flag, a security measure designed to prevent client-side scripts from accessing sensitive cookies, raising concerns among cybersecurity professionals. The attack hinges on exploiting legacy cookie parsing mechanisms and manipulating the structure of the
Cookie header to trick servers into revealing protected information.
The “cookie sandwich” attack leverages the fact that some browsers and servers still support older cookie specifications, allowing attackers to craft cookie values with special characters and quotes. By strategically placing these manipulated cookies within the Cookie header, attackers can create overlapping cookie boundaries. This causes vulnerable servers to misinterpret the cookie structure, treating the enclosed cookies as part of a single value. When the server reflects these manipulated cookies in its response, sensitive information like session IDs can be exposed to attackers.
Real-world examples demonstrate the potential impact of this vulnerability.
In one instance, a tracking domain with a reflected session parameter was successfully exploited using the “cookie sandwich” technique. By leveraging legacy cookie parsing in Apache Tomcat, the attacker was able to expose an HttpOnly protected PHPSESSID cookie. This vulnerability highlights the importance of robust cookie handling, response sanitization, and adherence to modern security practices.
To mitigate the risk of “cookie sandwich” attacks, organizations should prioritize several key measures. These include using modern parsing standards like RFC6265, disabling support for legacy cookie parsing, enabling HttpOnly and Secure flags on sensitive cookies, sanitizing user input and server responses, implementing Content Security Policies (CSP), and using the SameSite attribute to restrict cross-site cookie transmission. By adopting these strategies, organizations can strengthen their defenses and protect against this evolving threat.
