When dealing with session cookies in a distributed web application, protecting them from unintended exposure is paramount. Two critical vulnerabilities to address are:
Client-side script access: If malicious scripts (e.g., from XSS attacks) can read session cookies, attackers can hijack active user sessions. The HttpOnly flag prevents JavaScript and other client-side scripts from accessing the cookie, greatly reducing the risk of session theft through script injection.
Unencrypted transmission: Sending session cookies over plain HTTP exposes them to potential interception by network attackers. To mitigate this risk, the Secure flag should be used, ensuring that the cookie is only transmitted over encrypted HTTPS connections. This keeps the cookie confidential and prevents eavesdropping or man-in-the-middle attacks.
Serving all traffic over HTTPS is crucial because the Secure flag is ineffective if the initial connection is made over HTTP. The browser won't send the secure cookie until a secure connection has been established.
SameSite=Lax is a cookie attribute that controls how cookies are sent in cross-site requests, with the goal of mitigating Cross-Site Request Forgery (CSRF) attacks. Setting a cookie’s SameSite attribute to Lax allows the cookie to be sent when:
> The user navigates within the same site (e.g., links from one page to another within the same domain).
> The user performs a top-level navigation from an external site via a regular link click, as long as it’s a simple GET request.
A) Restricting the cookie domain and using HttpOnly only:
While applying HttpOnly prevents the cookie from being accessed by client-side scripts, this option doesn’t ensure the cookie is only transmitted over encrypted connections. A malicious actor could potentially intercept the session cookie over an unencrypted HTTP connection. Restricting the domain is a good practice to reduce scope, but it doesn’t protect against network-level eavesdropping.
C) Using SameSite=Lax and a limited Path without additional flags:
SameSite and Path attributes help control when and where a cookie is sent, reducing the risk of CSRF and limiting exposure within your domain. However, they do nothing to prevent client-side script access (no HttpOnly flag) or stop transmission over plain HTTP (no Secure flag). Attackers can still intercept or manipulate the cookie if traffic isn’t always encrypted, and JavaScript can still read it.
D) Relying solely on Secure and hardened TLS:
Just setting Secure ensures cookies won’t be sent over plain HTTP, which is good. However, it doesn’t prevent client-side scripts from reading the cookie if an attacker injects malicious JavaScript (no HttpOnly). Without HttpOnly, even a well-encrypted connection can’t stop a malicious script from stealing the session cookie once it’s on the client’s machine.
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It might seem counterintuitive from a security perspective, and modern best practices do strongly encourage always using HttpOnly and Secure flags for sensitive cookies. However, there are historical and practical reasons why developers might set cookies without these flags like legacy apps, development enviroment or non-sensitive data like UI / lang preferences.