Published: Exclusive Long-Read
CVSS Score: 4.3 (Medium)
Impacted software:
Firefox ESR < 115.1
Web developers and end-users depend on cookies every day—for logins, personalized settings, and session tracking. But what happens when the browser messes up managing those cookies? In the case of CVE-2023-4055, a quirky bug in Mozilla Firefox left web requests missing critical cookies, all because the browser lost track of the actual contents of its own "cookie jar".
In this exclusive long-read, I'll break down what went wrong, why it matters, show you exactly how the bug manifests, and provide example code to illustrate the issue. All explained with simple terms you can understand, even if cookies usually just mean snacks to you.
1. What is CVE-2023-4055?
Summary:
When too many cookies are set for a single domain in Firefox (versions before 116, or certain ESR versions), the browser's internal cookie management could lose synchronization. The next time a request was sent to the server, expected cookies could go missing—even though the web page JavaScript *thought* those cookies were still there.
In other words:
- Normally, when you run document.cookie = ..., the browser behind the scenes updates the cookie storage for the domain.
- There's a limit to how many cookies each domain can set (usually 20 cookies per domain).
If you try to set cookie #21, browsers must decide which old cookie to drop.
The bug? After hitting the per-domain cookie limit, Firefox would silently drop some cookies but not update its internal shadow copy. So the site's JavaScript *saw* more cookies than would actually be sent to the server.
Or, in rare cases, breaking app logic in ways that could be manipulated by an attacker.
While this bug wasn't directly exploitable for remote code execution or privilege escalation, it could cause subtle, annoying, and sometimes security-impacting bugs in sensitive workflows.
Set Cookies Beyond the Limit
The site (or an attacker) sets cookies over the allowed per-domain limit (say, tries to set 25 cookies).
4. Code Snipplet: Reproducing the Issue
Try this in a Firefox version below 116 (hint: use a VM or Docker container for older browsers!).
// Set 25 cookies in one domain. Most browsers only allow 20 per domain.
// After setting, try to read and compare the cookie 'jar'.
for (let i = 1; i <= 25; i++) {
document.cookie = cookie${i}=value${i}; path=/;
}
// List all cookies available to JS
console.log("document.cookie:", document.cookie);
// Now fetch a resource and inspect the network:
fetch('/echo-my-cookies').then(resp => resp.text()).then(console.log);
// Use your browser's network tab to check which cookies really got sent!
Expected:
The low-level cookie jar compiled and sent in HTTP headers.
Consequences:
An attacker abuses document.cookie to flood the site with extra cookies.
2. The browser juggles cookies, unpredictably dropping some (possibly including session/auth cookies).
Victim is now interacting without their authentication or CSRF cookie.
4. Application logic may get confused, deny access, or—if designed poorly—leak information under some race condition.
Proof-of-concept:
Using repeated document.cookie=... statements, an attacker can disrupt which cookies are actually sent, possibly causing denial-of-service or app instability for the user.
7. Mitigation and Fix
Fixed in:
- Firefox 116 release notes
Firefox ESR 102.14, 115.1
Browser now keeps the internal JS and HTTP cookie jars in sync, strictly capping the cookies and updating both sides.
If you're vulnerable:
Reduce usage of excessive cookies in applications
- Add monitoring/logging for missing auth/session cookies
8. Useful External References
- CVE-2023-4055 (NVD)
- Mozilla security advisory 2023-30
- MDN: Cookie Limits
Updating to the latest Firefox patches is essential to avoid mysterious cookie behaviors.
Stay safe out there—and watch your cookies! 🍪
Timeline
Published on: 08/01/2023 16:15:00 UTC
Last modified on: 08/09/2023 21:15:00 UTC