CVE-2023-4639

What it actually does

The flaw allows incorrect parsing of cookies containing value-delimiting characters in incoming requests, enabling construction of cookie values that exfiltrate HttpOnly cookie values or spoof arbitrary additional cookie values.

Why it matters to the business

Attackers may obtain unauthorized access to session data or modify application state, potentially exposing user information or enabling session manipulation in web applications.

How an attack unfolds

An attacker crafts an HTTP request containing a specially formed cookie value that uses value-delimiting characters. The vulnerable Undertow parser mishandles the value, allowing the attacker to read HttpOnly cookies or inject additional cookies that the application trusts.

How to fix it

Upgrade Undertow to version 2.3.18-1 or later on all affected Debian 13 systems.

How to confirm the fix worked

Verify the installed Undertow package version is 2.3.18-1 or newer; test cookie parsing behavior with known delimiting characters to confirm correct handling.

Typical attack path

1. Identify a target endpoint using Undertow
2. Craft a cookie value containing value-delimiting characters
3. Send the malicious request to the server
4. Observe exfiltrated HttpOnly cookie data or successfully spoofed cookie values

How to detect it

Inspect web server access logs for cookie values containing unusual delimiter characters; look for patterns indicating cookie exfiltration or unexpected additional cookies in requests.

What your security team should do

• Apply the 2.3.18-1 upgrade across all instances
• Review application cookie usage for HttpOnly and SameSite attributes
• Add logging for anomalous cookie structures in incoming requests