CVE-2023-4639: Undertow Cookie Parsing Vulnerability

CVE-2023-4639 Overview

A significant vulnerability has been identified in Undertow, the popular Java-based web server used extensively in Red Hat JBoss Enterprise Application Server and other enterprise platforms. This flaw stems from Undertow's incorrect parsing of cookies containing certain value-delimiting characters in incoming HTTP requests. Attackers can exploit this weakness to construct specially crafted cookie values that exfiltrate HttpOnly cookie values or spoof arbitrary additional cookie values, leading to unauthorized data access or modification.

Critical Impact

This vulnerability poses a significant threat to data confidentiality and integrity, allowing attackers to bypass HttpOnly cookie protections and potentially steal session tokens or other sensitive authentication data.

Affected Products

• Undertow (multiple versions)
• Red Hat JBoss Enterprise Application Platform
• Products utilizing Undertow as the underlying web server component

Discovery Timeline

• 2024-11-17 - CVE-2023-4639 published to NVD
• 2025-02-07 - Last updated in NVD database

Technical Details for CVE-2023-4639

Vulnerability Analysis

This vulnerability is classified as HTTP Request Smuggling (CWE-444), which occurs when Undertow's cookie parsing logic fails to properly handle certain delimiter characters within cookie values. The flaw allows an attacker to inject malicious content into cookie headers that the server interprets differently than intended.

The vulnerability exploits the inconsistency between how Undertow parses cookie boundaries versus how downstream applications interpret them. When an attacker sends a crafted HTTP request with specially constructed cookie values containing delimiter characters, Undertow may incorrectly split or merge cookie values, leading to a mismatch in cookie interpretation.

This attack can be executed remotely over the network without requiring authentication, though it does require specific conditions to be met in the target environment, making exploitation somewhat complex.

Root Cause

The root cause lies in Undertow's cookie parsing implementation, which does not properly validate and sanitize value-delimiting characters within cookie values. According to RFC 6265, certain characters should be handled with specific escaping or quoting mechanisms. Undertow's parser fails to correctly implement these standards, allowing specially crafted cookie values to be misinterpreted.

When processing incoming HTTP requests, the cookie parsing logic splits cookie name-value pairs based on delimiter characters. However, the parser does not properly account for scenarios where these delimiters appear within the cookie value itself, leading to incorrect parsing behavior.

Attack Vector

The attack is network-based and can be executed by any remote attacker who can send HTTP requests to an Undertow-based application. The attacker constructs a malicious HTTP request containing cookie headers with carefully placed delimiter characters.

The attack scenario typically involves:

1. The attacker identifies an application using Undertow as its web server
2. A crafted HTTP request is sent with cookie values containing special delimiter characters
3. Undertow incorrectly parses these cookies, potentially exposing HttpOnly cookie values
4. The attacker can exfiltrate sensitive session data or inject spoofed cookie values
5. This can lead to session hijacking or authentication bypass

The vulnerability specifically targets the HttpOnly flag protection, which is designed to prevent client-side JavaScript from accessing sensitive cookies. By exploiting the parsing flaw, attackers can circumvent this protection mechanism.

Detection Methods for CVE-2023-4639

Indicators of Compromise

• Unusual or malformed cookie headers in HTTP request logs containing unexpected delimiter characters
• Multiple cookies appearing where only one was expected in application logs
• Session anomalies where user sessions appear to be hijacked or spoofed
• HTTP requests with abnormally long or complex cookie values

Detection Strategies

• Implement Web Application Firewall (WAF) rules to detect malformed cookie headers with suspicious delimiter patterns
• Monitor HTTP request logs for cookie values containing unusual combinations of semicolons, commas, or other delimiter characters
• Enable detailed logging on Undertow instances to capture full cookie header content for analysis
• Deploy intrusion detection signatures that identify HTTP request smuggling patterns

Monitoring Recommendations

• Configure SentinelOne Singularity to monitor Java application processes for suspicious HTTP parsing behavior
• Set up alerts for anomalous session token activity that may indicate cookie exfiltration
• Implement log correlation rules to identify patterns consistent with cookie manipulation attacks
• Monitor for unusual authentication patterns that could indicate session hijacking

How to Mitigate CVE-2023-4639

Immediate Actions Required

• Apply the latest security patches from Red Hat for Undertow and JBoss EAP immediately
• Review and update Undertow to a patched version as specified in the vendor advisories
• Implement additional cookie validation at the application layer as a defense-in-depth measure
• Consider deploying a WAF with HTTP request smuggling protection rules

Patch Information

Red Hat has released multiple security advisories addressing this vulnerability. Organizations should review and apply the appropriate patches based on their deployed products:

• Red Hat Security Advisory RHSA-2024:1674
• Red Hat Security Advisory RHSA-2024:1675
• Red Hat Security Advisory RHSA-2024:1676
• Red Hat Security Advisory RHSA-2024:1677
• Red Hat Security Advisory RHSA-2024:2763
• Red Hat Security Advisory RHSA-2024:2764
• Red Hat Security Advisory RHSA-2024:3919

For additional technical details, consult the Red Hat CVE-2023-4639 Details page and Red Hat Bugzilla Report #2166022.

NetApp customers should refer to NetApp Security Advisory NTAP-20250207-0001 for guidance on affected products.

Workarounds

• Implement strict cookie validation at the reverse proxy or load balancer level to sanitize incoming cookie headers before they reach Undertow
• Deploy a Web Application Firewall configured to detect and block HTTP request smuggling attempts
• Add application-layer validation that rejects cookies containing unexpected delimiter characters
• Consider temporarily disabling cookie-based session management in favor of alternative authentication mechanisms until patches can be applied

# Example: Configure a reverse proxy to sanitize cookie headers
# Add to NGINX configuration to implement strict cookie validation
# This provides defense-in-depth while patching is in progress
proxy_cookie_flags ~ secure httponly samesite=strict;