CVE-2024-4027: Undertow DoS Vulnerability via Parameters

CVE-2024-4027 Overview

A vulnerability has been identified in Undertow, a lightweight Java web server. The flaw exists in servlets that utilize methods calling HttpServletRequestImpl.getParameterNames(). When a client sends a request with excessively large parameter names, the server can experience an OutOfMemoryError, leading to service disruption. This vulnerability can be exploited remotely by unauthorized users to conduct denial-of-service (DoS) attacks against affected Undertow deployments.

Critical Impact

Remote unauthenticated attackers can exhaust server memory by sending crafted HTTP requests with oversized parameter names, causing service outages and denial of service conditions.

Affected Products

• Undertow (affected versions not specified in advisory)
• Java web applications using Undertow as the underlying web server
• Applications utilizing HttpServletRequestImpl.getParameterNames() method

Discovery Timeline

• 2026-01-30 - CVE-2024-4027 published to NVD
• 2026-02-04 - Last updated in NVD database

Technical Details for CVE-2024-4027

Vulnerability Analysis

This vulnerability is classified as an Improper Input Validation issue (CWE-20). The core problem lies in how Undertow handles HTTP request parameters. When a servlet processes incoming requests and invokes the getParameterNames() method on the HttpServletRequestImpl object, Undertow parses and stores parameter names in memory. The vulnerability arises because Undertow does not implement adequate size restrictions on parameter names, allowing malicious actors to craft requests containing parameter names of arbitrary length.

When such malformed requests are processed, the parameter name data accumulates in the JVM heap memory. With sufficiently large or numerous oversized parameter names, the Java Virtual Machine's available heap space becomes exhausted, triggering an OutOfMemoryError. This exception typically causes the servlet thread to terminate abnormally and can destabilize the entire application server, potentially affecting all hosted applications.

Root Cause

The root cause of CVE-2024-4027 is the lack of proper bounds checking and memory management when parsing HTTP request parameter names in Undertow's HttpServletRequestImpl class. The implementation does not enforce maximum length limits on individual parameter names or impose aggregate limits on the total memory consumed by all parameter names in a single request. This design oversight allows attackers to control memory allocation on the server by simply manipulating the size of request parameters.

Attack Vector

The attack can be executed over the network without authentication or user interaction. An attacker crafts an HTTP request containing one or more parameter names of extreme length and sends it to a vulnerable Undertow server. When the servlet attempts to enumerate or access parameter names using getParameterNames(), the server allocates memory to store these oversized strings. By sending multiple requests or using sufficiently large parameter names, the attacker can rapidly deplete available heap memory, resulting in an OutOfMemoryError that crashes the application or makes it unresponsive.

The attack is particularly effective because:

• No authentication is required to send malicious requests
• Standard HTTP requests are used, making the attack difficult to distinguish from legitimate traffic initially
• The memory exhaustion can affect the entire JVM, impacting all applications running on the same server

Detection Methods for CVE-2024-4027

Indicators of Compromise

• Unexpected OutOfMemoryError exceptions in Undertow or application server logs
• Abnormal JVM heap usage spikes coinciding with incoming HTTP requests
• HTTP requests with unusually long query string parameters or POST body parameters
• Repeated application crashes or restarts without apparent cause

Detection Strategies

• Monitor JVM heap memory utilization and alert on rapid consumption patterns
• Implement application-level logging to capture request parameter sizes before processing
• Deploy web application firewalls (WAF) configured to block requests with oversized parameters
• Review Undertow access logs for requests with abnormally long URLs or POST bodies

Monitoring Recommendations

• Configure JVM memory monitoring with thresholds for heap utilization alerts
• Implement request size logging at the reverse proxy or load balancer level
• Set up automated alerts for OutOfMemoryError occurrences in application logs
• Monitor application availability and response times for degradation patterns

How to Mitigate CVE-2024-4027

Immediate Actions Required

• Review and apply available patches from Red Hat or the Undertow project
• Implement request size limits at the web application firewall or reverse proxy layer
• Configure maximum parameter name length restrictions where supported
• Consider implementing rate limiting to reduce the impact of potential DoS attempts

Patch Information

Refer to the Red Hat CVE-2024-4027 Advisory for official patch information and affected product versions. Additional technical details are available in the Red Hat Bugzilla Entry #2276410.

Organizations using Undertow should monitor vendor channels for security updates and apply patches as they become available.

Workarounds

• Deploy a reverse proxy (such as nginx or Apache) in front of Undertow to enforce maximum URL and parameter length limits
• Configure web application firewall rules to reject requests with parameter names exceeding reasonable thresholds
• Increase JVM heap size as a temporary measure to raise the attack complexity (note: this does not fix the vulnerability)
• Implement application-level validation to reject requests with oversized parameters before invoking getParameterNames()

# Example nginx configuration to limit request URI and body size
# Add to server or location block

# Limit client request body size
client_max_body_size 1m;

# Limit URI length
large_client_header_buffers 4 8k;

# Limit query string length
if ($query_string ~* ".{8192,}") {
    return 414;
}