CVE-2024-4027: 
Java vulnerability analysis and mitigation

CVE-2024-4027 is a denial-of-service vulnerability in the Undertow Java web server framework, specifically in the HttpServletRequestImpl.getParameterNames() method. When a servlet calls this method, an unauthenticated remote attacker can send HTTP requests with excessively large parameter names, triggering an OutOfMemoryError and crashing the application. Affected versions include io.undertow:undertow-core < 2.2.39.Final, >= 2.3.0.Alpha1 and < 2.3.21.Final, and = 2.4.0.Alpha1. The vulnerability was published on January 30, 2026, and carries a CVSS v3.1 base score of 7.5 (High) (Red Hat Advisory, Github Advisory).

The root cause is improper input validation (CWE-20) — specifically, the failure to impose limits on the size of HTTP request parameter names before processing them (also characterized as CWE-770: Allocation of Resources Without Limits or Throttling). When HttpServletRequestImpl.getParameterNames() is invoked by a servlet, it processes all parameter names from the incoming request without enforcing a size cap, allowing a crafted request with abnormally large parameter names to exhaust JVM heap memory. The attack requires no authentication, no user interaction, and is exploitable over the network with low complexity. Relevant upstream commits and issues are tracked at UNDERTOW-2377 and referenced in the GitHub advisory (Github Advisory, Red Hat Bugzilla).

Successful exploitation causes an OutOfMemoryError in the JVM hosting the Undertow-based application, resulting in a complete loss of availability for the affected service. Because the attack targets memory exhaustion, it can crash the entire application server process, denying service to all legitimate users. There is no impact on confidentiality or data integrity, and no known potential for lateral movement or data exfiltration from this vulnerability alone (Github Advisory, Red Hat Advisory).

1. Reconnaissance: Identify internet-facing applications running on Undertow (e.g., JBoss EAP, WildFly, Quarkus-based services) using tools like Shodan or Censys, targeting versions prior to 2.2.39.Final or 2.3.21.Final.
2. Identify servlet endpoints: Enumerate HTTP endpoints that invoke servlets likely to call HttpServletRequestImpl.getParameterNames(), such as form-processing or parameter-parsing endpoints.
3. Craft malicious request: Construct an HTTP POST or GET request containing one or more parameter names of extremely large size (e.g., megabytes in length) to overwhelm the server's memory allocation.
4. Send the request: Transmit the crafted request to the target endpoint. No authentication or special headers are required.
5. Trigger OutOfMemoryError: The server's JVM attempts to allocate memory for the oversized parameter names during getParameterNames() processing, exhausting heap memory and causing an OutOfMemoryError, crashing the application or rendering it unresponsive (Github Advisory).

• Network: Unusual HTTP requests with abnormally large parameter names (multi-kilobyte or megabyte-sized query string keys) targeting servlet endpoints; high-volume or repeated requests from a single source IP to form-processing endpoints.
• Logs: Java application logs showing java.lang.OutOfMemoryError: Java heap space or GC overhead limit exceeded errors correlated with incoming HTTP requests; Undertow access logs showing requests with unusually large Content-Length or query string sizes.
• Process: JVM process for the Undertow-based application crashing or restarting unexpectedly; elevated garbage collection activity or heap usage spikes visible in JVM monitoring tools (e.g., JConsole, Prometheus JMX exporter).
• Application: Sudden service unavailability or HTTP 503 responses following a burst of requests to parameter-handling endpoints.

Upgrade io.undertow:undertow-core to one of the patched versions: 2.2.39.Final, 2.3.21.Final, or 2.4.0.Beta1 (Github Advisory). IBM has also released a fix for affected IBM DevOps Test UI / Rational Functional Tester products (IBM Advisory). As interim mitigations, deploy a Web Application Firewall (WAF) or reverse proxy configured to reject requests with parameter names exceeding a defined size threshold, and implement network-level rate limiting. Monitor JVM heap usage and configure alerting for OutOfMemoryError events to detect exploitation attempts early.

The vulnerability received standard coverage from vulnerability aggregators and security blogs upon its January 2026 disclosure, with no notable controversy or significant researcher commentary beyond routine advisories. Red Hat, as the CNA, published the initial advisory and bug report (Red Hat Advisory, Red Hat Bugzilla). IBM subsequently issued its own advisory for affected downstream products (IBM Advisory).