CVE-2026-3260: Undertow DoS Vulnerability

CVE-2026-3260 Overview

A resource exhaustion vulnerability has been discovered in Undertow, a flexible, performant web server written in Java. A remote attacker could exploit this vulnerability by sending an HTTP GET request containing multipart/form-data content. If the underlying application processes parameters using methods like getParameterMap(), the server prematurely parses and stores this content to disk. This could lead to resource exhaustion, potentially resulting in a Denial of Service (DoS).

Critical Impact

Remote attackers can cause Denial of Service by exhausting server disk resources through crafted multipart/form-data requests, potentially rendering web applications unavailable.

Affected Products

• Undertow (affected versions not specified in advisory)
• Applications using Undertow that process request parameters via getParameterMap() or similar methods
• Red Hat products utilizing Undertow as the underlying web server

Discovery Timeline

• 2026-03-24 - CVE CVE-2026-3260 published to NVD
• 2026-03-24 - Last updated in NVD database

Technical Details for CVE-2026-3260

Vulnerability Analysis

This vulnerability falls under CWE-770 (Allocation of Resources Without Limits or Throttling). The flaw exists in how Undertow handles HTTP GET requests that contain multipart/form-data content in an unexpected context.

Under normal circumstances, multipart/form-data is typically used with POST requests for file uploads. However, when an attacker sends this content type with a GET request and the application invokes parameter parsing methods such as getParameterMap(), Undertow's request processing logic prematurely parses the multipart content and writes temporary files to disk.

The vulnerability is exploitable over the network without requiring authentication or user interaction. However, exploitation requires specific conditions to be met in the target application's request handling logic, which introduces complexity in successful attacks.

Root Cause

The root cause lies in Undertow's insufficient validation of content types relative to HTTP methods. When multipart/form-data content is received on a GET request, the server fails to properly validate whether parsing is appropriate before invoking the multipart parser. Combined with a lack of resource limits on temporary file storage, this allows attackers to exhaust disk space by sending numerous malicious requests.

Attack Vector

The attack is conducted over the network and targets applications running on Undertow that invoke parameter parsing methods regardless of the HTTP method used. An attacker can craft HTTP GET requests with multipart/form-data bodies containing large amounts of data. When the target application calls methods like getParameterMap(), the server parses the multipart content and stores temporary files to disk.

By sending a high volume of these requests, an attacker can fill the server's disk space, leading to resource exhaustion and denial of service. The attack does not require authentication and can be launched remotely against any exposed endpoint that processes request parameters.

The vulnerability manifests in Undertow's multipart request handling when certain parameter parsing methods are invoked. Detailed technical information is available in the Red Hat CVE-2026-3260 Advisory and the associated Red Hat Bugzilla Report #2443010.

Detection Methods for CVE-2026-3260

Indicators of Compromise

• Unusual volume of HTTP GET requests with Content-Type: multipart/form-data headers
• Rapid disk space consumption on application servers running Undertow
• Accumulation of temporary files in Undertow's temp directory
• Application performance degradation or service unavailability without corresponding legitimate traffic increase

Detection Strategies

• Monitor HTTP request logs for GET requests containing multipart/form-data content type, which is an unusual combination
• Implement Web Application Firewall (WAF) rules to flag or block GET requests with multipart/form-data content
• Configure disk space monitoring alerts on servers running Undertow-based applications
• Review application logs for excessive parameter parsing operations

Monitoring Recommendations

• Set up disk utilization thresholds with automated alerting when temporary storage exceeds normal baselines
• Monitor request patterns for anomalous combinations of HTTP methods and content types
• Implement rate limiting on endpoints that process request parameters
• Track temporary file creation rates in Undertow's working directories

How to Mitigate CVE-2026-3260

Immediate Actions Required

• Apply vendor patches as they become available from Red Hat or the Undertow project
• Implement WAF rules to reject HTTP GET requests containing multipart/form-data content
• Configure disk quotas for the user account running the application server
• Review application code to ensure parameter parsing methods are only called when necessary

Patch Information

Refer to the Red Hat CVE-2026-3260 Advisory for official patch information and updates. Additional details can be found in Red Hat Bugzilla Report #2443010. Organizations should monitor these resources for security updates addressing this vulnerability.

Workarounds

• Deploy a reverse proxy or WAF to filter incoming requests and block GET requests with multipart/form-data content type
• Implement application-level validation to check HTTP method before invoking parameter parsing methods
• Configure temporary file storage limits on the Undertow server
• Consider moving temporary file storage to a separate partition to prevent system-wide disk exhaustion

# Example WAF rule concept for Apache ModSecurity
# Block GET requests with multipart/form-data content type
SecRule REQUEST_METHOD "GET" "chain,id:1001,deny,status:400,msg:'Suspicious GET with multipart'"
SecRule REQUEST_HEADERS:Content-Type "multipart/form-data"