CVE-2024-43398: Ruby-lang Rexml DoS Vulnerability

CVE-2024-43398 Overview

CVE-2024-43398 is a Denial of Service (DoS) vulnerability in REXML, the XML toolkit for Ruby. The vulnerability exists in REXML gem versions prior to 3.3.6 and can be triggered when parsing XML documents containing many deeply nested elements that share the same local name attributes. This algorithmic complexity attack allows remote attackers to exhaust system resources by sending specially crafted XML documents to applications using the vulnerable tree parser API.

Critical Impact

Applications parsing untrusted XML content using REXML's tree parser API (such as REXML::Document.new) are vulnerable to resource exhaustion attacks that can render services unavailable.

Affected Products

• ruby-lang rexml (versions prior to 3.3.6)
• netapp bootstrap_os
• netapp hci_compute_node

Discovery Timeline

• 2024-08-22 - CVE-2024-43398 published to NVD
• 2025-11-03 - Last updated in NVD database

Technical Details for CVE-2024-43398

Vulnerability Analysis

This vulnerability is classified as CWE-776 (Improper Restriction of Recursive Entity References in DTDs), which relates to XML Entity Expansion attacks. The flaw manifests when REXML processes XML documents with deeply nested elements sharing identical local name attributes. The parser's handling of these structures leads to excessive computational overhead, causing algorithmic complexity issues that can exhaust CPU and memory resources.

The vulnerability specifically affects the tree parser API implementation in REXML, which builds an in-memory representation of the XML document. When processing maliciously crafted XML with recursive or deeply nested structures, the parser's resource consumption grows disproportionately to the input size, making it susceptible to denial of service attacks.

Importantly, this vulnerability does not affect all REXML parsing modes. The stream parser API and SAX2 parser API are not impacted because they process XML documents sequentially without building complete in-memory tree structures.

Root Cause

The root cause lies in the tree parser's algorithm for handling deeply nested XML elements with identical local name attributes. When parsing such structures, the implementation lacks proper bounds checking and resource limits, allowing attackers to craft XML payloads that cause exponential or polynomial resource consumption relative to input size.

Attack Vector

The attack vector is network-based, requiring the attacker to submit a maliciously crafted XML document to an application using the vulnerable REXML::Document.new API or similar tree parser methods. The attack does not require authentication or user interaction, though it has high attack complexity due to the specific conditions required to trigger the vulnerability effectively.

An attacker would craft an XML document containing numerous deeply nested elements with matching local name attributes. When this document is parsed by a vulnerable application, the server experiences significant resource consumption that can lead to service degradation or complete unavailability.

The vulnerability can be exploited by sending specially crafted XML with deeply nested elements sharing identical local name attributes to applications using REXML's tree parser API. For detailed technical information, see the GitHub Security Advisory.

Detection Methods for CVE-2024-43398

Indicators of Compromise

• Unusual CPU or memory spikes in Ruby application processes when processing XML input
• Application timeouts or hangs during XML parsing operations
• Large XML payloads in request logs with deeply nested element structures
• Service availability issues correlated with XML processing activity

Detection Strategies

• Monitor Ruby application resource utilization for anomalous spikes during XML processing
• Implement request logging to capture XML payload sizes and processing times
• Deploy web application firewalls (WAF) with rules to detect oversized or deeply nested XML structures
• Use application performance monitoring (APM) to track REXML::Document.new call durations

Monitoring Recommendations

• Set up alerts for Ruby process memory and CPU usage thresholds
• Monitor application response times for endpoints that accept XML input
• Track error rates related to XML parsing timeouts or failures
• Implement logging for rejected or rate-limited XML requests

How to Mitigate CVE-2024-43398

Immediate Actions Required

• Upgrade REXML gem to version 3.3.6 or later immediately
• Audit applications to identify usage of REXML tree parser API with untrusted input
• Implement input validation to restrict XML document depth and size
• Consider switching to stream parser or SAX2 parser APIs for processing untrusted XML

Patch Information

The REXML development team has released version 3.3.6 which includes the security patch addressing this vulnerability. Organizations should upgrade to this version or later to remediate the issue. The fix is available through standard Ruby gem distribution channels.

For additional guidance, refer to the GitHub Release v3.3.6, Debian LTS Announcement, and NetApp Security Advisory.

Workarounds

• Use REXML stream parser API or SAX2 parser API instead of tree parser for untrusted XML
• Implement request size limits at the web server or application level for XML endpoints
• Deploy reverse proxy or WAF rules to filter potentially malicious XML payloads
• Consider using alternative XML parsing libraries with built-in protection against entity expansion attacks

# Upgrade REXML gem to patched version
gem update rexml

# Or specify minimum version in Gemfile
# gem 'rexml', '>= 3.3.6'
bundle update rexml