CVE-2021-33813: JDOM SAXBuilder XXE Vulnerability

CVE-2021-33813 Overview

CVE-2021-33813 is an XML External Entity (XXE) vulnerability discovered in the SAXBuilder component of JDOM through version 2.0.6. This vulnerability allows remote attackers to cause a denial of service (DoS) condition by sending specially crafted HTTP requests containing malicious XML payloads. The flaw exists due to improper handling of external entity references during XML parsing operations.

Critical Impact

Attackers can exploit this XXE vulnerability to cause service disruption through resource exhaustion, potentially affecting dependent applications including Apache Solr, Apache Tika, and Oracle Communications Messaging Server.

Affected Products

• JDOM versions through 2.0.6
• Apache Solr versions 8.8.1 and 8.9
• Apache Tika version 1.25
• Debian Linux 9.0
• Fedora 35
• Oracle Communications Messaging Server 8.1

Discovery Timeline

• June 16, 2021 - CVE CVE-2021-33813 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-33813

Vulnerability Analysis

This vulnerability stems from the SAXBuilder class in JDOM failing to properly disable external entity processing by default when parsing XML documents. When an application uses SAXBuilder to parse untrusted XML input without explicitly configuring secure parsing features, an attacker can inject malicious XML containing external entity declarations. These declarations can reference external resources, causing the parser to attempt recursive entity expansion or fetch remote resources, leading to denial of service through resource exhaustion.

The issue is particularly concerning because JDOM is a widely-used Java library for XML manipulation, and the vulnerable SAXBuilder class is commonly used for reading XML documents from various input sources including HTTP requests, files, and streams. Applications that process XML from untrusted sources without proper validation are at risk.

Root Cause

The root cause of CVE-2021-33813 lies in the default configuration of the SAXBuilder class, which does not disable external entity and DTD processing by default. According to secure coding practices, XML parsers should have external entity processing disabled unless explicitly required. The JDOM library's SAXBuilder constructor creates instances that allow external entity processing, exposing applications to XXE attacks when processing untrusted XML content.

Attack Vector

The attack is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by sending a crafted HTTP request containing malicious XML payload with external entity declarations to any endpoint that uses the vulnerable JDOM SAXBuilder to parse XML. The malicious XML can contain entity definitions that trigger recursive expansion (billion laughs attack) or reference external resources, causing the target system to exhaust memory, CPU resources, or network connections, resulting in denial of service.

Common attack scenarios include:

• Recursive entity expansion causing memory exhaustion
• External entity references pointing to large or slow-responding resources
• Entity expansion loops that consume CPU cycles

Detection Methods for CVE-2021-33813

Indicators of Compromise

• Unusual memory consumption spikes in Java applications processing XML
• HTTP requests containing suspicious XML payloads with DOCTYPE declarations and entity definitions
• Application logs showing XML parsing errors or stack traces related to entity resolution
• Network connections from the application to unexpected external hosts during XML processing

Detection Strategies

• Monitor application dependencies for JDOM versions 2.0.6 and earlier using software composition analysis (SCA) tools
• Implement web application firewall (WAF) rules to detect and block requests containing potentially malicious XML entity declarations
• Review application code for usage of SAXBuilder without secure configuration settings
• Deploy runtime application self-protection (RASP) solutions to detect XXE exploitation attempts

Monitoring Recommendations

• Enable verbose logging for XML parsing operations to capture entity resolution attempts
• Set up alerts for abnormal resource consumption patterns in services that process XML
• Monitor outbound network connections from XML-processing applications for unauthorized external entity fetches
• Implement application performance monitoring (APM) to detect parsing-related slowdowns

How to Mitigate CVE-2021-33813

Immediate Actions Required

• Upgrade JDOM to version 2.0.6.1 or later which addresses this vulnerability
• Configure SAXBuilder to disable external entity processing by setting appropriate features
• Review and update dependent applications including Apache Solr, Apache Tika deployments
• Apply vendor-specific patches from Oracle and Linux distributions

Patch Information

The JDOM project has addressed this vulnerability through a fix detailed in the GitHub Pull Request Discussion. Updated versions are available via the GitHub Release Notes. For Oracle products, security patches are available through the Oracle Security Alert - April 2022 and Oracle Security Alert - July 2022. Linux distribution users should apply patches announced in the Debian LTS Announcement - June 2021 and Fedora Package Announcements.

Workarounds

• Configure SAXBuilder to disable external entities by setting the feature http://xml.org/sax/features/external-general-entities to false
• Set the feature http://xml.org/sax/features/external-parameter-entities to false on SAXBuilder instances
• Disable DTD processing entirely by setting http://apache.org/xml/features/disallow-doctype-decl to true
• Implement input validation to reject XML documents containing DOCTYPE declarations from untrusted sources

// Secure SAXBuilder Configuration Example
SAXBuilder builder = new SAXBuilder();
builder.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true);
builder.setFeature("http://xml.org/sax/features/external-general-entities", false);
builder.setFeature("http://xml.org/sax/features/external-parameter-entities", false);
builder.setFeature("http://apache.org/xml/features/nonvalidating/load-external-dtd", false);