CVE-2024-55887: Ucum-java XXE Vulnerability in XML Parsing

CVE-2024-55887 Overview

CVE-2024-55887 is an XML External Entity (XXE) injection vulnerability affecting Ucum-java, a FHIR Java library that provides Unified Code for Units of Measure (UCUM) services. In versions prior to 1.0.9, XML parsing performed by the UcumEssenceService is vulnerable to XXE attacks. A processed XML file containing a malicious DTD tag could enable attackers to extract data from the host system, potentially exposing sensitive configuration files, credentials, or other critical information.

Critical Impact

Attackers can exploit this XXE vulnerability to read arbitrary files from the host system and potentially perform Server-Side Request Forgery (SSRF) attacks when external clients can submit XML to applications using Ucum-java.

Affected Products

• Ucum-java versions prior to 1.0.9
• Applications using UcumEssenceService to process untrusted XML input
• FHIR implementations utilizing the vulnerable Ucum-java library

Discovery Timeline

• 2024-12-13 - CVE-2024-55887 published to NVD
• 2024-12-13 - Last updated in NVD database

Technical Details for CVE-2024-55887

Vulnerability Analysis

This vulnerability falls under CWE-611 (Improper Restriction of XML External Entity Reference). The UcumEssenceService component in Ucum-java fails to properly configure the XML parser to disable external entity processing. When parsing XML input, the vulnerable parser processes Document Type Definition (DTD) declarations, including external entity references that can point to local files or remote resources.

The impact primarily affects confidentiality, as attackers can read sensitive files from the server's file system. This vulnerability is particularly dangerous in healthcare environments where FHIR implementations may process XML from external sources, potentially exposing patient data, system credentials, or internal network resources.

Root Cause

The root cause is the insecure default configuration of the XML parser used by UcumEssenceService. The parser does not disable DTD processing or external entity resolution, allowing malicious XML documents to include references to external resources. When the parser encounters these references, it attempts to resolve them, leading to information disclosure or SSRF vulnerabilities.

Attack Vector

The attack requires network access where an attacker can submit malicious XML content to an application using the vulnerable Ucum-java library. The attacker crafts an XML document containing a malicious DTD that defines an external entity pointing to a local file (such as /etc/passwd on Linux systems) or an internal network resource. When the vulnerable parser processes this XML, it resolves the external entity and includes the content in the parsed output, which may then be returned to the attacker or logged in an accessible location.

The vulnerability mechanism works as follows: An attacker submits XML with a DOCTYPE declaration containing an external entity definition. The malicious entity references a file path or URL that the server can access. When UcumEssenceService parses the XML, the parser resolves the entity, reads the referenced resource, and substitutes it into the document. For detailed technical information, refer to the GitHub Security Advisory.

Detection Methods for CVE-2024-55887

Indicators of Compromise

• Unusual XML parsing errors in application logs referencing external files or URLs
• Unexpected outbound network connections from the application server to internal resources
• Log entries showing attempts to access sensitive system files like /etc/passwd, /etc/shadow, or Windows credential stores
• HTTP requests containing DOCTYPE declarations with ENTITY definitions in XML payloads

Detection Strategies

• Monitor application logs for XML parsing exceptions related to file access or network connections
• Implement Web Application Firewall (WAF) rules to detect and block XML payloads containing DOCTYPE declarations
• Use intrusion detection systems to identify patterns of XXE exploitation attempts in network traffic
• Audit dependencies to identify applications using Ucum-java versions prior to 1.0.9

Monitoring Recommendations

• Enable verbose logging for XML parsing operations in affected applications
• Set up alerts for file access attempts from the XML parser to sensitive system locations
• Monitor outbound network traffic from application servers for unusual internal network scanning patterns
• Implement runtime application self-protection (RASP) to detect XXE exploitation attempts

How to Mitigate CVE-2024-55887

Immediate Actions Required

• Upgrade Ucum-java to version 1.0.9 or later immediately
• Audit all applications using Ucum-java to identify vulnerable deployments
• Ensure that XML input to UcumEssenceService comes only from trusted sources
• Implement input validation to reject XML documents containing DOCTYPE declarations

Patch Information

The vulnerability has been fixed in Ucum-java version 1.0.9. Organizations should update their Maven or Gradle dependencies to reference the patched version. For more information about the fix, consult the GitHub Security Advisory.

Workarounds

• Ensure that the source XML for instantiating UcumEssenceService is from a trusted source only
• Implement application-level filtering to strip DOCTYPE declarations from incoming XML before processing
• Deploy network segmentation to limit the impact of potential SSRF attacks
• Use a security-hardened XML parser wrapper that disables external entity processing at the application layer

To configure secure XML parsing at the application level if upgrading is not immediately possible, ensure your XML parser factory disables DTD processing by setting the appropriate parser features. Specifically, disable http://apache.org/xml/features/disallow-doctype-decl and http://xml.org/sax/features/external-general-entities features. Consult the OWASP XXE Prevention Cheat Sheet for language-specific configuration guidance.