CVE-2021-20353: IBM WebSphere Application Server XXE Flaw

CVE-2021-20353 Overview

IBM WebSphere Application Server versions 7.0, 8.0, 8.5, and 9.0 contain an XML External Entity (XXE) Injection vulnerability that occurs when processing XML data. A remote attacker could exploit this vulnerability to expose sensitive information from the server or consume memory resources, potentially leading to denial of service conditions. This vulnerability was assigned IBM X-Force ID: 194882.

Critical Impact

Remote unauthenticated attackers can exploit this XXE vulnerability to read sensitive files from the server, access internal network resources, or cause denial of service through memory exhaustion.

Affected Products

• IBM WebSphere Application Server 7.0
• IBM WebSphere Application Server 8.0
• IBM WebSphere Application Server 8.5
• IBM WebSphere Application Server 9.0

Discovery Timeline

• February 10, 2021 - CVE-2021-20353 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-20353

Vulnerability Analysis

This vulnerability is classified as CWE-611 (Improper Restriction of XML External Entity Reference). The IBM WebSphere Application Server fails to properly configure its XML parser to disable external entity processing, allowing attackers to inject malicious XML content that references external resources.

When the application processes XML input, the parser resolves external entity references without proper restrictions. This allows an attacker to craft malicious XML documents that can read local files from the server, perform server-side request forgery (SSRF) attacks against internal systems, or trigger denial of service conditions by referencing resources that consume excessive memory.

The attack can be performed remotely over the network without authentication, and the attacker does not require any special privileges or user interaction to exploit this vulnerability.

Root Cause

The root cause of this vulnerability lies in the insecure default configuration of the XML parser used by IBM WebSphere Application Server. The XML parser does not properly disable external entity processing, DTD processing, or external parameter entity expansion. When XML input is processed, these features allow the parser to fetch and process external resources, leading to information disclosure and resource consumption vulnerabilities.

Attack Vector

The attack is performed over the network against IBM WebSphere Application Server instances that accept and process XML input. An attacker can exploit this vulnerability by sending a specially crafted XML document containing malicious external entity definitions.

A typical XXE attack payload would include a Document Type Definition (DTD) that declares an external entity pointing to a sensitive file on the server (such as /etc/passwd on Linux systems or configuration files containing credentials). When the XML parser processes this document, it resolves the external entity and includes the contents of the referenced file in the response, effectively leaking sensitive information to the attacker.

For denial of service attacks, the attacker can reference resources that cause excessive memory consumption, such as the /dev/random device on Linux systems or recursive entity expansion (billion laughs attack).

Detection Methods for CVE-2021-20353

Indicators of Compromise

• Unusual XML requests containing DTD declarations or external entity references in server logs
• Attempts to access sensitive files like /etc/passwd, /etc/shadow, or application configuration files through XML parsing
• HTTP requests with XML payloads containing suspicious keywords such as <!ENTITY, SYSTEM, or file:// protocols
• Unexpected outbound network connections from the WebSphere server to external or internal resources

Detection Strategies

• Monitor WebSphere Application Server logs for XML parsing errors or unusual file access attempts
• Implement Web Application Firewall (WAF) rules to detect and block XXE attack patterns in incoming XML requests
• Configure network intrusion detection systems (IDS) to alert on XML payloads containing external entity definitions
• Review application logs for failed attempts to access system files or internal network resources

Monitoring Recommendations

• Enable detailed logging for XML processing components within WebSphere Application Server
• Monitor for abnormal memory consumption patterns that could indicate XXE-based denial of service attempts
• Implement file integrity monitoring on sensitive configuration files and system resources
• Configure alerts for unexpected outbound connections from application servers to internal network segments

How to Mitigate CVE-2021-20353

Immediate Actions Required

• Apply the security patch provided by IBM immediately to all affected WebSphere Application Server installations
• Review and audit all XML processing functionality within deployed applications
• Implement network segmentation to limit the impact of potential SSRF attacks
• Enable Web Application Firewall rules to filter malicious XML payloads

Patch Information

IBM has released security updates to address this vulnerability. Administrators should consult the IBM Support Document for detailed patching instructions and download links for their specific WebSphere Application Server version. Additional technical details are available in the IBM X-Force Vulnerability Database and the Zero Day Initiative Advisory ZDI-21-174.

Workarounds

• Configure XML parsers to disable external entity processing, DTD processing, and external parameter entities where possible
• Implement input validation to reject XML documents containing DTD declarations or external entity references
• Use allowlisting for expected XML structures and reject any XML that deviates from the expected schema
• Deploy a Web Application Firewall (WAF) with XXE attack detection capabilities as an interim protection measure

# Example WebSphere XML parser hardening (consult IBM documentation for specific implementation)
# Disable external entity processing in XML parsers
# Set the following JVM system properties:
-Djavax.xml.accessExternalDTD=""
-Djavax.xml.accessExternalSchema=""
-Djavax.xml.accessExternalStylesheet=""