CVE-2021-27635: SAP NetWeaver Java XXE Vulnerability

CVE-2021-27635 Overview

CVE-2021-27635 is an XML External Entity (XXE) vulnerability affecting SAP NetWeaver Application Server for JAVA. The vulnerability arises from missing XML validation in the application, allowing an authenticated administrator to submit specially crafted XML files over the network. Successful exploitation enables attackers to read arbitrary files from the filesystem or cause a complete denial of service by crashing the system.

Critical Impact

Authenticated administrators can leverage this XXE vulnerability to fully compromise system confidentiality through arbitrary file read access or completely disrupt availability by causing system crashes.

Affected Products

• SAP NetWeaver Application Server for JAVA version 7.20
• SAP NetWeaver Application Server for JAVA version 7.30
• SAP NetWeaver Application Server for JAVA version 7.31
• SAP NetWeaver Application Server for JAVA version 7.40
• SAP NetWeaver Application Server for JAVA version 7.50

Discovery Timeline

• June 9, 2021 - CVE-2021-27635 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-27635

Vulnerability Analysis

This vulnerability is classified under CWE-611 (Improper Restriction of XML External Entity Reference). The application fails to properly validate XML input submitted through network connections, allowing malicious external entity references to be processed. When an authenticated administrator submits a specially crafted XML file, the parser processes external entity declarations without restriction, leading to server-side request forgery-like behavior within the XML parsing context.

The attack requires network access and high privileges (administrator authentication), which limits the potential attacker pool. However, once exploited, the impact on confidentiality and availability is severe. The vulnerability does not allow modification of data, so integrity remains unaffected. This XXE flaw enables attackers to reference external entities pointing to local file system resources or trigger resource exhaustion attacks leading to denial of service conditions.

Root Cause

The root cause of CVE-2021-27635 is the absence of proper XML validation and secure parser configuration in SAP NetWeaver AS for JAVA's system connections handling. The XML parser is configured to allow external entity resolution without restrictions, enabling attackers to include malicious entity references. Secure XML parsing requires disabling DTD processing, external entity resolution, and external parameter entities, none of which appear to be properly configured in the affected versions.

Attack Vector

The attack is network-based and requires the attacker to first obtain valid administrator credentials to the SAP NetWeaver AS for JAVA instance. Once authenticated, the attacker can interact with the system connections functionality and submit malicious XML payloads containing external entity declarations. These declarations can reference local files using file:// protocol handlers to exfiltrate sensitive configuration files, credentials, or other system data. Alternatively, the attacker can craft payloads designed to cause XML parser exhaustion or infinite recursion, resulting in denial of service.

The exploitation mechanism involves crafting XML documents with DOCTYPE declarations containing ENTITY definitions that reference external resources. When the server parses these documents without proper validation, it attempts to resolve the external references, exposing file contents or consuming excessive resources.

Detection Methods for CVE-2021-27635

Indicators of Compromise

• Unusual XML parsing errors in SAP NetWeaver AS for JAVA logs, particularly involving external entity resolution failures
• Unexpected file access attempts originating from the Java process running SAP NetWeaver, especially to sensitive files like /etc/passwd or SAP configuration files
• System connection configuration changes made by administrator accounts during unusual hours
• Application crashes or service restarts preceded by XML processing activities

Detection Strategies

• Monitor SAP NetWeaver AS for JAVA application logs for XML parsing exceptions and external entity resolution attempts
• Implement network traffic analysis to detect XML payloads containing DOCTYPE declarations with ENTITY references
• Configure SIEM rules to alert on file read operations by the SAP NetWeaver process to sensitive system paths
• Deploy web application firewall (WAF) rules to inspect and block XML payloads containing suspicious external entity patterns

Monitoring Recommendations

• Enable verbose logging for XML processing components in SAP NetWeaver AS for JAVA
• Establish baseline behavior for administrator actions in system connections and alert on deviations
• Monitor for signs of denial of service conditions such as memory exhaustion or excessive CPU usage during XML parsing operations
• Review administrator account access patterns and implement anomaly detection for privileged user activities

How to Mitigate CVE-2021-27635

Immediate Actions Required

• Apply the security patch referenced in SAP Support Note #3053066 immediately
• Review administrator account access and ensure principle of least privilege is enforced
• Audit recent system connection configuration changes for signs of exploitation
• Consider temporarily restricting network access to the affected functionality until patching is complete

Patch Information

SAP has released a security patch addressing this vulnerability. Administrators should obtain the fix from SAP Support Note #3053066. Additional information about this and related security updates can be found on the SAP Wiki Security Advisory page. Technical details about the exploitation technique are documented in the Packet Storm Security advisory and the Full Disclosure mailing list.

Workarounds

• Restrict network access to SAP NetWeaver AS for JAVA administrative interfaces using firewall rules or network segmentation
• Implement strict access controls limiting which accounts have administrator privileges
• Deploy a web application firewall (WAF) configured to block XML payloads containing external entity declarations
• If feasible, disable or restrict the system connections functionality until the patch can be applied
• Consider implementing additional authentication requirements for administrative functions

# Example: Network restriction for SAP administrative interfaces
# Restrict access to SAP NetWeaver management ports to trusted networks only
iptables -A INPUT -p tcp --dport 50000 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 50000 -j DROP

# Review active administrator sessions
# Check SAP NetWeaver user management for unusual admin accounts