CVE-2021-40722: Adobe Experience Manager XXE RCE Flaw

CVE-2021-40722 Overview

CVE-2021-40722 is an XML External Entity (XXE) injection vulnerability affecting Adobe Experience Manager (AEM) Forms Cloud Service and AEM version 6.5.10.0 and below. This vulnerability allows an unauthenticated attacker to inject malicious XML entities that are processed by the application's XML parser, potentially leading to Remote Code Execution (RCE) on affected systems.

XXE vulnerabilities occur when an application parses XML input containing references to external entities without proper validation or restrictions. In this case, the vulnerable XML parser in AEM Forms can be exploited to execute arbitrary code on the server, read sensitive files, perform server-side request forgery (SSRF), or cause denial of service conditions.

Critical Impact

This vulnerability enables unauthenticated attackers to achieve Remote Code Execution on Adobe Experience Manager servers, potentially compromising the entire content management infrastructure and any sensitive data managed by the platform.

Affected Products

• Adobe Experience Manager version 6.5.10.0 and earlier
• Adobe Experience Manager Cloud Service
• AEM Forms Cloud Service offering

Discovery Timeline

• 2022-01-13 - CVE-2021-40722 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-40722

Vulnerability Analysis

This vulnerability is classified under CWE-611 (Improper Restriction of XML External Entity Reference). The root cause lies in the improper handling of XML input within Adobe Experience Manager Forms, where the XML parser is configured to process external entity references without adequate security restrictions.

When an XML parser processes a document containing an external entity declaration, it attempts to resolve and include the referenced resource. Attackers can exploit this behavior to force the parser to access local files, internal network resources, or execute code through various XXE attack techniques.

The vulnerability is particularly severe because it requires no authentication, can be exploited remotely over the network, and results in full system compromise through code execution capabilities. The attack complexity is low, meaning minimal technical knowledge is required to successfully exploit this flaw.

Root Cause

The vulnerability stems from insecure XML parser configuration in the AEM Forms component. The XML parser fails to disable external entity processing and Document Type Definition (DTD) processing, allowing attackers to inject malicious XML content that references external resources or leverages XXE-to-RCE techniques.

Secure XML parsing requires explicitly disabling features such as external general entities, external parameter entities, and DTD processing. The absence of these security controls in the affected AEM versions creates the exploitable condition.

Attack Vector

An attacker can exploit this vulnerability by submitting specially crafted XML content to vulnerable AEM Forms endpoints. The attack can be conducted remotely over the network without requiring any user interaction or prior authentication.

The exploitation typically involves:

1. Identifying a vulnerable AEM Forms endpoint that processes XML input
2. Crafting malicious XML content with external entity declarations
3. Submitting the payload to the target application
4. The XML parser processes the malicious entities, leading to code execution

The attacker can leverage various XXE techniques including external DTD loading, parameter entity injection, or blind XXE with out-of-band data exfiltration to achieve Remote Code Execution on the target system.

Detection Methods for CVE-2021-40722

Indicators of Compromise

• Unexpected outbound connections from AEM servers to external hosts or IP addresses
• Unusual file access patterns in AEM Forms processing logs, particularly attempts to read system files
• XML parsing errors or exceptions in application logs referencing external entities or DTDs
• Unexpected processes spawned by the AEM application server

Detection Strategies

• Monitor web application firewall (WAF) logs for XML payloads containing DOCTYPE declarations, ENTITY definitions, or SYSTEM/PUBLIC keywords
• Implement network monitoring to detect unusual outbound connections from AEM servers to internal or external resources
• Review AEM Forms access logs for suspicious requests to XML processing endpoints
• Deploy endpoint detection and response (EDR) solutions to identify code execution attempts originating from the AEM process

Monitoring Recommendations

• Enable verbose logging for AEM Forms XML processing components
• Configure alerts for file access attempts to sensitive system files from the AEM application
• Monitor for DNS queries to suspicious external domains originating from AEM servers
• Implement network segmentation monitoring to detect lateral movement attempts following potential exploitation

How to Mitigate CVE-2021-40722

Immediate Actions Required

• Apply the latest security patches from Adobe as outlined in Adobe Security Advisory APSB21-103
• Upgrade Adobe Experience Manager to a version newer than 6.5.10.0 that contains the security fix
• For AEM Cloud Service customers, ensure the latest service release has been applied
• Review access controls and network segmentation for AEM servers to limit potential impact

Patch Information

Adobe has released security updates to address this vulnerability in Adobe Security Advisory APSB21-103. Organizations running Adobe Experience Manager version 6.5.10.0 or earlier should upgrade to a patched version immediately. AEM Cloud Service customers should verify that the latest security updates have been automatically applied to their instances.

Administrators should reference the Adobe Security Advisory APSB21-103 for detailed patching instructions and affected version information.

Workarounds

• Implement Web Application Firewall (WAF) rules to block XML requests containing DOCTYPE, ENTITY, or SYSTEM declarations
• Restrict network access to AEM Forms endpoints to trusted IP ranges only
• Configure network egress filtering to prevent the AEM server from making outbound connections to untrusted hosts
• If AEM Forms functionality is not required, consider disabling the affected components until patches can be applied

Mitigation through WAF rules involves blocking common XXE payload patterns. Configure your WAF to inspect XML content and reject requests containing external entity references or DTD declarations. However, this should be considered a temporary measure, and applying the official Adobe security patch remains the recommended remediation approach.