CVE-2021-27610: SAP NetWeaver ABAP Auth Bypass Flaw

CVE-2021-27610 Overview

CVE-2021-27610 is an authentication bypass vulnerability affecting SAP NetWeaver ABAP Server and ABAP Platform. The vulnerability arises from the system's failure to create information about internal and external RFC (Remote Function Call) users in a consistent and distinguished format. This inconsistency in user differentiation could lead to improper authentication, allowing malicious users to obtain illegitimate access to the system.

SAP NetWeaver is a critical enterprise application platform that serves as the foundation for many SAP business applications. The ABAP (Advanced Business Application Programming) server handles RFC communications, which are essential for system-to-system integration in SAP environments. This vulnerability poses significant risk to enterprise environments where RFC connectivity is extensively used for business process automation and inter-system communication.

Critical Impact

Attackers can exploit inconsistent RFC user formatting to bypass authentication mechanisms, potentially gaining unauthorized access to SAP NetWeaver systems and the sensitive business data they contain.

Affected Products

• SAP NetWeaver ABAP versions 700, 701, 702, 731, 740, 750, 751, 752, 753, 754, 755, 804
• SAP NetWeaver Application Server ABAP versions 700, 701, 702, 731, 740, 750, 751, 752, 753, 754, 755, 804

Discovery Timeline

• June 16, 2021 - CVE-2021-27610 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-27610

Vulnerability Analysis

This vulnerability is classified under CWE-287 (Improper Authentication), which encompasses flaws where an application fails to properly verify the identity of a user attempting to access resources. In the context of SAP NetWeaver ABAP, the system does not adequately differentiate between internal and external RFC users when creating authentication-related information.

The RFC protocol is fundamental to SAP system communication, enabling synchronous and asynchronous calls between distributed systems. When user information is formatted inconsistently, the authentication layer may incorrectly validate credentials or grant access permissions based on ambiguous user identity data. This can lead to authentication bypass scenarios where an attacker masquerades as a legitimate internal user.

The vulnerability requires network access but does not require prior authentication or user interaction, making it particularly dangerous in environments where SAP systems are accessible across network boundaries.

Root Cause

The root cause of CVE-2021-27610 lies in the inconsistent formatting of RFC user information within the SAP NetWeaver ABAP authentication subsystem. The system fails to maintain a clear distinction between internal RFC users (system accounts used for trusted RFC connections) and external RFC users (standard user accounts accessing via RFC). This ambiguity in user classification creates opportunities for authentication confusion.

When the authentication mechanism cannot reliably distinguish between user types, it may apply incorrect trust relationships or permission models. Internal RFC connections typically operate with elevated trust levels, and if an external user can be misidentified as an internal user due to formatting inconsistencies, they may inherit these elevated privileges.

Attack Vector

The attack leverages the network-accessible RFC interface of SAP NetWeaver ABAP systems. An attacker can exploit the vulnerability by crafting RFC requests that exploit the inconsistent user formatting behavior. The attack does not require valid credentials or user interaction, as the vulnerability exists in the pre-authentication phase where user type determination occurs.

Exploitation typically involves:

1. Identifying exposed SAP NetWeaver ABAP systems with RFC interfaces accessible over the network
2. Crafting malformed or specially formatted RFC authentication requests that exploit the user type ambiguity
3. Gaining unauthorized access by having the system misclassify the attacker as an internal or trusted RFC user
4. Leveraging the obtained access to perform unauthorized operations within the SAP environment

Due to the nature of this authentication bypass, successful exploitation grants attackers high-impact access affecting confidentiality, integrity, and availability of the target system.

Detection Methods for CVE-2021-27610

Indicators of Compromise

• Unusual RFC login attempts from unexpected source IP addresses or systems
• Authentication log entries showing successful RFC connections without corresponding legitimate business transactions
• RFC connection patterns that bypass normal user authentication workflows
• Anomalous access to sensitive transactions or data by RFC user accounts

Detection Strategies

• Enable comprehensive RFC security logging via transaction SM21 and STRFCTRACE to capture detailed RFC activity
• Configure SAP Security Audit Log (transaction SM19) to monitor RFC authentication events and failures
• Implement network-level monitoring for RFC traffic (typically ports 3300-3399) to identify suspicious connection patterns
• Deploy SIEM rules to correlate RFC authentication events with expected user behavior baselines

Monitoring Recommendations

• Regularly review RFC destination configurations (SM59) for unauthorized trusted connections
• Monitor RFC user accounts for privilege escalation attempts or unexpected permission changes
• Implement real-time alerting for failed authentication attempts followed by successful RFC logins
• Track RFC Gateway activity via SMGW for signs of connection manipulation or abuse

How to Mitigate CVE-2021-27610

Immediate Actions Required

• Apply the security patch documented in SAP Support Note #3007182 immediately
• Review and audit all RFC destinations configured in transaction SM59 for unnecessary trusted connections
• Restrict network access to SAP RFC ports (3300-3399) using firewall rules to limit exposure
• Enable detailed RFC security logging to detect any exploitation attempts

Patch Information

SAP has released a security patch to address CVE-2021-27610. The patch corrects the inconsistent formatting of internal and external RFC user information, ensuring proper authentication validation. Organizations should apply the patch documented in SAP Support Note #3007182 to all affected SAP NetWeaver ABAP Server and Application Server ABAP instances.

The patch is applicable to all affected versions: 700, 701, 702, 731, 740, 750, 751, 752, 753, 754, 755, and 804. For detailed patch application guidance and compatibility information, refer to the SAP Security Wiki Page.

Workarounds

• Restrict RFC Gateway access by configuring gw/acl_mode and related security parameters to limit RFC connectivity
• Implement Secure Network Communications (SNC) for RFC connections to add an additional authentication layer
• Review and remove unnecessary RFC destinations, particularly those configured as trusted systems
• Enable RFC logging at the application and network levels to maintain visibility into RFC traffic until patching is complete

# SAP profile parameter configuration to enhance RFC security
# Add to instance profile or DEFAULT.PFL

# Enable RFC gateway access control
gw/acl_mode = 1

# Restrict RFC registrations
gw/reg_no_conn_info = 1

# Enable security audit logging for RFC
rsau/enable = 1