CVE-2021-41617: OpenSSH Privilege Escalation Vulnerability

CVE-2021-41617 Overview

CVE-2021-41617 is a privilege escalation vulnerability affecting sshd in OpenSSH versions 6.2 through 8.x before 8.8. When certain non-default configurations are used, the vulnerability allows privilege escalation because supplemental groups are not initialized as expected. Helper programs for AuthorizedKeysCommand and AuthorizedPrincipalsCommand may run with privileges associated with group memberships of the sshd process, if the configuration specifies running the command as a different user.

Critical Impact

Attackers with local access can potentially escalate privileges by exploiting improper supplemental group handling in OpenSSH configurations using AuthorizedKeysCommand or AuthorizedPrincipalsCommand directives.

Affected Products

• OpenBSD OpenSSH 6.2 through 8.x before 8.8
• Fedora 33, 34, and 35
• NetApp Active IQ Unified Manager, Clustered Data ONTAP, HCI Management Node, ONTAP Select Deploy Administration Utility, SolidFire
• NetApp AFF A250 and AFF 500F (firmware)
• Oracle HTTP Server 12.2.1.2.0, 12.2.1.3.0, 12.2.1.4.0
• Oracle ZFS Storage Appliance Kit 8.8
• StarWind Virtual SAN v8r13

Discovery Timeline

• 2021-09-26 - CVE-2021-41617 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-41617

Vulnerability Analysis

This vulnerability affects OpenSSH's handling of supplemental groups when executing external helper commands. When administrators configure AuthorizedKeysCommand or AuthorizedPrincipalsCommand to run as a specific user, the sshd daemon fails to properly initialize the supplemental groups for that user. Instead, the helper program inherits the supplemental group memberships of the sshd process itself, which typically runs with elevated privileges.

The exploitation requires specific non-default configurations to be present in the sshd_config file. Systems that do not use AuthorizedKeysCommand or AuthorizedPrincipalsCommand directives are not affected by this vulnerability. However, environments that leverage these features for centralized key management or directory-based authentication may be at risk.

Root Cause

The root cause lies in improper privilege handling during the execution of external authentication helper programs. When sshd spawns a helper process for AuthorizedKeysCommand or AuthorizedPrincipalsCommand, it correctly changes the effective user ID but fails to reinitialize the supplemental group list. This results in the helper process retaining group memberships from the parent sshd process, potentially granting access to files and resources that should be restricted.

Attack Vector

The attack vector is local and requires the attacker to have existing access to the target system. Exploitation involves leveraging the inherited supplemental groups to access resources that would normally be restricted. The attacker would need to identify a system using the vulnerable configuration and then craft an authentication attempt that triggers the execution of the misconfigured helper command. Once executed, the helper process runs with unintended group privileges, enabling access to sensitive files or execution of privileged operations through group-based access controls.

The vulnerability requires specific conditions: a non-default sshd configuration using AuthorizedKeysCommand or AuthorizedPrincipalsCommand with a user different from the one running sshd, combined with file system resources protected by group permissions that align with sshd's supplemental groups.

Detection Methods for CVE-2021-41617

Indicators of Compromise

• Unexpected access to group-protected files by processes spawned from sshd
• Authentication log entries showing helper command execution with unusual group associations
• Process traces revealing supplemental group inheritance from sshd parent process

Detection Strategies

• Review sshd_config for use of AuthorizedKeysCommand or AuthorizedPrincipalsCommand directives with user specifications
• Use the Tenable Nessus Plugin #154174 to scan for vulnerable OpenSSH versions
• Audit OpenSSH version across all systems using package management or configuration management tools
• Monitor process group memberships during SSH authentication events

Monitoring Recommendations

• Implement file integrity monitoring on sensitive group-protected files that could be targeted
• Enable detailed sshd logging and forward to SIEM for correlation analysis
• Set up alerts for SSH configuration changes, particularly to AuthorizedKeysCommand and AuthorizedPrincipalsCommand settings
• Monitor for privilege escalation attempts following SSH authentication events

How to Mitigate CVE-2021-41617

Immediate Actions Required

• Upgrade OpenSSH to version 8.8 or later to fully remediate the vulnerability
• Audit current sshd_config files for use of AuthorizedKeysCommand and AuthorizedPrincipalsCommand directives
• Review and restrict file system permissions on resources accessible via sshd's supplemental groups
• Consider temporarily disabling AuthorizedKeysCommand or AuthorizedPrincipalsCommand if upgrade is not immediately possible

Patch Information

OpenSSH 8.8 addresses this vulnerability by properly initializing supplemental groups when executing helper commands. Organizations should apply vendor-specific patches based on their distribution:

• OpenSSH: Upgrade to version 8.8 or later as documented in the OpenSSH Release Notes 8.8
• Fedora: Apply updates per the Fedora Package Announcements for Fedora 33, 34, and 35
• NetApp: Review the NetApp Security Advisory 2021 for product-specific guidance
• Oracle: Apply patches per the Oracle CPU April 2022 Advisory and Oracle CPU July 2022 Advisory
• Debian: Apply updates per Debian Security Advisory DSA-5586
• StarWind: Review the StarWind Security Advisory SW-20220805-0001

Workarounds

• Remove or comment out AuthorizedKeysCommand and AuthorizedPrincipalsCommand directives in sshd_config if not required
• Ensure helper commands specified in these directives run as the same user as sshd to minimize group inheritance impact
• Restrict group memberships of the sshd process to minimize exposure of group-protected resources
• Apply strict file system permissions to limit the impact of unintended group access

# Check for vulnerable configuration in sshd_config
grep -E "^(AuthorizedKeysCommand|AuthorizedPrincipalsCommand)" /etc/ssh/sshd_config

# Verify current OpenSSH version
ssh -V

# Restart sshd after configuration changes (system-dependent)
systemctl restart sshd