CVE-2021-44141: Samba Information Disclosure Vulnerability

CVE-2021-44141 Overview

CVE-2021-44141 is an information disclosure vulnerability affecting all versions of Samba prior to 4.15.5. A malicious client can exploit server symlinks to determine if a file or directory exists in an area of the server file system that is not exported under the share definition. This symlink attack requires SMB1 with unix extensions to be enabled for successful exploitation.

Critical Impact

Authenticated attackers can enumerate files and directories outside the exported share, potentially revealing sensitive system structure and configuration information that could facilitate further attacks.

Affected Products

• Samba versions prior to 4.15.5
• Red Hat Storage 3.0
• Fedora 34 and 35

Discovery Timeline

• 2022-02-21 - CVE-2021-44141 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-44141

Vulnerability Analysis

This vulnerability falls under Information Disclosure (CWE-200) and Symlink Attack (CWE-59) categories. The flaw exists in how Samba handles symbolic links when SMB1 with unix extensions is enabled. An authenticated attacker with access to a Samba share can create or follow symlinks to probe areas of the server's file system that should be inaccessible.

The attack allows an adversary to determine the existence of files and directories outside the boundaries of the configured share export. While this does not directly allow reading the contents of those files, the ability to enumerate file system structure can provide valuable reconnaissance information for subsequent attacks.

Root Cause

The root cause stems from improper validation of symlink targets when unix extensions are enabled in SMB1 protocol mode. Samba fails to adequately restrict symlink traversal, allowing authenticated clients to create symlinks pointing to arbitrary locations on the server file system. The server's response when accessing these symlinks reveals whether the target path exists, enabling file and directory enumeration beyond the share boundaries.

Attack Vector

The attack requires network access and low-privilege authentication to a Samba share. The attacker must have the ability to create or manipulate symlinks on the share, and the server must have SMB1 with unix extensions enabled. By creating symlinks to various paths and observing the server's response, the attacker can systematically probe the file system to discover what files and directories exist in unexported areas.

The vulnerability mechanism works by exploiting the server's handling of symlink resolution. When a client requests information about a symlink, the server's response differs based on whether the symlink target exists, allowing the attacker to infer the presence of files without direct access to their contents.

Detection Methods for CVE-2021-44141

Indicators of Compromise

• Unusual symlink creation activity on Samba shares by authenticated users
• SMB access logs showing repeated attempts to access symlinks pointing outside share boundaries
• Abnormal file system traversal patterns in Samba audit logs
• Multiple failed or denied access attempts following symlink paths

Detection Strategies

• Enable detailed Samba audit logging to capture symlink operations and share access patterns
• Monitor for SMB1 protocol usage, particularly with unix extensions enabled
• Implement file integrity monitoring on Samba shares to detect suspicious symlink creation
• Review Samba logs for patterns indicating systematic file system probing

Monitoring Recommendations

• Configure Samba's vfs_full_audit module to log symlink-related operations
• Set up alerts for SMB1 connections, which should be rare in modern environments
• Monitor authentication logs for accounts with unusual share access patterns
• Deploy network-level monitoring for SMB traffic anomalies

How to Mitigate CVE-2021-44141

Immediate Actions Required

• Upgrade Samba to version 4.15.5 or later immediately
• Disable SMB1 protocol if not required for legacy compatibility
• Disable unix extensions on shares where not explicitly needed
• Review share configurations to ensure proper access controls are in place
• Audit existing symlinks on Samba shares for suspicious targets

Patch Information

Samba has released version 4.15.5 which addresses this vulnerability. Organizations should apply this update as the primary remediation measure. For detailed patch information, refer to the Samba Security Report for CVE-2021-44141. Additional guidance is available in the Gentoo Security Advisory GLSA-202309-06.

Workarounds

• Disable SMB1 protocol by setting server min protocol = SMB2 in smb.conf
• Disable unix extensions by adding unix extensions = no to the global section of smb.conf
• Use the wide links = no setting to prevent following symlinks outside the share
• Implement strict share-level permissions to limit which users can create symlinks

# Configuration example - Add to smb.conf [global] section
[global]
# Disable SMB1 protocol
server min protocol = SMB2

# Disable unix extensions
unix extensions = no

# Prevent following symlinks outside share path
wide links = no
follow symlinks = no