CVE-2021-20254: Samba Information Disclosure Vulnerability

CVE-2021-20254 Overview

A flaw was found in Samba. The Samba smbd file server must map Windows group identities (SIDs) into unix group ids (gids). The code that performs this had a flaw that could allow it to read data beyond the end of the array in the case where a negative cache entry had been added to the mapping cache. This could cause the calling code to return those values into the process token that stores the group membership for a user. The highest threat from this vulnerability is to data confidentiality and integrity.

Critical Impact

Attackers with low privileges can exploit this out-of-bounds read vulnerability over the network to potentially gain unauthorized access to sensitive data or modify group membership tokens, compromising both confidentiality and integrity of the affected systems.

Affected Products

• Samba Samba (multiple versions)
• Fedora 32 and 33
• Red Hat Enterprise Linux 7.0 and 8.0
• Debian Linux 9.0

Discovery Timeline

• May 5, 2021 - CVE CVE-2021-20254 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-20254

Vulnerability Analysis

This vulnerability is classified as CWE-125: Out-of-Bounds Read. The flaw exists within the Samba smbd file server's SID-to-GID mapping functionality. When Samba processes Windows Security Identifiers (SIDs) and maps them to Unix group IDs (gids), it relies on a caching mechanism to improve performance. The vulnerability emerges when a negative cache entry—indicating a failed previous lookup—exists in the mapping cache.

Under these conditions, the code fails to properly validate array bounds before reading, allowing it to access memory beyond the intended array boundaries. The data read from this out-of-bounds location can then be incorrectly incorporated into the process token that stores user group memberships. This could allow an attacker to manipulate their effective group membership, potentially gaining access to resources they should not be authorized to access.

The attack requires network access and low-level privileges, but no user interaction is necessary. While high attack complexity is required to exploit this vulnerability, successful exploitation could result in significant impact to both data confidentiality and integrity.

Root Cause

The root cause of this vulnerability lies in insufficient bounds checking within the SID-to-GID mapping code path. Specifically, when iterating through cached mapping entries, the code does not properly handle the scenario where a negative cache entry is present. This causes the array index to reference memory locations outside the allocated buffer, reading arbitrary data that subsequently gets processed as legitimate group membership information.

Attack Vector

The attack vector for CVE-2021-20254 is network-based. An authenticated attacker with low privileges can exploit this vulnerability by triggering the SID-to-GID mapping functionality under conditions where a negative cache entry exists in the mapping cache.

The exploitation flow involves:

1. An attacker establishes a connection to a vulnerable Samba smbd server
2. The attacker performs operations that trigger the SID-to-GID mapping functionality
3. If a negative cache entry exists in the mapping cache, the out-of-bounds read occurs
4. The arbitrary data read from memory is incorporated into the process token
5. The attacker potentially gains unauthorized group memberships, allowing access to restricted resources

The vulnerability is exploitable without user interaction, though the attack complexity is considered high due to the requirement for specific cache state conditions.

Detection Methods for CVE-2021-20254

Indicators of Compromise

• Unusual access patterns to files or directories that should be restricted based on group membership
• Anomalous smbd process behavior or unexpected memory access patterns in system logs
• Authentication events followed by access to resources outside the user's normal group permissions

Detection Strategies

• Monitor Samba logs for unusual authentication patterns or unexpected group membership changes during sessions
• Implement file integrity monitoring on sensitive directories to detect unauthorized access attempts
• Deploy network monitoring to identify abnormal SMB/CIFS traffic patterns that may indicate exploitation attempts

Monitoring Recommendations

• Enable detailed Samba logging (log level = 3 or higher) to capture SID-to-GID mapping operations
• Configure audit logging for sensitive file shares to track access by user and group
• Implement SIEM rules to correlate Samba authentication events with subsequent file access patterns

How to Mitigate CVE-2021-20254

Immediate Actions Required

• Apply vendor-provided security patches immediately to all affected Samba installations
• Review access logs for any signs of unauthorized access to restricted resources
• Audit current group memberships and access controls to identify any potential compromises
• Consider temporarily restricting network access to Samba servers until patches are applied

Patch Information

Security patches addressing CVE-2021-20254 are available from Samba and major Linux distributions. Organizations should apply updates from their respective distribution vendors:

• Samba: Refer to the Samba CVE-2021-20254 Security Advisory for official patches
• Red Hat: Updates available through standard Red Hat update channels (see Red Hat Bug Report)
• Debian: Security updates available via Debian repositories (see Debian LTS Announcement)
• Fedora: Package updates available through Fedora update system
• Gentoo: See Gentoo GLSA 2021-05-22 for guidance
• NetApp: Refer to the NetApp Security Advisory for affected products

Workarounds

• Implement network segmentation to limit exposure of Samba servers to trusted networks only
• Apply strict access controls and principle of least privilege for Samba share access
• Consider disabling or restricting access to affected Samba services until patches can be applied
• Enable enhanced logging to detect potential exploitation attempts while awaiting patch deployment

# Example: Restrict Samba access to trusted networks in smb.conf
[global]
    hosts allow = 192.168.1.0/24 10.0.0.0/8
    hosts deny = ALL
    
# Enable enhanced logging for detection
    log level = 3
    log file = /var/log/samba/log.%m