CVE-2021-44142: Samba vfs_fruit Module RCE Vulnerability

CVE-2021-44142 Overview

CVE-2021-44142 is a critical out-of-bounds heap read and write vulnerability in the Samba vfs_fruit module, which provides enhanced compatibility with Apple SMB clients and interoperability with Netatalk 3 AFP fileservers. The vulnerability exists in how Samba handles extended file attributes (EA, xattr) when the vfs_fruit module is configured.

A remote attacker with write access to extended file attributes can exploit this vulnerability to execute arbitrary code with the privileges of the smbd process, which typically runs as root. This makes it an extremely dangerous vulnerability for organizations running Samba file servers with Apple client compatibility enabled.

Critical Impact

Remote attackers with write access to extended file attributes can achieve arbitrary code execution with root privileges on vulnerable Samba servers configured with the vfs_fruit module.

Affected Products

• Samba versions prior to 4.13.17
• Samba versions prior to 4.14.12
• Samba versions prior to 4.15.5
• Debian Linux 10.0 and 11.0
• Canonical Ubuntu Linux 14.04 ESM, 16.04 ESM, 18.04 LTS, 20.04 LTS, and 21.10
• Synology DiskStation Manager
• Fedora 34 and 35
• Red Hat Enterprise Linux 7.0 and 8.0 (including various variants)
• Red Hat Gluster Storage 3.5
• Red Hat Virtualization Host 4.0

Discovery Timeline

• 2022-02-21 - CVE-2021-44142 published to NVD
• 2025-04-23 - Last updated in NVD database

Technical Details for CVE-2021-44142

Vulnerability Analysis

This vulnerability was demonstrated at Pwn2Own Austin, showcasing its severity and exploitability. The flaw resides in the vfs_fruit module, a Samba Virtual File System (VFS) module designed to enable macOS clients to access files stored on Samba servers while preserving Apple-specific file attributes and resource forks.

When processing specially crafted extended file attributes, the vfs_fruit module fails to properly validate the size and boundaries of the data being read and written to heap memory. This allows an attacker to read or write data beyond the allocated buffer boundaries, leading to heap corruption that can be leveraged for arbitrary code execution.

The vulnerability is particularly dangerous because the smbd daemon typically runs with root privileges, meaning successful exploitation grants the attacker complete control over the affected system. Additionally, the attack can be performed remotely over the network by any authenticated user with write access to extended file attributes on a Samba share.

Root Cause

The root cause is insufficient bounds checking when parsing extended file attributes in the vfs_fruit module. Specifically, the module does not properly validate the length fields in extended attribute data structures before performing memory operations. This allows specially crafted extended attributes to trigger both out-of-bounds read (CWE-125) and out-of-bounds write conditions on the heap.

The vulnerable code path is triggered when the vfs_fruit module processes AFP-style extended attributes used for Apple-specific metadata such as Finder information and resource forks.

Attack Vector

The attack requires network access to a Samba server configured with the vfs_fruit module enabled. The attacker must have authenticated access with permission to write extended file attributes on at least one share.

The exploitation process involves crafting malicious extended file attributes that trigger the out-of-bounds heap operations. By carefully controlling the content of these attributes, an attacker can:

1. Corrupt heap metadata to gain control over memory allocation
2. Overwrite function pointers or other critical data structures
3. Redirect execution flow to attacker-controlled code
4. Achieve arbitrary code execution with the privileges of the smbd process

For detailed technical analysis of the exploitation technique, refer to the ZDI Blog Post on CVE-2021-44142.

Detection Methods for CVE-2021-44142

Indicators of Compromise

• Unexpected crashes or restarts of the smbd process indicating potential exploitation attempts
• Anomalous extended file attribute operations in Samba audit logs, particularly on shares accessed by untrusted users
• Evidence of memory corruption or heap spray patterns in crash dumps
• Unusual process spawning or network connections originating from the smbd process

Detection Strategies

• Monitor Samba logs for unusual extended attribute operations, especially those involving AFP-style metadata
• Implement file integrity monitoring on Samba configuration files and binaries
• Deploy network intrusion detection rules to identify malformed SMB traffic targeting extended attribute handling
• Use endpoint detection and response (EDR) solutions to monitor for suspicious process behavior from smbd

Monitoring Recommendations

• Enable verbose logging for Samba VFS operations to capture extended attribute access patterns
• Configure alerting on smbd process crashes or unexpected terminations
• Monitor for privilege escalation attempts originating from Samba-related processes
• Implement baseline monitoring for extended attribute usage patterns to detect anomalies

How to Mitigate CVE-2021-44142

Immediate Actions Required

• Upgrade Samba to version 4.13.17, 4.14.12, or 4.15.5 or later immediately
• If immediate patching is not possible, disable the vfs_fruit VFS module as a temporary mitigation
• Review Samba share configurations to identify systems with vfs_fruit enabled
• Restrict write access to extended file attributes to trusted users only

Patch Information

Samba has released patched versions that address this vulnerability. Organizations should upgrade to the following minimum versions:

• Samba 4.13.x: Upgrade to 4.13.17 or later
• Samba 4.14.x: Upgrade to 4.14.12 or later
• Samba 4.15.x: Upgrade to 4.15.5 or later

For detailed patch information, consult the Samba Security CVE-2021-44142 advisory and the Samba Bugzilla Entry #14914.

Linux distribution-specific patches are available from Debian, Ubuntu, Fedora, Red Hat, and other vendors. Refer to the CERT Vulnerability ID #119678 for comprehensive vendor response information.

Workarounds

• Remove fruit from the vfs objects configuration line in smb.conf to disable the vulnerable module
• Restrict SMB share access to only trusted authenticated users with a legitimate need
• Implement network segmentation to limit exposure of Samba servers to untrusted networks
• Consider disabling extended attribute support if not required for your environment

# Temporary workaround: Disable vfs_fruit module in smb.conf
# Before (vulnerable):
# vfs objects = fruit streams_xattr

# After (mitigated):
vfs objects = streams_xattr

# Then restart Samba service:
systemctl restart smbd