CVE-2021-25216: Debian Linux Buffer Overflow Vulnerability

CVE-2021-25216 Overview

CVE-2021-25216 is a critical vulnerability affecting ISC BIND DNS servers configured to use GSS-TSIG features. The vulnerability exists in the ISC SPNEGO (Simple and Protected GSSAPI Negotiation Mechanism) implementation and can lead to either a buffer over-read (causing server crash) on 64-bit platforms or a buffer overflow (potentially enabling remote code execution) on 32-bit platforms. While BIND's default configuration is not vulnerable, servers become exposed when explicitly configuring the tkey-gssapi-keytab or tkey-gssapi-credential options.

Critical Impact

This vulnerability enables remote attackers to crash BIND DNS servers or potentially achieve remote code execution on 32-bit systems without authentication, severely impacting DNS infrastructure availability and integrity.

Affected Products

• ISC BIND 9.5.0 through 9.11.29
• ISC BIND 9.12.0 through 9.16.13
• ISC BIND 9.17.0 through 9.17.1 (development branch)
• ISC BIND Supported Preview Edition 9.11.3-S1 through 9.11.29-S1
• ISC BIND Supported Preview Edition 9.16.8-S1 through 9.16.13-S1
• Debian Linux 9.0 and 10.0
• Siemens SINEC Infrastructure Network Services
• NetApp Active IQ Unified Manager, Cloud Backup, and various hardware firmware

Discovery Timeline

• April 29, 2021 - CVE-2021-25216 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-25216

Vulnerability Analysis

The vulnerability resides in ISC's custom SPNEGO implementation used for GSS-TSIG authentication in BIND DNS servers. GSS-TSIG is commonly deployed in environments where BIND integrates with Samba or operates alongside Active Directory domain controllers. The flaw manifests differently depending on the target architecture: on 64-bit platforms, an out-of-bounds read condition occurs that causes the named daemon to crash, while on 32-bit platforms, a more severe buffer overflow condition exists that could potentially allow attackers to execute arbitrary code remotely.

ISC determined that their SPNEGO implementation was unnecessary given the availability of standard implementations in MIT and Heimdal Kerberos libraries. Consequently, ISC announced the removal of their SPNEGO implementation from BIND 9.11 and 9.16 releases in April, following its earlier removal from the 9.17 development branch.

Root Cause

The root cause is an out-of-bounds read vulnerability (CWE-125) in ISC's proprietary SPNEGO implementation. The code fails to properly validate buffer boundaries when processing SPNEGO authentication tokens, leading to memory access violations. This implementation flaw exists because the ISC SPNEGO code does not adequately check the length and bounds of input data during GSS-TSIG negotiation sequences.

Attack Vector

Exploitation requires network access to a vulnerable BIND server configured with GSS-TSIG features enabled. An attacker can send specially crafted DNS requests containing malicious SPNEGO tokens to the target server. The attack does not require authentication or user interaction. On 64-bit systems, the attack results in an immediate service crash due to the buffer over-read. On 32-bit systems, the buffer overflow could potentially be leveraged for remote code execution, giving attackers control over the DNS server.

The vulnerability is exposed when either tkey-gssapi-keytab or tkey-gssapi-credential configuration options are explicitly set, which is common in Samba-integrated environments and mixed Active Directory deployments.

Detection Methods for CVE-2021-25216

Indicators of Compromise

• Unexpected crashes or restarts of the named BIND daemon
• Segmentation faults in BIND logs related to SPNEGO or GSS-TSIG processing
• Unusual DNS TKEY query patterns targeting the server
• Core dumps indicating memory access violations in SPNEGO-related code paths
• Anomalous network traffic on DNS ports containing malformed SPNEGO tokens

Detection Strategies

• Monitor BIND server logs for crashes with stack traces referencing SPNEGO or GSSAPI functions
• Implement network intrusion detection rules to identify malformed GSS-TSIG negotiation attempts
• Use SentinelOne's behavioral AI to detect process crashes and anomalous memory access patterns
• Deploy DNS query logging to capture suspicious TKEY requests targeting GSS-TSIG functionality

Monitoring Recommendations

• Enable detailed logging in BIND to capture GSS-TSIG related events and errors
• Configure alerting for repeated named daemon restarts or unexpected terminations
• Implement network monitoring for DNS traffic anomalies, particularly focusing on TKEY record queries
• Review system logs for segmentation faults or memory-related errors in the named process

How to Mitigate CVE-2021-25216

Immediate Actions Required

• Update ISC BIND to version 9.11.31, 9.16.15, or later which remove the vulnerable ISC SPNEGO implementation
• If immediate patching is not possible, disable GSS-TSIG by removing tkey-gssapi-keytab and tkey-gssapi-credential configuration options
• Configure BIND to use system Kerberos libraries (MIT or Heimdal) for SPNEGO functionality instead of the ISC implementation
• Apply vendor-specific patches from Debian, NetApp, and Siemens for affected downstream products

Patch Information

ISC has released patched versions that completely remove the vulnerable SPNEGO implementation. The fix is available in BIND 9.11.31, 9.16.15, and subsequent releases. Users should upgrade to these versions or later to fully remediate the vulnerability. For Debian systems, security updates are available through DSA-4909. NetApp and Siemens have also released advisories with specific guidance for their affected products. See the ISC Knowledge Base and Zero Day Initiative Advisory ZDI-21-657 for additional technical details.

Workarounds

• Remove or comment out the tkey-gssapi-keytab configuration option from named.conf to disable the vulnerable code path
• Remove or comment out the tkey-gssapi-credential configuration option if present
• If GSS-TSIG functionality is required, configure BIND to use system-provided MIT or Heimdal Kerberos libraries for SPNEGO
• Implement network-level access controls to restrict which clients can perform GSS-TSIG negotiations

# Configuration example - Disable GSS-TSIG in named.conf
# Comment out or remove the following options to disable vulnerable code path:
# tkey-gssapi-keytab "/path/to/keytab";
# tkey-gssapi-credential "DNS/server.example.com@EXAMPLE.COM";

# Verify BIND version after patching
named -v
# Expected output should show 9.11.31, 9.16.15, or later

# Restart BIND after configuration changes
systemctl restart named