CVE-2020-8625: ISC BIND GSS-TSIG DOS Vulnerability

CVE-2020-8625 Overview

CVE-2020-8625 is a buffer overflow vulnerability (CWE-120) affecting ISC BIND DNS servers configured to use GSS-TSIG features. While BIND's default configuration is not vulnerable, servers become exposed when explicitly setting valid values for the tkey-gssapi-keytab or tkey-gssapi-credential configuration options. This vulnerability is particularly concerning in enterprise environments where BIND is integrated with Samba or in mixed-server environments combining BIND servers with Active Directory domain controllers.

The vulnerability allows remote attackers to potentially crash the named process, causing a denial of service. While remote code execution has not been proven, ISC acknowledges it is theoretically possible, making this a critical security concern for affected organizations.

Critical Impact

Remote attackers can crash DNS servers or potentially achieve remote code execution on BIND servers configured with GSS-TSIG, affecting DNS availability in enterprise environments integrated with Active Directory or Samba.

Affected Products

• ISC BIND 9.5.0 through 9.11.27
• ISC BIND 9.12.0 through 9.16.11
• ISC BIND 9.11.3-S1 through 9.11.27-S1 (Supported Preview Edition)
• ISC BIND 9.16.8-S1 through 9.16.11-S1 (Supported Preview Edition)
• ISC BIND 9.17.0 through 9.17.1 (Development Branch)
• Debian Linux 9.0 and 10.0
• Fedora 32, 33, and 34
• Siemens SINEC Infrastructure Network Services
• NetApp Cloud Backup, A250, and 500F systems

Discovery Timeline

• February 17, 2021 - CVE-2020-8625 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-8625

Vulnerability Analysis

This vulnerability resides in the GSS-TSIG (Generic Security Service Algorithm for Secret Key Transaction) implementation within BIND. GSS-TSIG is an authentication mechanism used to secure DNS updates and zone transfers, commonly deployed in environments requiring Kerberos-based authentication between DNS servers and Active Directory or Samba domain controllers.

The buffer overflow occurs when processing specially crafted GSS-TSIG requests. The vulnerability is network-exploitable without requiring authentication or user interaction, though exploitation complexity is considered high. A successful attack could result in complete compromise of confidentiality, integrity, and availability of the affected DNS server.

Root Cause

The root cause is a classic buffer overflow (CWE-120) in the BIND code path that handles GSS-TSIG transactions. When the tkey-gssapi-keytab or tkey-gssapi-credential options are explicitly configured, the vulnerable code path becomes active. Insufficient bounds checking when processing GSS-TSIG data allows an attacker to write beyond allocated buffer boundaries, potentially corrupting adjacent memory structures.

Attack Vector

The attack is conducted remotely over the network by sending malicious DNS requests to a vulnerable BIND server. The attacker targets the GSS-TSIG handling routines by crafting specially formed TKEY resource record requests. Since this is a network-based attack requiring no authentication, any BIND server with GSS-TSIG enabled and exposed to the network is a potential target.

The most likely exploitation outcome is crashing the named process, causing DNS service disruption. However, careful exploitation could potentially achieve remote code execution by manipulating memory to redirect program flow.

Detection Methods for CVE-2020-8625

Indicators of Compromise

• Unexpected crashes of the named process with memory corruption signatures in core dumps
• Anomalous TKEY resource record requests in DNS query logs
• Unusual GSS-TSIG negotiation attempts from external or unexpected source IPs
• Multiple DNS service restarts without administrative action

Detection Strategies

• Monitor for BIND/named process crashes and analyze crash dumps for buffer overflow indicators
• Implement DNS query logging and alert on malformed or suspicious TKEY requests
• Deploy network intrusion detection signatures for GSS-TSIG exploitation attempts
• Enable BIND query logging with querylog yes; to capture detailed request information

Monitoring Recommendations

• Configure centralized logging for all BIND servers to aggregate and correlate DNS events
• Set up automated alerting for named process termination or restart events
• Monitor network traffic for unusual volumes of DNS TKEY queries
• Review system logs regularly for BIND-related error messages indicating memory issues

How to Mitigate CVE-2020-8625

Immediate Actions Required

• Verify if your BIND configuration uses tkey-gssapi-keytab or tkey-gssapi-credential options
• Update affected BIND installations to patched versions immediately
• If unable to patch, consider temporarily disabling GSS-TSIG features where operationally feasible
• Restrict network access to DNS servers to trusted sources only

Patch Information

ISC has released security updates to address this vulnerability. Organizations should upgrade to the following patched versions:

• BIND 9.11.28 or later for the 9.11.x branch
• BIND 9.16.12 or later for the 9.16.x branch
• BIND 9.11.28-S1 or later for Supported Preview Edition 9.11.x
• BIND 9.16.12-S1 or later for Supported Preview Edition 9.16.x

For detailed patch information, refer to the ISC Knowledge Base CVE-2020-8625 advisory. Linux distribution users should apply updates through their respective package managers - see the Debian Security DSA-4857 and Fedora package announcements for distribution-specific guidance.

Workarounds

• Remove or comment out tkey-gssapi-keytab and tkey-gssapi-credential configuration options if GSS-TSIG is not required
• Implement network-level access controls to restrict DNS queries to trusted clients only
• Deploy a Web Application Firewall (WAF) or DNS firewall capable of inspecting and filtering malicious DNS traffic
• Consider running BIND in a chroot environment to limit potential impact of exploitation

# Check if GSS-TSIG is configured (vulnerable configuration)
grep -E "tkey-gssapi-keytab|tkey-gssapi-credential" /etc/named.conf /etc/bind/named.conf 2>/dev/null

# If found and not needed, comment out these lines:
# tkey-gssapi-keytab "/path/to/keytab";
# tkey-gssapi-credential "DNS/server.example.com";

# Restart BIND after configuration changes
systemctl restart named
# or
rndc reconfig