CVE-2024-4076: BIND 9 Denial of Service Vulnerability

CVE-2024-4076 Overview

CVE-2024-4076 is a denial of service vulnerability affecting ISC BIND 9, one of the most widely deployed DNS server software solutions. The vulnerability occurs when client queries trigger the serving of stale data while simultaneously requiring lookups in local authoritative zone data. This specific combination of operations can cause an assertion failure, leading to an unexpected termination of the DNS server process.

Critical Impact

Successful exploitation allows remote attackers to crash BIND 9 DNS servers without authentication, causing service disruption for all dependent network services and potentially impacting entire organizational DNS infrastructure.

Affected Products

• BIND 9 versions 9.16.13 through 9.16.50
• BIND 9 versions 9.18.0 through 9.18.27
• BIND 9 versions 9.19.0 through 9.19.24
• BIND 9 versions 9.11.33-S1 through 9.11.37-S1 (Supported Preview Edition)
• BIND 9 versions 9.16.13-S1 through 9.16.50-S1 (Supported Preview Edition)
• BIND 9 versions 9.18.11-S1 through 9.18.27-S1 (Supported Preview Edition)

Discovery Timeline

• 2024-07-23 - CVE-2024-4076 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2024-4076

Vulnerability Analysis

This vulnerability is classified under CWE-617 (Reachable Assertion), indicating that the software contains an assertion that can be triggered by a remote attacker. In BIND 9, assertions are used to verify internal program state and assumptions during execution. When an assertion fails, the program immediately terminates to prevent undefined behavior.

The vulnerability manifests when two specific conditions occur simultaneously: the DNS server is configured to serve stale data (a feature that allows DNS servers to return expired cached data when authoritative servers are unreachable), and an incoming query requires lookups in local authoritative zone data. The interaction between these two code paths triggers a logic error that causes an assertion check to fail.

The network-accessible nature of this vulnerability means that any remote attacker capable of sending DNS queries to a vulnerable BIND 9 server can potentially exploit this flaw. No authentication or special privileges are required to trigger the condition, making it particularly dangerous for internet-facing DNS servers.

Root Cause

The root cause is an assertion failure (CWE-617) that occurs due to improper handling of concurrent operations involving stale data serving and local authoritative zone lookups. The assertion mechanism in BIND 9 is designed to catch programming errors during development, but in this case, an edge condition can be triggered by carefully crafted client queries.

When the stale data serving feature (stale-answer-enable) is active and a query arrives that requires both accessing stale cache data and performing lookups against locally configured authoritative zones, the internal state machine enters an unexpected state that violates the programmed assertions.

Attack Vector

The attack vector is network-based, requiring the attacker to send specially crafted DNS queries to a vulnerable BIND 9 server. The exploitation requires the following conditions:

The attacker must be able to send DNS queries to the target server (either directly if the server is public, or from within the network if the server is internal). The target server must have stale data serving enabled, which is a common configuration for improving DNS reliability. The server must also have local authoritative zone data configured, which is typical for organizations hosting their own DNS zones.

The attack does not require any user interaction or authentication. An attacker can repeatedly exploit this vulnerability to maintain a denial of service condition, as each successful exploitation will crash the named daemon. While most BIND deployments include automatic restart mechanisms, the repeated crashes can still cause significant service degradation and potential data loss in DNS query logs.

Detection Methods for CVE-2024-4076

Indicators of Compromise

• Unexpected named process terminations or restarts in system logs
• Assertion failure messages in BIND logs containing references to stale data handling or zone lookups
• Increased DNS query failures reported by clients during attack windows
• Unusual patterns of DNS queries targeting specific zone combinations

Detection Strategies

• Monitor BIND log files for assertion failure messages, particularly those related to stale or auth code paths
• Implement DNS query rate limiting and anomaly detection to identify potential exploitation attempts
• Configure alerting on unexpected named service restarts or crashes
• Deploy network monitoring to detect unusual DNS query patterns from single sources

Monitoring Recommendations

• Enable detailed BIND logging with appropriate severity levels to capture assertion failures
• Implement centralized log aggregation for all DNS servers to correlate potential attack patterns
• Set up automated alerting when DNS server availability drops below expected thresholds
• Monitor system-level metrics for unexpected process terminations of the named service

How to Mitigate CVE-2024-4076

Immediate Actions Required

• Upgrade BIND 9 to patched versions: 9.16.51, 9.18.28, 9.20.0, or later
• For Supported Preview Editions, upgrade to 9.16.51-S1, 9.18.28-S1, or later
• If immediate patching is not possible, consider temporarily disabling stale data serving as a workaround
• Review DNS server configurations to identify all affected BIND 9 installations in your environment

Patch Information

ISC has released security patches addressing this vulnerability. Organizations running affected versions should upgrade to the latest stable release. Detailed patch information and upgrade instructions are available in the ISC Knowledge Base CVE-2024-4076. Additional vendor advisories are available from NetApp Security Advisory NTAP-20240731-0001 for systems incorporating BIND.

Further technical discussion and community updates can be found in the Openwall OSS-Security Mailing List Post and subsequent Openwall OSS-Security Mailing List Update.

Workarounds

• Disable the stale answer feature by setting stale-answer-enable no; in the BIND configuration
• Implement network-level access controls to restrict which hosts can send DNS queries to vulnerable servers
• Deploy redundant DNS infrastructure to maintain availability during potential exploitation attempts
• Consider temporary migration to unaffected DNS software while patching is scheduled

# Temporary workaround - Disable stale answer feature in named.conf
# Add or modify the following option in your BIND configuration:
options {
    stale-answer-enable no;
};
# Then reload the configuration:
# rndc reconfig