CVE-2020-8623: ISC BIND DNS Server DoS Vulnerability

CVE-2020-8623 Overview

CVE-2020-8623 is a denial of service vulnerability affecting ISC BIND, the widely deployed DNS server software. The vulnerability exists in BIND installations that were built with the --enable-native-pkcs11 configuration option and are actively signing zones using RSA keys. An attacker who can send specially crafted query packets to a vulnerable BIND server can trigger a crash, resulting in service disruption for DNS resolution.

The vulnerability is classified under CWE-617 (Reachable Assertion), indicating that the flaw involves an assertion that can be triggered by external input, causing the application to terminate unexpectedly. This type of vulnerability is particularly impactful in DNS infrastructure where availability is critical.

Critical Impact

Remote attackers can crash vulnerable BIND DNS servers through network-accessible query packets, potentially disrupting DNS resolution services for entire organizations or networks.

Affected Products

• ISC BIND 9.10.0 through 9.11.21
• ISC BIND 9.12.0 through 9.16.5
• ISC BIND 9.17.0 through 9.17.3
• ISC BIND 9.10.5-S1 through 9.11.21-S1 (Supported Preview Edition)
• Fedora 31 and 32
• openSUSE Leap 15.1 and 15.2
• Debian Linux 9.0 and 10.0
• Canonical Ubuntu Linux 16.04 ESM, 18.04 LTS, and 20.04 LTS
• Synology DNS Server
• NetApp SteelStore Cloud Integrated Storage

Discovery Timeline

• 2020-08-21 - CVE-2020-8623 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-8623

Vulnerability Analysis

This vulnerability is a reachable assertion flaw that affects BIND DNS servers under specific configuration conditions. For a system to be vulnerable, three conditions must be met simultaneously: the BIND installation must have been compiled with native PKCS#11 support enabled (--enable-native-pkcs11), the server must be actively signing one or more DNS zones with RSA keys, and the server must be network-accessible to potential attackers.

The flaw resides in the cryptographic signing operations related to PKCS#11 and RSA key handling. When a malformed or specially crafted DNS query is processed, it triggers an assertion failure in the BIND code path responsible for cryptographic operations, causing the named daemon to terminate abnormally.

The network-based attack vector with low complexity requirements makes this vulnerability particularly concerning for organizations running DNSSEC with hardware security modules or software-based PKCS#11 implementations. The vulnerability only affects availability, with no impact on confidentiality or integrity of DNS data.

Root Cause

The root cause of CVE-2020-8623 is a reachable assertion (CWE-617) in the BIND DNS server code. Specifically, the vulnerability exists in the code path that handles PKCS#11-based RSA signing operations. When processing certain query packets, the code reaches an assertion condition that was not properly guarded against malicious or unexpected input.

Assertions are typically used as debugging tools to verify internal program state and are expected to never be triggered during normal operation. However, when an assertion can be reached through external input (a "reachable assertion"), it becomes a denial of service vector since assertion failures cause immediate program termination.

Attack Vector

The attack vector for CVE-2020-8623 is network-based, requiring the attacker to send specially crafted DNS query packets to a vulnerable BIND server. The attack requires:

1. Network Access: The attacker must be able to reach the DNS server on its listening port (typically UDP/TCP port 53)
2. Vulnerable Configuration: The target must run BIND compiled with --enable-native-pkcs11
3. Active RSA Zone Signing: The server must be signing at least one zone with an RSA key

When these conditions are met, a crafted query can trigger the assertion failure in the PKCS#11 RSA signing code path. The attack does not require authentication, user interaction, or specialized privileges, making it relatively straightforward to execute once a vulnerable target is identified.

The vulnerability mechanism involves sending malformed DNS queries that exploit weaknesses in how BIND handles cryptographic operations during zone signing. When the vulnerable code path processes the crafted input, it fails an internal assertion check, causing the named daemon to crash. For detailed technical information, refer to the ISC CVE-2020-8623 Documentation.

Detection Methods for CVE-2020-8623

Indicators of Compromise

• Unexpected crashes or restarts of the BIND named daemon without apparent cause
• Core dumps or crash logs indicating assertion failures in PKCS#11 or RSA-related functions
• Log entries showing abnormal termination of the named process
• Unusual DNS query patterns preceding service outages, particularly malformed or unexpected query types

Detection Strategies

• Monitor BIND logs for assertion failure messages and abnormal termination events
• Deploy network-based intrusion detection rules to identify malformed DNS query patterns targeting BIND servers
• Implement process monitoring to detect unexpected named daemon restarts or crashes
• Conduct vulnerability scanning to identify BIND servers compiled with --enable-native-pkcs11 that are running affected versions

Monitoring Recommendations

• Configure automated alerting for BIND process crashes or assertion failures
• Monitor DNS service availability metrics and establish baseline response time patterns
• Review system logs for core dump generation events associated with the named process
• Implement DNS query logging to capture traffic patterns during security incidents for forensic analysis

How to Mitigate CVE-2020-8623

Immediate Actions Required

• Identify all BIND installations in your environment that were built with --enable-native-pkcs11 support
• Prioritize patching DNS servers that are actively signing zones with RSA keys
• Implement network segmentation to limit exposure of vulnerable BIND servers to untrusted networks
• Enable enhanced logging on BIND servers to capture potential exploitation attempts

Patch Information

ISC has released patches addressing this vulnerability. Organizations should upgrade to patched versions of BIND as documented in the ISC security advisory. Multiple Linux distributions have also released security updates:

• Debian: Updates available via DSA-4752 and Debian LTS announcement
• Ubuntu: Updates available via USN-4468-1
• Fedora: Updates available via Fedora package announcements
• openSUSE: Updates available via openSUSE security announcements
• Gentoo: Updates available via GLSA 202008-19
• Synology: Updates available via Synology Security Advisory
• NetApp: Review the NetApp Security Advisory for affected products

Workarounds

• If patching is not immediately possible, consider temporarily disabling PKCS#11-based zone signing and switching to software-based signing methods
• Implement strict network access controls to limit which hosts can send DNS queries to vulnerable servers
• Deploy DNS proxy or filtering solutions in front of vulnerable BIND servers to detect and block malicious query patterns
• Consider temporarily taking affected servers out of production if they are not critical and cannot be patched or protected

# Verify BIND compilation options to check for PKCS#11 support
named -V | grep -i pkcs11

# Check BIND version
named -v

# Review zone signing configuration
grep -r "dnssec-signzone\|auto-dnssec" /etc/bind/