CVE-2021-25220: ISC BIND DNS Cache Poisoning Vulnerability

CVE-2021-25220 Overview

CVE-2021-25220 is a DNS cache poisoning vulnerability affecting ISC BIND 9, one of the most widely deployed DNS server software packages. The vulnerability allows the DNS resolver cache to become poisoned with incorrect records, which can lead to DNS queries being directed to malicious or unintended servers. This could result in false information being returned to clients, potentially enabling man-in-the-middle attacks, redirection to malicious sites, or disruption of legitimate network services.

The vulnerability affects a broad range of BIND 9 versions including 9.11.0 through 9.11.36, 9.12.0 through 9.16.26, and 9.17.0 through 9.18.0. Additionally, BIND Supported Preview Editions from 9.11.4-S1 through 9.11.36-S1 and 9.16.8-S1 through 9.16.26-S1 are impacted. ISC notes that versions earlier than those listed, going back to 9.1.0, are also believed to be affected but have not been tested as they have reached end-of-life status.

Critical Impact

DNS cache poisoning can redirect users and applications to attacker-controlled servers, enabling data interception, credential theft, and delivery of malicious content without detection by end users.

Affected Products

• ISC BIND 9.11.0 through 9.11.36, 9.12.0 through 9.16.26, 9.17.0 through 9.18.0
• BIND Supported Preview Editions 9.11.4-S1 through 9.11.36-S1 and 9.16.8-S1 through 9.16.26-S1
• Fedora 34, 35, and 36
• NetApp H-Series firmware (H300S, H500S, H700S, H300E, H500E, H700E, H410S, H410C)
• Siemens SINEC INS
• Juniper Junos OS on SRX Series devices

Discovery Timeline

• 2022-03-23 - CVE-2021-25220 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-25220

Vulnerability Analysis

This vulnerability relates to how BIND 9 handles DNS response processing and caching when operating as a forwarding resolver. The flaw is classified under CWE-444 (Inconsistent Interpretation of HTTP Requests), though in this DNS context it manifests as inconsistent interpretation of DNS responses that can lead to cache contamination.

When a BIND resolver configured with forwarders receives DNS responses, the software may improperly cache records that should be rejected or validated more strictly. This allows an attacker who can influence DNS responses—either by controlling an upstream DNS server or through network position—to inject malicious records into the resolver's cache. These poisoned cache entries are then served to clients making queries to the affected resolver.

The attack is particularly dangerous in enterprise environments where internal resolvers serve many clients, as a single cache poisoning event can affect all downstream users. The scope of impact extends beyond the vulnerable component itself, as clients relying on the poisoned resolver receive incorrect DNS information without any indication of compromise.

Root Cause

The root cause lies in insufficient validation of DNS response data when the resolver processes answers from forwarders. BIND's caching logic fails to adequately verify that response records are authoritative and legitimate for the queried domain, allowing forged or manipulated records to be stored in the cache. This is especially problematic in configurations using the forward only directive, where the resolver depends entirely on upstream forwarders for DNS resolution.

Attack Vector

An attacker can exploit this vulnerability through network-based attacks without requiring authentication. The exploitation scenario typically involves:

1. Positioning: The attacker gains ability to influence DNS responses reaching the target BIND resolver, either by compromising an upstream forwarder, performing network interception, or controlling a malicious authoritative server
2. Response Manipulation: Crafted DNS responses containing malicious record mappings are sent to the vulnerable resolver
3. Cache Injection: The resolver caches the poisoned records without proper validation
4. Client Impact: All clients querying the compromised resolver receive the attacker's forged DNS responses, potentially for an extended period based on TTL values

The attack does not require user interaction and can be performed remotely. While high privileges are needed to position the attack (controlling upstream DNS infrastructure), the impact crosses security boundaries by affecting all clients of the poisoned resolver.

Detection Methods for CVE-2021-25220

Indicators of Compromise

• Unexpected DNS resolution results where legitimate domains resolve to unfamiliar IP addresses
• Cache entries pointing to IP addresses not associated with the expected authoritative DNS servers
• Anomalous DNS response patterns from configured forwarders showing inconsistent or suspicious records
• Client systems connecting to unexpected servers after DNS lookups for known services

Detection Strategies

• Monitor DNS resolver cache contents for entries that map well-known domains to suspicious IP addresses
• Implement DNS response validation tools that compare resolver answers against authoritative sources
• Deploy network monitoring to detect DNS traffic patterns indicative of cache poisoning attempts
• Audit BIND configuration for forward only directives which increase susceptibility to this vulnerability

Monitoring Recommendations

• Enable verbose query logging on BIND resolvers to track DNS resolution patterns and identify anomalies
• Implement DNSSEC validation where possible to cryptographically verify DNS response authenticity
• Monitor resolver performance metrics for unusual cache hit/miss ratios that could indicate cache manipulation
• Set up alerts for DNS responses from forwarders containing records for domains outside expected zones

How to Mitigate CVE-2021-25220

Immediate Actions Required

• Upgrade ISC BIND to patched versions: 9.11.37, 9.16.27, 9.18.1, or later releases as specified in the ISC advisory
• Review and restrict forwarder configurations to trusted upstream DNS servers only
• Enable DNSSEC validation to provide cryptographic verification of DNS responses
• Implement network segmentation to limit exposure of internal DNS resolvers

Patch Information

ISC has released security updates to address this vulnerability. Administrators should upgrade to BIND 9.11.37, 9.16.27, 9.18.1, or later versions. For Supported Preview Editions, upgrade to 9.11.37-S1 or 9.16.27-S1. Detailed patch information and upgrade guidance is available in the ISC CVE-2021-25220 advisory.

Additional vendor-specific patches are available:

• Siemens Security Advisory SSA-637483 for SINEC INS
• Juniper Security Bulletin for Junos OS SRX Series
• NetApp Security Advisory NTAP-20220408 for affected NetApp products

Workarounds

• Configure BIND to avoid using forwarders if direct recursive resolution is feasible for your environment
• Implement strict network access controls limiting which systems can send DNS responses to your resolver
• Deploy DNS firewalling solutions to filter suspicious DNS responses before they reach the resolver
• Consider implementing split-horizon DNS to reduce attack surface for internal zone resolution

# Example: BIND configuration to enable DNSSEC validation
# Add to named.conf options block
options {
    dnssec-validation auto;
    dnssec-enable yes;
    
    # Restrict forwarders to trusted servers only
    forwarders {
        192.0.2.1;  # Trusted upstream DNS 1
        192.0.2.2;  # Trusted upstream DNS 2
    };
    
    # Limit who can query this resolver
    allow-query { 
        localhost; 
        192.168.0.0/16; 
    };
};