CVE-2025-40776: BIND 9 Cache Poisoning Vulnerability

CVE-2025-40776 Overview

A DNS cache-poisoning vulnerability exists in BIND 9 caching resolvers configured to send EDNS Client Subnet (ECS) options. When ECS is enabled, attackers may exploit the improper acceptance of data from remote sources (CWE-349) to inject malicious DNS records into the resolver's cache, potentially redirecting traffic to attacker-controlled servers.

Critical Impact

Attackers can poison DNS caches of vulnerable BIND 9 resolvers, enabling large-scale phishing attacks, traffic interception, and man-in-the-middle scenarios affecting all clients relying on the compromised resolver.

Affected Products

• BIND 9 versions 9.11.3-S1 through 9.16.50-S1
• BIND 9 versions 9.18.11-S1 through 9.18.37-S1
• BIND 9 versions 9.20.9-S1 through 9.20.10-S1

Discovery Timeline

• July 16, 2025 - CVE-2025-40776 published to NVD
• July 16, 2025 - Last updated in NVD database

Technical Details for CVE-2025-40776

Vulnerability Analysis

This vulnerability stems from improper handling of EDNS Client Subnet (ECS) options in BIND 9's caching resolver implementation. ECS is a DNS extension that allows recursive resolvers to include partial client IP address information in queries to authoritative nameservers, enabling geographically-optimized responses.

The flaw exists in how the resolver processes and caches responses when ECS is configured. Due to insufficient validation of incoming DNS responses in relation to ECS data, an attacker can exploit this weakness to inject forged DNS records into the resolver's cache. This is classified under CWE-349 (Acceptance of Extraneous Untrusted Data With Trusted Data), indicating that the resolver improperly accepts malicious data alongside legitimate DNS response data.

Successful exploitation allows attackers to redirect DNS queries to arbitrary IP addresses without requiring authentication or user interaction. The attack can be executed remotely over the network, making it particularly dangerous for public-facing resolvers.

Root Cause

The root cause lies in the BIND 9 resolver's failure to properly validate the relationship between ECS scope information and cached DNS responses. When processing DNS responses containing ECS data, the resolver does not adequately verify that the response data corresponds to the original query context, allowing attackers to craft specially-formed responses that bypass validation checks and persist malicious records in the cache.

Attack Vector

The attack is network-based and requires no privileges or user interaction. An attacker must be positioned to intercept or inject DNS traffic between the vulnerable resolver and authoritative nameservers. The attack works by sending crafted DNS responses that exploit the ECS handling weakness, causing the resolver to cache poisoned records that will be served to all subsequent clients querying for the affected domains.

The vulnerability affects cache integrity without impacting confidentiality or availability directly, but the integrity impact is significant as poisoned cache entries can redirect traffic across an entire network or organization relying on the compromised resolver.

Detection Methods for CVE-2025-40776

Indicators of Compromise

• Unexpected DNS resolution results for known domains returning different IP addresses than expected
• Anomalous DNS response patterns with mismatched ECS scope data in resolver logs
• Sudden increases in DNS cache entries with unusual TTL values or scope configurations
• Client reports of being redirected to suspicious or unexpected websites

Detection Strategies

• Monitor BIND 9 query logs for DNS responses with inconsistent ECS scope parameters
• Implement DNS response validation by comparing resolver answers against known authoritative records
• Deploy network intrusion detection systems (IDS) rules to identify suspicious DNS traffic patterns targeting ECS-enabled resolvers
• Configure DNS monitoring tools to alert on unexpected cache entry modifications

Monitoring Recommendations

• Enable detailed query logging on BIND 9 resolvers to capture ECS-related response data
• Implement DNSSEC validation where possible to detect tampered DNS responses
• Deploy SentinelOne Singularity platform for endpoint visibility to detect malicious redirections resulting from cache poisoning
• Establish baseline DNS resolution metrics and alert on significant deviations

How to Mitigate CVE-2025-40776

Immediate Actions Required

• Identify all BIND 9 resolvers in your environment running affected versions (9.11.3-S1 through 9.16.50-S1, 9.18.11-S1 through 9.18.37-S1, 9.20.9-S1 through 9.20.10-S1)
• Review the ISC CVE-2025-40776 Advisory for vendor-provided guidance and patches
• Assess whether ECS is required for your deployment and consider disabling it as a temporary mitigation
• Flush DNS caches on potentially affected resolvers after applying mitigations

Patch Information

ISC has released security advisories addressing this vulnerability. Administrators should consult the ISC Knowledge Base for specific patch versions and upgrade instructions. Upgrade to the latest patched version of BIND 9 for your respective release branch immediately.

Workarounds

• Disable ECS functionality in BIND 9 configuration if not required for your environment by removing or commenting out ECS-related options in named.conf
• Implement network-level filtering to restrict DNS traffic to trusted sources only
• Enable DNSSEC validation to provide an additional layer of integrity verification for DNS responses
• Consider deploying a secondary DNS validation layer to cross-check resolver responses

# Configuration example - Disable ECS in BIND 9 named.conf
# Add to options block to disable sending ECS to upstream servers
options {
    send-client-subnet no;
    ecs-forward no;
};