CVE-2025-40778: BIND DNS Cache Poisoning Vulnerability

CVE-2025-40778 Overview

CVE-2025-40778 is a DNS cache poisoning vulnerability in ISC BIND 9 that occurs when the resolver is too lenient in accepting records from DNS responses. Under certain circumstances, an attacker can exploit this permissive behavior to inject forged data into the BIND cache, potentially redirecting DNS queries to malicious destinations and compromising the integrity of DNS resolution for affected systems.

Critical Impact
Attackers can inject forged DNS records into the cache, enabling redirection of legitimate traffic to attacker-controlled servers, facilitating phishing, malware distribution, or man-in-the-middle attacks across entire networks.

Affected Products

BIND 9 versions 9.11.0 through 9.16.50
BIND 9 versions 9.18.0 through 9.18.39
BIND 9 versions 9.20.0 through 9.20.13
BIND 9 versions 9.21.0 through 9.21.12
BIND 9 Supported Preview Edition versions 9.11.3-S1 through 9.16.50-S1
BIND 9 Supported Preview Edition versions 9.18.11-S1 through 9.18.39-S1
BIND 9 Supported Preview Edition versions 9.20.9-S1 through 9.20.13-S1

Discovery Timeline

2025-10-22 - CVE-2025-40778 published to NVD
2025-11-04 - Last updated in NVD database

Technical Details for CVE-2025-40778

Vulnerability Analysis

This vulnerability stems from insufficient validation of DNS response records by BIND 9 resolvers. When processing DNS answers, BIND fails to properly verify that returned records are legitimate and appropriately scoped to the original query. This weakness falls under CWE-349 (Acceptance of Extraneous Untrusted Data With Trusted Data), where the resolver accepts injected records alongside legitimate response data.

The network-accessible nature of DNS services means attackers do not require authentication or user interaction to exploit this vulnerability. A successful attack compromises the integrity of the DNS cache, allowing the attacker to redirect any DNS query to arbitrary IP addresses of their choosing. This can affect all clients relying on the poisoned resolver for DNS lookups.

Root Cause

The root cause is BIND's overly permissive acceptance policy when parsing DNS response records. The resolver does not adequately enforce bailiwick rules or validate that additional and authority section records are properly associated with the queried domain. This allows an attacker to include malicious records in DNS responses that are then cached by the resolver as if they were legitimate.

Attack Vector

The attack is conducted over the network against BIND resolvers. An attacker positions themselves to respond to DNS queries (either as a malicious authoritative server or through network interception) and includes forged records in the response. The vulnerable BIND resolver accepts these records without sufficient scrutiny and caches them for the configured TTL period.

Once cached, any client querying the poisoned resolver for the affected domain will receive the attacker-controlled response. This enables various attack scenarios including:

Redirecting users to phishing sites
Intercepting sensitive communications via man-in-the-middle attacks
Distributing malware through fake software update servers
Bypassing security controls that rely on DNS for policy enforcement

Technical details and proof-of-concept code are available through the GitHub Gist Exploit Code. For complete vulnerability documentation, refer to the ISC CVE-2025-40778 Advisory.

Detection Methods for CVE-2025-40778

Indicators of Compromise

Unexpected DNS record changes for critical domains in resolver cache
DNS responses containing records that differ from authoritative server data
Anomalous TTL values in cached DNS records
User reports of being redirected to unexpected websites
Security tool alerts for domain reputation changes without corresponding authoritative updates

Detection Strategies

Implement DNS response validation by cross-referencing cached records with authoritative servers periodically
Monitor for DNS cache inconsistencies using tools like dnstop or passivedns
Deploy DNSSEC validation to cryptographically verify DNS response authenticity
Analyze DNS query logs for unusual patterns or responses to domains not typically queried

Monitoring Recommendations

Enable detailed BIND query logging with querylog yes; to capture DNS resolution activity
Set up alerts for changes to high-value domain records (authentication services, banking, email providers)
Monitor resolver performance metrics for unusual cache hit/miss ratios
Implement continuous DNS integrity monitoring using external validation services

How to Mitigate CVE-2025-40778

Immediate Actions Required

Identify all BIND 9 installations in your environment running affected versions
Prioritize patching for internet-facing recursive resolvers
Review DNS cache contents for signs of compromise using rndc dumpdb -cache
Consider temporarily restricting recursive queries to trusted networks until patching is complete

Patch Information

ISC has released security updates to address this vulnerability. Administrators should upgrade BIND to the latest patched version for their respective branch. Detailed patch information and download links are available in the ISC CVE-2025-40778 Advisory. Additional discussion and context can be found on the Openwall OSS-Security Mailing List.

Workarounds

Enable DNSSEC validation if not already active to reject unsigned or improperly signed responses
Reduce the attack surface by restricting recursive queries using allow-recursion ACLs
Implement Response Rate Limiting (RRL) to mitigate injection attempts
Consider deploying DNS firewall solutions to filter known malicious response patterns

bash

# Configuration example - Enable DNSSEC validation and restrict recursion
# Add to named.conf options block:

options {
    dnssec-validation auto;
    allow-recursion { trusted-networks; };
    allow-query-cache { trusted-networks; };
};

acl trusted-networks {
    10.0.0.0/8;
    192.168.0.0/16;
    172.16.0.0/12;
    localhost;
};