Overview
In this post, we will be discussing the grave vulnerability identified as CVE-2025-40776, which affects the `named` caching resolver configured to deploy EDNS Client Subnet (ECS) options. This vulnerability has a significant impact on BIND 9 versions, thereby posing a serious threat to organizations worldwide. The severity of this vulnerability emerges from its potential to compromise entire systems or leak sensitive data, which can have devastating ramifications for businesses, particularly those handling large volumes of sensitive data.
Vulnerability Summary
CVE ID: CVE-2025-40776
Severity: High (CVSS 8.6)
Attack Vector: Network
Privileges Required: None
User Interaction: None
Impact: Potential system compromise or data leakage
Affected Products
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Product | Affected Versions
BIND 9 | 9.11.3-S1 to 9.16.50-S1
BIND 9 | 9.18.11-S1 to 9.18.37-S1
BIND 9 | 9.20.9-S1 to 9.20.10-S1
How the Exploit Works
This exploit works by sending malicious requests to a vulnerable caching resolver that has been configured to use EDNS Client Subnet (ECS) options. This, in turn, manipulates the caching mechanism of the resolver leading to incorrect data being stored and served to the clients. This form of attack is commonly known as a cache-poisoning attack, which can result in system compromise or data leakage.
Conceptual Example Code
Below is a conceptual example of a malicious DNS request that could exploit this vulnerability:
GET /dns-query?dns=AAABAAABAAAAAAAAA3d3dwdleGFtcGxlA2NvbQAAAQAB HTTP/1.1
Host: vulnerable-dns-resolver.example.com
Accept: application/dns-message
X-Forwarded-For: malicious-ip-addressIn this example, the attacker sends a GET request with a DNS query. The `X-Forwarded-For` header is used to spoof the client’s IP address, tricking the DNS resolver into thinking the request is coming from a different source. This could lead to the cache poisoning, as the resolver might end up storing and serving incorrect data.
To mitigate the risk of this vulnerability, it is recommended to apply the vendor-supplied patch or use WAF/IDS as a temporary mitigation strategy. Regular patch management and monitoring of network activity can also help in identifying any unusual patterns that may signify an attack.