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DNS cache poisoning is an attack that puts false DNS records in a resolver’s cache. The resolver can then give users or systems an incorrect address for a legitimate domain, potentially sending them to an attacker-controlled or compromised destination.
What DNS cache poisoning means
DNS resolvers cache answers so they can reuse DNS information rather than look it up every time. In cache poisoning, a resolver accepts false DNS data and stores it. Later requests that use the poisoned cache may receive the incorrect answer.
Some guidance uses “DNS spoofing” for this attack. “DNS hijacking” is broader and can describe other ways of subverting DNS resolution. Terminology is not consistent across all sources, so this article uses cache poisoning specifically for false data stored and reused by a resolver. FIRST’s guidance describes the core issue as corrupting a resolver’s cache so it returns incorrect responses, often to redirect users or machines.
How DNS cache poisoning works
A caching resolver sends a DNS query and checks incoming responses against the query and protocol rules. In a forged-response attack, an adversary tries to get a false answer accepted before the legitimate reply arrives. The forged response must match the outstanding query closely enough to be accepted. MITRE CAPEC-142 describes crafted replies matching a transaction ID and arriving before the authorized answer.
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The 2008 Kaminsky vulnerability highlighted the risk of relying on a 16-bit DNS Query ID to match replies to queries. That is historical context, not a current list of vulnerable software or a patch guide: administrators should maintain supported resolver software and follow its vendor’s current security guidance. ISC’s advisory for CVE-2008-1447 explains the historical weakness.
What a poisoned DNS cache can do
A poisoned resolver can return the wrong address for a real domain. A client that relies on that answer may connect to a malicious or compromised host. If an upstream resolver is affected, inaccurate data may also reach downstream resolvers that request the affected records. The result can enable phishing or divert legitimate traffic, as described in RFC 7873.
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Redirection does not by itself mean that HTTPS has been defeated or that credentials will necessarily be stolen. What happens after a connection is redirected depends on the destination and the application’s other security checks.
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How DNSSEC, DNS Cookies, and resolver hardening differ
These defenses address different parts of the problem; they are most useful as complementary controls rather than substitutes for one another.
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| Defense | What it helps protect | Coverage and role |
|---|---|---|
| DNSSEC validation | Authenticates DNS data and supports authenticated denial of existence. | A validating resolver can reject data that fails validation when the relevant zone is signed and the trust chain is valid. It does not authenticate unsigned data or prevent every kind of compromise. See RFC 3833 for threat background. |
| DNS Cookies | Helps authenticate DNS transactions and deter certain forged off-path replies. | Benefits depend on support by both sides and are limited and complementary. RFC 7873 says, “With the use of DNS Cookies, a resolver can generally reject such forged replies.” |
| Query unpredictability | Makes it harder to guess the details needed to forge an acceptable response. | Standards-based measures, including unpredictable query parameters and source-port variation, improve resilience but do not provide DNSSEC’s cryptographic validation of signed data. See RFC 5452. |
| Supported, patched resolver software | Reduces exposure to known implementation weaknesses. | Keep the resolver on a supported release and follow current guidance for that specific implementation. ISC’s 2008 advisory is historical and its old version instructions should not be used as current deployment advice. |
How to investigate a suspicious DNS answer
- Preserve the evidence. Record the resolver’s response and relevant logs before changing cache state, so the observed answer can be reviewed.
- Compare with authoritative data. Check the suspicious result against an authoritative answer for the same name, accounting for the possibility of legitimate differences such as caching or configuration.
- Check DNSSEC validation. Review whether validation succeeded or failed and whether the relevant zone is signed. A failure is a useful signal, but it can also result from domain misconfiguration.
- Investigate before treating it as an attack. A mismatch or validation failure alone does not prove poisoning. Incorrect data can persist in caches until they are flushed; use the resolver vendor’s current instructions for any cache-clearing action because procedures differ by product.
FIRST’s practical guidance likewise cautions that DNSSEC validation failures can have non-malicious causes.
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