DNS Cache Poisoning

  • Fabian Sinner
  • March 22, 2024

Table of content

    DNS Cache Poisoning

    DNS cache poisoning, also known as DNS spoofing, is a form of cyberattack in which an attacker injects data into the DNS resolver cache to trick the resolver into returning a false IP address. When users then want to visit a website, they are redirected to a fraudulent page instead.

    How does DNS cache poisoning work?

    DNS cache poisoning works by exploiting vulnerabilities in the DNS protocol to inject false entries into the DNS cache of a server or resolver client. The attack aims to trick the resolver into returning a false IP address for a request, unknowingly redirecting users to a site controlled by the attacker.

    It all starts when a user wants to visit a website. Their device sends a request to the DNS resolver to get the IP address of the requested domain. The attacker listens for requests to the DNS resolver or initiates a request from the user themselves; for example, by clicking on a link or opening an infected email. The attacker then prepares fake responses to send to the resolver.

    Before the legitimate DNS server can respond, the attacker sends a flood of fake responses to the resolver. These responses are designed to look like they come from the real DNS server. However, they contain fake IP addresses that point to servers controlled by the attacker.

    If the resolver does not correctly check which server an answer comes from and considers the fake answer to be valid, it saves the false IP address in the cache. Future requests for the same domain will then be answered with the fake IP address from the cache.

    The user is redirected to the wrong IP address and ends up on a fraudulent website controlled by the attacker. This site can be used for phishing, malware distribution, or other malicious purposes.

    As long as the fake entry remains in the resolver’s cache, all users making the same request will be redirected to the fraudulent website. This will continue until the cache entry expires or the cache is purged.

    Why is DNS cache poisoning so dangerous?

    A successful attack can affect a large number of users. Once a DNS cache record is poisoned, all users making requests through that cache will be redirected to the wrong address. This can affect thousands or even millions of users, depending on how widespread the poisoned DNS resolver is.

    It is often difficult for the end user to recognize DNS cache poisoning. The fake websites can look very similar to the real ones, causing users to unknowingly reveal sensitive information such as login details, personal information, or credit card details.

    Attackers can use DNS cache poisoning to carry out a wide range of malicious activities, including phishing, malware distribution, Distributed Denial of Service (DDoS) attacks by redirecting traffic, and man-in-the-middle attacks, where the attacker intercepts communications between the user and the targeted website.

    Once poisoned, cache entries can remain in the cache for the duration of their TTL (time-to-live), meaning that the effects of the attack can persist even after the original attack has been stopped. In some cases, this can last for hours or even days.

    DNS cache poisoning can weaken trust in the Internet and its core infrastructure. It can have serious reputational consequences for companies whose names are misused, and it can weaken overall trust in online transactions and communications.

    Defending against DNS cache poisoning requires the implementation of security measures such as DNSSEC, which can be both technically complex and resource-intensive. Not all organizations or ISPs are willing or able to implement these measures, leaving their networks vulnerable.

    How can you protect yourself from those attacks?

    To protect against DNS cache poisoning, individuals and organizations can take measures from their respective sides. These measures are aimed at strengthening the security of the DNS system and reducing the likelihood of successful attacks.

    • Implement DNSSEC (Domain Name System Security Extensions)

    DNSSEC provides a way to verify the authenticity of DNS responses. It uses digital signatures to ensure that the data returned by the DNS resolver has not been tampered with. The use of DNSSEC can significantly improve the security of DNS queries.

    • Perform validation on the resolver

    DNS resolvers can be configured to validate the authenticity of DNS responses. This can be achieved by using software that supports DNSSEC validation. Validation helps to ensure that the data comes from a trusted source.

    • Minimize cache poisoning vulnerabilities

    DNS server software should always be kept up to date to close known vulnerabilities that could be exploited by attackers. Regular updates and patches are crucial to ensure security.

    • Use cryptographic measures

    Techniques can be used to improve the security of communication between DNS servers and resolvers, such as the use of TLS (Transport Layer Security) for DNS queries (known as DNS-over-TLS) or HTTPS (DNS-over-HTTPS). These methods encrypt the data traffic, making it more difficult for attackers to manipulate the communication.

    • Limit the source rate

    Limiting the rate at which responses are accepted from unknown or untrusted DNS servers can limit an attacker’s ability to inject spoofed DNS responses into the cache.

    • Network monitoring and analysis

    Network behavior should be actively monitored for anomalies that could indicate a DNS cache poisoning attack, such as unexpected DNS queries or traffic patterns. Advanced security systems can help identify and alert to suspicious activity.

    • Create awareness

    Employees and end users should be educated on the risks of DNS-based attacks and cybersecurity best practices. Awareness can help minimize the risk of phishing and other fraudulent tactics often used in connection with DNS cache poisoning.

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