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DNS rebinding is a form of computer attack. In this attack, a malicious web page causes visitors to run a client-side script that attacks machines elsewhere on the network. In theory, the same-origin policy prevents this from happening: client-side scripts are only allowed to access content on the same host that served the script. Comparing domain names is an essential part of enforcing this policy, so DNS rebinding circumvents this protection by abusing the Domain Name System (DNS).

This attack can be used to breach a private network by causing the victim's web browser to access machines at private IP addresses and return the results to the attacker. It can also be employed to use the victim machine for spamming, distributed denial-of-service attacks or other malicious activities.


How DNS rebinding worksEdit

The attacker registers a domain (such as and delegates it to a DNS server under the attacker's control. The server is configured to respond with a very short time to live (TTL) record, preventing the response from being cached. When the victim browses to the malicious domain, the attacker's DNS server first responds with the IP address of a server hosting the malicious client-side code. For instance, they could point the victim's browser to a website that contains malicious JavaScript or Flash scripts that are intended to execute on the victim's computer.

The malicious client-side code makes additional accesses to the original domain name (such as These are permitted by the same-origin policy. However, when the victim's browser runs the script it makes a new DNS request for the domain, and the attacker replies with a new IP address. For instance, they could reply with an internal IP address or the IP address of a target somewhere else on the Internet.


The following techniques attempt to prevent DNS rebinding attacks:[1][2][3][4]

  • Web browsers can implement DNS pinning:[5] the IP address is locked to the value received in the first DNS response. This technique may block some legitimate uses of Dynamic DNS, and may not work against all attacks. However, it is important to fail safe (stop rendering) if the IP address does change, because using an IP address past the TTL expiration can open the opposite vulnerability when the IP address has legitimately changed and the expired IP address may now be controlled by an attacker.
  • Private IP addresses can be filtered out of DNS responses.
    • External public DNS servers with this filtering e.g. OpenDNS.[6]
    • Local sysadmins can configure the organization's local nameservers to block the resolution of external names into internal IP addresses. This has the downside of allowing an attacker to map the internal address ranges in use.
    • DNS filtering in a firewall or daemon e.g. dnswall.[7][8]
  • Web servers can reject HTTP requests with an unrecognized Host header.
  • The Firefox NoScript extension provides partial protection (for private networks) using its ABE feature, which blocks web traffic from external addresses to local addresses.

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