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A domain can resolve to different IP addresses over time because DNS answers can change. DNS rebinding turns that normal behavior into an attack: a name first points a victim’s browser to an attacker-controlled web server, then points the same name to an address on the victim’s private network. The browser may still treat requests as coming from the same web origin, allowing the attacker’s page to probe an internal service through the browser.
How can one domain point to different IP addresses?
DNS translates a hostname into an IP address so a client can connect to the corresponding server. A DNS answer is not necessarily permanent: a domain’s operator can change the answer, and resolvers may ask again after the answer’s time to live (TTL) expires. This is ordinary DNS behavior. Rebinding is the deliberate use of changing answers to redirect a browser between an external server and an internal address.
MITRE’s CAPEC-275 describes the basic sequence: an attacker arranges for a controlled name to return the public address of a server hosting malicious content, often with a short TTL. After the browser loads that content and looks up the name again, the attacker’s DNS service returns an address in the victim’s internal network instead. The page’s script then sends requests to that address using the same hostname. MITRE CAPEC-275
Why does DNS rebinding matter to browser security?
Browsers use the same-origin policy to limit how a page can interact with other resources. RFC 6454 defines an origin using the scheme, host, and port. When those values remain unchanged, the browser may consider requests to be within the same origin even if a new DNS lookup sends the connection to a different IP address. The name used for the request and the network destination have changed in different ways: the browser’s origin check is based on the name, while DNS determines where the connection goes.
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RFC 6454 states: “In practice, the same-origin policy relies upon the Domain Name System (DNS) for security because many commonly used URI schemes, such as http, use DNS-based naming authorities.” That reliance helps explain the risk: if the same hostname is redirected to an internal service, browser-origin protections may not provide the separation an administrator expects. RFC 6454
What can an attacker do with a successful rebinding attempt?
The attacker is trying to use a victim’s browser as a route to services that may be unreachable from the public internet. If an internal service accepts the browser’s requests and its behavior permits the attack, malicious code may be able to read or change data, trigger commands, or scan internal hosts. The precise impact depends on the target service and its defenses; a changed DNS answer alone does not guarantee access.
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For the sequence to work, the victim must load executable content from the attacker-controlled name, the browser must make a later request that results in a new resolution, and the internal target must accept or mishandle the resulting request. Stanford’s Web Security Research page summarizes the technique as subverting the same-origin policy and turning browsers into open network proxies. Stanford Web Security Research
Which defenses reduce DNS rebinding risk?
No single control is a universal fix. The useful question is where a control is enforced, which destination it restricts, and whether it protects the actual internal service path.
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| Control | Where it acts | What it constrains | Trade-off or limitation |
|---|---|---|---|
| IP pinning | Browser or client | Keeps using the recorded IP address rather than switching destinations when DNS answers change | MITRE notes a compatibility trade-off: pinning can conflict with legitimate sites. A client must enforce the behavior for it to help. |
| DNS answer filtering | DNS resolver | Blocks external names from resolving to internal addresses | Protects only clients using the filtering resolver; it does not itself validate requests at internal services. |
| HTTP Host-header validation | Application server | Rejects requests whose Host header names an unexpected host | Must be implemented by the relevant service and cover the request path being protected. |
| Firewall restrictions | Network boundary | Limits which traffic can reach sensitive internal services or default virtual hosts | Rules need to protect the actual service path and resist circumvention; a firewall is not a substitute for application checks. |
MITRE lists IP pinning, Host-header validation, and filtering DNS answers as mitigations. Stanford’s historical research page also emphasizes server-side Host-header checks for attacks against the browser and a firewall resistant to circumvention for sensitive content on default virtual hosts. It identifies dnswall as a daemon for filtering private IP addresses in DNS responses; that historical mention is not a current endorsement or status report. MITRE CAPEC-275 · Stanford Web Security Research
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What should administrators check?
- Identify internal services that trust requests based on hostnames or expose sensitive actions to browser-originated requests.
- Validate the HTTP Host header at the application server and reject unexpected hostnames.
- Use DNS resolvers that prevent public names from resolving to private internal addresses where appropriate.
- Review network rules around sensitive services, including default virtual hosts, and verify that the intended controls cannot be bypassed through another route.
- Test the complete path: the client’s DNS behavior, the resolver’s answers, the network boundary, and the application’s handling of requests.
The available standards and attack descriptions explain the mechanism and documented mitigations, but do not establish how current individual browsers, DNS providers, or routers behave by default. Administrators should verify their own clients, resolvers, and services rather than assuming a particular default is protective.
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