DOM clobbering is a browser security issue in which named HTML elements collide with properties that JavaScript expects to find on objects such as window or document. The browser may return an element or collection where application code expects a configuration value, so the code can behave unexpectedly or become vulnerable. It does not rewrite every JavaScript variable, and it does not automatically create cross-site scripting (XSS).
How can an HTML element change what JavaScript receives?
Browsers expose some elements by their id or name through named properties on browser objects. If an application reads a property such as window.redirectTo, an element with a matching name may affect what that lookup returns. The value can be an element or a collection rather than the application value the code expected. OWASP describes this browser behavior and its security implications in its DOM Clobbering Prevention Cheat Sheet.
The key condition is the application’s lookup: ordinary JavaScript variables are not all reassigned simply because matching markup exists. Risk arises when code relies on a clobberable global or DOM property and then trusts the returned value.
When does DOM clobbering become a vulnerability?
An attacker needs a way to influence HTML that reaches the page, and the application must use a colliding property unsafely. This can matter even when filtering blocks script tags, because the injected markup can be non-script HTML. PortSwigger explains the pattern in its DOM clobbering guide.
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Redirects and script URLs
OWASP illustrates code that uses window.redirectTo || '/profile/' as a navigation destination. A matching named element can provide a value that changes the destination. In another pattern, a clobbered configuration property influences the URL used to create a script element. PortSwigger describes a related case in which repeated anchor IDs expose a collection and a named child property is then used as a script URL. These examples show why values from named properties must not be treated as trusted configuration.
Interference with DOM filtering
DOM clobbering can also disrupt code that manipulates or filters markup. PortSwigger describes a form-based example where an injected input named attributes interferes with code expecting a form’s attributes collection. The broader lesson is that security-sensitive filtering should verify the type and expected behavior of DOM properties it uses.
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What can an attacker achieve?
Consequences depend on the affected code path. A collision may cause unexpected behavior or navigation; in a vulnerable data flow, it can contribute to script execution. The presence of DOM clobbering alone does not establish that a page has XSS: the attacker needs a reachable HTML injection point and useful application behavior that trusts the clobbered value. OWASP and PortSwigger document these risks without establishing a general prevalence rate.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to prevent DOM clobbering
Sanitize untrusted HTML
Sanitize HTML before inserting it into the DOM. OWASP recommends DOMPurify or the Sanitizer API. DOMPurify’s default SANITIZE_DOM setting addresses collisions with built-in APIs and properties; setting SANITIZE_NAMED_PROPS: true also isolates custom names by prefixing them with user-content-. Choose settings that preserve the legitimate IDs and names your feature needs.
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If you use the Sanitizer API, configure it to block id and name attributes when that suits the feature. OWASP cautions that the API’s default configuration does not itself prevent DOM clobbering. Check support in the browsers your application targets before relying on the API.
Keep sensitive state out of named globals
Store security-sensitive configuration in local lexical variables or encapsulated state rather than looking it up through window or document. Explicit let and const declarations help prevent accidental globals, but they do not protect a separate property such as window.NAME if code still reads that property.
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Validate values where they are used
Before using a value from window, document, or a DOM property in a sensitive operation, check that it has the expected type and interface. For example, code expecting a particular DOM object should reject an unexpected element or collection instead of treating it as configuration or a trusted URL.
Use CSP as an additional layer
A Content Security Policy (CSP) can restrict some attempts to load a new script, but it does not correct unsafe use of a value by code that is already running. Combine CSP with safe data flow and HTML sanitization rather than treating it as a complete fix.
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Which defense should you use?
| Defense | Where it helps | Important limit |
|---|---|---|
| HTML sanitization | At the point untrusted markup enters the DOM; can remove or isolate risky names. | Configuration must account for legitimate uses of id and name; defaults vary by tool. |
| Safer scoping and encapsulated state | In application code, by keeping sensitive values out of clobberable named globals. | Does not protect code that separately reads a named window or document property. |
| Type and interface validation | At the point a value is used, by rejecting unexpected elements, collections, or other values. | Each sensitive use site needs an appropriate check. |
| CSP | Can restrict certain paths that attempt to load scripts. | Does not prevent every misuse of values by already-running code. |
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