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Editorial review: Document __Http- and __Host-Http- cookie prefixes #40959

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Document __Http- and __Host-Http- cookie prefixes
chrisdavidmills Sep 1, 2025
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Fixes for yoavweiss review comments
chrisdavidmills Sep 1, 2025
e3b4ee4
Add security boundary line to __Host- description
chrisdavidmills Sep 1, 2025
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Fixes for bsmth review comments
chrisdavidmills Sep 1, 2025
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13 changes: 6 additions & 7 deletions files/en-us/web/api/document/cookie/index.md
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Expand Up @@ -53,12 +53,11 @@ You can also assign to this property a string of the form `"key=value"`, specify
- `;secure`: Specifies that the cookie should only be transmitted over a secure protocol.

- The cookie value string can use {{jsxref("Global_Objects/encodeURIComponent", "encodeURIComponent()")}} to ensure that the string does not contain any commas, semicolons, or whitespace (which are disallowed in cookie values).
- Some user agent implementations support the following cookie prefixes:
- `__Secure-` Signals to the browser that it should only include the cookie in requests transmitted over a secure channel.
- `__Host-` Signals to the browser that in addition to the restriction to only use the cookie from a secure origin, the scope of the cookie is limited to a path attribute passed down by the server.
If the server omits the path attribute the "directory" of the request URI is used.
It also signals that the domain attribute must not be present, which prevents the cookie from being sent to other domains.
For Chrome the path attribute must always be the origin.
- The cookie name can have a prefix that imposes specific restrictions on the cookie's attributes in supporting user-agents. All cookie prefixes start with a double-underscore (`__`) and end in a dash (`-`). The following prefixes are defined:
- **`__Secure-`**: Cookies with names starting with `__Secure-` must be set with the `Secure` attribute by a secure page (HTTPS).
- **`__Host-`**: Cookies with names starting with `__Host-` must be set with the `Secure` attribute by a secure page (HTTPS). In addition, they must not have a `Domain` attribute specified, and the `Path` attribute must be set to `/`. This guarantees that such cookies are only sent to the host that set them, and not to any other host on the domain. It also guarantees that they are set host-wide and cannot be overridden on any path on that host. This combination yields a cookie that is as close as can be to treating the origin as a security boundary.
- **`__Http-`**: Cookies with names starting with `__Http-` must be set with the `Secure` flag by a secure page (HTTPS) and in addition must have the `HttpOnly` attribute set to prove that they were set via the `Set-Cookie` header (they can't be set or modified via JavaScript features such as `Document.cookie` or the [Cookie Store API](/en-US/docs/Web/API/Cookie_Store_API)).
- **`__Host-Http-`**: Cookies with names starting with `__Host-Http-` must be set with the `Secure` flag by a secure page (HTTPS) and must have the `HttpOnly` attribute set to prove that they were set via the `Set-Cookie` header. In addition, they also have the same restrictions as `__Host-`-prefixed cookies. This combination yields a cookie that is as close as can be to treating the origin as a security boundary while at the same time ensuring developers and server operators know that its scope is limited to HTTP requests.

> [!NOTE]
> The dash is considered part of the prefix.
Expand All @@ -67,7 +66,7 @@ You can also assign to this property a string of the form `"key=value"`, specify
> These flags are only settable with the `secure` attribute.

> [!NOTE]
> As you can see from the code above, `document.cookie` is an [accessor property](/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperty#description) with native _setter_ and _getter_ functions, and consequently is _not_ a [data property](/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperty#description) with a value: what you write is not the same as what you read, everything is always mediated by the JavaScript interpreter.
> The `document.cookie` property is an [accessor property](/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperty#description) with native _setter_ and _getter_ functions, and consequently is _not_ a [data property](/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperty#description) with a value: what you write is not the same as what you read, everything is always mediated by the JavaScript interpreter.

## Examples

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16 changes: 10 additions & 6 deletions files/en-us/web/http/guides/cookies/index.md
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Expand Up @@ -113,7 +113,7 @@ document.cookie = "yummy_cookie=chocolate";
document.cookie = "tasty_cookie=strawberry";
```

You can also access existing cookies and set new values for them, provided the [`HttpOnly`](/en-US/docs/Web/HTTP/Reference/Headers/Set-Cookie#httponly) attribute isn't set on them (i.e., in the `Set-Cookie` header that created it):
You can also access existing cookies and set new values for them:

```js
console.log(document.cookie);
Expand All @@ -125,7 +125,9 @@ console.log(document.cookie);
// logs "tasty_cookie=strawberry; yummy_cookie=blueberry"
```

Note that, for security purposes, you can't change cookie values by sending an updated `Cookie` header directly when initiating a request, i.e., via {{domxref("Window/fetch", "fetch()")}} or {{domxref("XMLHttpRequest")}}. Note that there are also good reasons why you shouldn't allow JavaScript to modify cookies — i.e., set `HttpOnly` during creation. See the [Security](#security) section for more details.
For security purposes, you can't change cookie values by sending an updated `Cookie` header directly when initiating a request, for example, via {{domxref("Window/fetch", "fetch()")}} or {{domxref("XMLHttpRequest")}}.

There are good reasons why you shouldn't allow JavaScript to modify cookies at all. You can prevent JavaScript from accessing a cookie by specifying the [`HttpOnly`](/en-US/docs/Web/HTTP/Reference/Headers/Set-Cookie#httponly) attribute during its creation. See the [Security](#security) section for more details.

## Security

Expand Down Expand Up @@ -219,12 +221,14 @@ Because of the design of the cookie mechanism, a server can't confirm that a coo

An application on a subdomain can set a cookie with the `Domain` attribute, which gives access to that cookie on all other subdomains. This mechanism can be abused in a [session fixation](https://owasp.org/www-community/attacks/Session_fixation) attack.

As a [defense-in-depth measure](<https://en.wikipedia.org/wiki/Defense_in_depth_(computing)>), however, you can use _cookie prefixes_ to assert specific facts about the cookie. Two prefixes are available:
As a [defense-in-depth measure](<https://en.wikipedia.org/wiki/Defense_in_depth_(computing)>), you can use _cookie prefixes_ to impose specific restrictions on a cookie's attributes in supporting user-agents. All cookie prefixes start with a double-underscore (`__`) and end in a dash (`-`). Four prefixes are available:

- `__Host-`: If a cookie name has this prefix, it's accepted in a {{HTTPHeader("Set-Cookie")}} header only if it's also marked with the `Secure` attribute, was sent from a secure origin, does _not_ include a `Domain` attribute, and has the `Path` attribute set to `/`. In other words, the cookie is _domain-locked_.
- `__Secure-`: If a cookie name has this prefix, it's accepted in a {{HTTPHeader("Set-Cookie")}} header only if it's marked with the `Secure` attribute and was sent from a secure origin. This is weaker than the `__Host-` prefix.
- **`__Secure-`**: Cookies with names starting with `__Secure-` must be set with the `Secure` attribute by a secure page (HTTPS).
- **`__Host-`**: Cookies with names starting with `__Host-` must be set with the `Secure` attribute by a secure page (HTTPS). In addition, they must not have a `Domain` attribute specified, and the `Path` attribute must be set to `/`. This guarantees that such cookies are only sent to the host that set them, and not to any other host on the domain. It also guarantees that they are set host-wide and cannot be overridden on any path on that host. This combination yields a cookie that is as close as can be to treating the origin as a security boundary.
- **`__Http-`**: Cookies with names starting with `__Http-` must be set with the `Secure` flag by a secure page (HTTPS) and in addition must have the `HttpOnly` attribute set to prove that they were set via the `Set-Cookie` header (they can't be set or modified via JavaScript features such as {{domxref("Document.cookie")}} or the [Cookie Store API](/en-US/docs/Web/API/Cookie_Store_API)).
- **`__Host-Http-`**: Cookies with names starting with `__Host-Http-` must be set with the `Secure` flag by a secure page (HTTPS) and must have the `HttpOnly` attribute set to prove that they were set via the `Set-Cookie` header. In addition, they also have the same restrictions as `__Host-`-prefixed cookies. This combination yields a cookie that is as close as can be to treating the origin as a security boundary while at the same time ensuring developers and server operators know that its scope is limited to HTTP requests.

The browser will reject cookies with these prefixes that don't comply with their restrictions. This ensures that subdomain-created cookies with prefixes are either confined to a subdomain or ignored completely. As the application server only checks for a specific cookie name when determining if the user is authenticated or a CSRF token is correct, this effectively acts as a defense measure against [session fixation](https://owasp.org/www-community/attacks/Session_fixation).
The browser will reject cookies with these prefixes that don't comply with their restrictions. As the application server only checks for a specific cookie name when determining if the user is authenticated or a CSRF token is correct, this effectively acts as a defense measure against [session fixation](https://owasp.org/www-community/attacks/Session_fixation).

> [!NOTE]
> On the server, the web application _must_ check for the full cookie name including the prefix. User agents _do not_ strip the prefix from the cookie before sending it in a request's {{HTTPHeader("Cookie")}} header.
Expand Down
38 changes: 23 additions & 15 deletions files/en-us/web/http/reference/headers/set-cookie/index.md
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Expand Up @@ -60,21 +60,14 @@ Set-Cookie: <cookie-name>=<cookie-value>; Domain=<domain-value>; Secure; HttpOnl
- : Defines the cookie name and its value.
A cookie definition begins with a name-value pair.

A `<cookie-name>` can contain any US-ASCII characters except for: control characters ({{Glossary("ASCII")}} characters 0 up to 31 and ASCII character 127) or separator characters (space, tab and the characters: `( ) < > @ , ; : \ " / [ ] ? = { }`)
A `<cookie-name>` can contain any US-ASCII characters except for control characters ({{Glossary("ASCII")}} characters 0 up to 31 and ASCII character 127) or separator characters (space, tab and the characters: `( ) < > @ , ; : \ " / [ ] ? = { }`)

A `<cookie-value>` can optionally be wrapped in double quotes and include any US-ASCII character excluding control characters (ASCII characters 0 up to 31 and ASCII character 127), {{glossary("Whitespace")}}, double quotes, commas, semicolons, and backslashes.

**Encoding**: Many implementations perform {{Glossary("Percent-encoding", "percent-encoding")}} on cookie values.
However, this is not required by the RFC specification.
The percent-encoding does help to satisfy the requirements of the characters allowed for `<cookie-value>`.
**Encoding**: Many implementations perform {{Glossary("Percent-encoding", "percent-encoding")}} on cookie values. However, this is not required by the RFC specification. The percent-encoding does help to satisfy the requirements of the characters allowed for `<cookie-value>`.

> [!NOTE]
> Some `<cookie-name>` have a specific semantic:
>
> **`__Secure-` prefix**: Cookies with names starting with `__Secure-` (dash is part of the prefix) must be set with the `secure` flag from a secure page (HTTPS).
>
> **`__Host-` prefix**: Cookies with names starting with `__Host-` are sent only to the host subdomain or domain that set them, and not to any other host.
> They must be set with the `secure` flag, must be from a secure page (HTTPS), must not have a domain specified, and the path must be `/`.
> Some cookie names contain prefixes that impose specific restrictions on the cookie's attributes in supporting user-agents. See [Cookie prefixes](#cookie_prefixes) for more information.

- `Domain=<domain-value>` {{optional_inline}}
- : Defines the host to which the cookie will be sent.
Expand Down Expand Up @@ -163,6 +156,18 @@ Set-Cookie: <cookie-name>=<cookie-value>; Domain=<domain-value>; Secure; HttpOnl
>
> Insecure sites (`http:`) cannot set cookies with the `Secure` attribute. The `https:` requirements are ignored when the `Secure` attribute is set by localhost.

## Cookie prefixes

Some cookie names contain prefixes that impose specific restrictions on the cookie's attributes in supporting user-agents. All cookie prefixes start with a double-underscore (`__`) and end in a dash (`-`). The following prefixes are defined:

- **`__Secure-`**: Cookies with names starting with `__Secure-` must be set with the `Secure` attribute by a secure page (HTTPS).
- **`__Host-`**: Cookies with names starting with `__Host-` must be set with the `Secure` attribute by a secure page (HTTPS). In addition, they must not have a `Domain` attribute specified, and the `Path` attribute must be set to `/`. This guarantees that such cookies are only sent to the host that set them, and not to any other host on the domain. It also guarantees that they are set host-wide and cannot be overridden on any path on that host. This combination yields a cookie that is as close as can be to treating the origin as a security boundary.
- **`__Http-`**: Cookies with names starting with `__Http-` must be set with the `Secure` flag by a secure page (HTTPS) and in addition must have the `HttpOnly` attribute set to prove that they were set via the `Set-Cookie` header (they can't be set or modified via JavaScript features such as `Document.cookie` or the [Cookie Store API](/en-US/docs/Web/API/Cookie_Store_API)).
- **`__Host-Http-`**: Cookies with names starting with `__Host-Http-` must be set with the `Secure` flag by a secure page (HTTPS) and must have the `HttpOnly` attribute set to prove that they were set via the `Set-Cookie` header. In addition, they also have the same restrictions as `__Host-`-prefixed cookies. This combination yields a cookie that is as close as can be to treating the origin as a security boundary while at the same time ensuring developers and server operators know that its scope is limited to HTTP requests.

> [!WARNING]
> You cannot count on these additional assurances on browsers that don't support cookie prefixes; in such cases, prefixed cookies will always be accepted.

## Examples

### Session cookie
Expand Down Expand Up @@ -205,12 +210,11 @@ Set-Cookie: sessionId=e8bb43229de9; Domain=foo.example.com

### Cookie prefixes
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I actually missed that this section is under "Examples", so there's some small duplication, but it's also okay in this state, IMO.


Cookie names prefixed with `__Secure-` or `__Host-` can be used only if they are set with the `secure` attribute from a secure (HTTPS) origin.
Cookie names prefixed with `__Secure-` or `__Host-` can be used only if they are set with the `Secure` attribute from a secure (HTTPS) origin.

In addition, cookies with the `__Host-` prefix must have a path of `/` (meaning any path at the host) and must not have a `Domain` attribute.
Cookie names prefixed with `__Http-` or `__Host-Http-` can be used only if they are set with the `Secure` attribute from a secure (HTTPS) origin and in addition must have the `HttpOnly` attribute set to prove that they were set via the `Set-Cookie` header and not on the client-side via JavaScript.

> [!WARNING]
> For clients that don't implement cookie prefixes, you cannot count on these additional assurances, and prefixed cookies will always be accepted.
In addition, cookies with the `__Host-` or `__Host-Http-` prefix must have a path of `/` (meaning any path at the host) and must not have a `Domain` attribute.

```http
// Both accepted when from a secure origin (HTTPS)
Expand All @@ -225,6 +229,10 @@ Set-Cookie: __Host-id=1; Secure

// Rejected due to setting a Domain
Set-Cookie: __Host-id=1; Secure; Path=/; Domain=example.com

// Only settable via Set-Cookie
Set-Cookie: __Http-ID=123; Secure; Domain=example.com
Set-Cookie: __Host-Http-ID=123; Secure; Path=/
```

### Partitioned cookie
Expand All @@ -234,7 +242,7 @@ Set-Cookie: __Host-example=34d8g; SameSite=None; Secure; Path=/; Partitioned;
```

> [!NOTE]
> Partitioned cookies must be set with `Secure`. In addition, it is recommended to use the `__Host` prefix when setting partitioned cookies to make them bound to the hostname and not the registrable domain.
> Partitioned cookies must be set with `Secure`. In addition, it is recommended to use a `__Host` or `__Host-Http-` prefix when setting partitioned cookies to make them bound to the hostname and not the registrable domain.

## Specifications

Expand Down