The HTTP Sec-From Header
draft-abarth-origin-01

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Abstract

This document defines the HTTP Sec-From header. The Sec-From header is added by the user agent to describe the security contexts that caused the user agent to initiate an HTTP request. HTTP servers can use the Sec-From header to mitigate against Cross-Site Request Forgery (CSRF) vulnerabilities.

Table of Contents

1.  Introduction
2.  Origin
3.  Comparing Origins
4.  Serializing Origins
    4.1.  Unicode Serialization of an Origin
    4.2.  ASCII Serialization of an Origin
5.  User Agent Behavior
6.  HTTP Server Behavior
7.  Privacy Considerations
8.  Security Considerations
9.  IANA Considerations
10.  TODO
§  Authors' Addresses

1.  Introduction

This document describes the HTTP Sec-From header. The Sec-From header identifies the security contexts that caused the user agent to initiate an HTTP request. HTTP servers can mitigate cross-site request forgery vulnerabilies by accepting requests only if the Sec-From header contains only white-listed origins.

TODO: Discuss other CSRF defenses.

2.  Origin

The following algoritm MUST be used to compute the origin of a URI.

1. Let /uri/ be the URI for which the origin is being determined.
2. Parse /uri/.
3. If /uri/ does not use a server-based naming authority, or if parsing /uri/ failed, or if /uri/ is not an absolute URI, then return an implementation-defined value.
4. Let /scheme/ be the scheme component of /uri/, converted to lowercase.
5. If the implementation doesn't support the protocol given by /scheme/, then return an implementaion-defined value.
6. If /scheme/ is "file", then the implementation MAY return a implementation-defined value.
7. Let /host/ be the host component of /uri/.
8. Apply the IDNA ToASCII algorithm to /host/, with both the AllowUnassigned and UseSTD3ASCIIRules flags set. Let /host/ be the result of the ToASCII algorithm.
9. If ToASCII fails to convert one of the components of the string (e.g. because it is too long or because it contains invalid characters), then return an implementation-defined value.
10. Let /host/ be the result of converting /host/ to lowercase.
11. If there is no port component of /uri/, then let /port/ be the default port for the protocol given by /scheme/. Otherwise, let /port/ be the port component of /uri/.
12. Return the tuple (/scheme/, /host/, /port/).

Implementations MAY define other types of origins in addition to the scheme/host/port tuple type defined above. (For example, user agents could implement globally unique origins or certificate-based origins.)

3.  Comparing Origins

Implementations MUST use the following algorithm to test whether two origins are the "same origin".

1. Let /A/ be the first origin being compared, and let /B/ be the second origin being compared.
2. If either /A/ or /B/ is not a scheme/host/port tuple, return an implementation-defined value.
3. If /A/ and /B/ have scheme components that are not identical, return false.
4. If /A/ and /B/ have host components that are not identical, return false.
5. If /A/ and /B/ have port components that are not identical, return false.
6. Return true.

4.  Serializing Origins

4.1.  Unicode Serialization of an Origin

Implementations MUST using the following algorithm to compute the Unicode serialization of an origin:

1. If the origin in question is not a scheme/host/port tuple, then return the string

   null

   (i.e., the character sequence U+006E, U+0075, U+006C, U+006C) and abort these steps.
2. Otherwise, let /result/ be the scheme part of the origin tuple.
3. Append the string "://" to /result/.
4. Apply the IDNA ToUnicode algorithm to each component of the host part of the origin tuple, and append the results of each component, in the same order, separated by U+002E FULL STOP characters (".") to /result/.
5. If the port part of the origin tuple gives a port that is different from the default port for the protocol given by the scheme part of the origin tuple, then append a U+003A COLON character (":") and the given port, in base ten, to /result/.
6. Return /result/.

TODO: Check that we handle IPv6 literals correctly.

4.2.  ASCII Serialization of an Origin

Implementations MUST using the following algorithm to compute the ASCII serialization of an origin:

1. If the origin in question is not a scheme/host/port tuple, then return the string

   null

   (i.e., the character sequence U+006E, U+0075, U+006C, U+006C) and abort these steps.
2. Otherwise, let /result/ be the scheme part of the origin tuple.
3. Append the string "://" to /result/.
4. Apply the IDNA ToASCII algorithm the host part of the origin tuple, with both the AllowUnassigned and UseSTD3ASCIIRules flags set, and append the result to /result/.
5. If ToASCII fails to convert one of the components of the string, e.g. because it is too long or because it contains invalid characters, then return the literal string "null" and abort these steps.
6. If the port part of the origin tuple gives a port that is different from the default port for the protocol given by the scheme part of the origin tuple, then append a U+003A COLON character (":") and the given port, in base ten, to /result/.
7. Return /result/.

5.  User Agent Behavior

Whenever a user agent issues an HTTP request, the user agent MUST include exactly one HTTP header named "Sec-From".

The value of the Sec-From header MUST either be

1. the string "null" (i.e., the character sequence U+006E, U+0075, U+006C, U+006C) or
2. a sequence of non-"null" ASCII serializations of origins separated by at least one LWS character.

Whenever a user agent would send a Sec-From header containing two consecutive, idential origin serializations, the user agent MAY remove one such origin serialization from the header.

Whenever a user agent issues an HTTP request from a "privacy-sensitive" context, the user agent MUST send the value "null" in the Sec-From header.

Whenever a user agent issues an HTTP request to URI /A/ as a result of an HTTP redirect from URI /B/, the user agent MUST either:

1. set the value of the Sec-From header in the HTTP request to /A/ to "null" (i.e., the character sequence U+006E, U+0075, U+006C, U+006C),
2. set the value of the Sec-From header in the /A/ request to the value of the Sec-From header in the /B/ request extended with a space and the ASCII serialization of the origin of /B/, unless this would result in the header containing the origin serialization "null".

Whenever a user agent issues an HTTP request that (1) is *not* the result of an HTTP redirect and (2) is *not* initiated from a "privacy-sensitive" context, the user agent SHOULD set the value of the Sec-From header to the ASCII serialization of the origin that initiated the HTTP request.

Note: This behavior differs from that of the HTTP Referer header, which user agents often suppress when an origin with an "https" scheme issues a request for a URI with an "http" scheme.

6.  HTTP Server Behavior

HTTP Servers MAY use the Sec-From header to "defend themselves against CSRF attacks." Such servers are known as "participating servers" in this section.

Let the /origin white list/ of a participating server be a set of strings selected by the operator of that server.

The string "null" MUST NOT be a member of the /origin white list/ for any participating server.

Example: The origin white list for the example.com Web server could be the strings "http://example.com", "https://example.com", "http://www.example.com", and "https://www.example.com".

A participating server MUST use the following algorithm when determining whether to modify state in response to an HTTP request:

1. If the request method is safe (as defined by RFC 2616, Section 9.1.1, e.g. either "GET" nor "HEAD"), return "MUST NOT modify state" and abort these steps.
2. If the request does not contain a header named "Sec-From", return "MAY modify state" abort these stepts.
3. For each request header named "Sec-From", let the /initiating origin list/ be the list of origins represented in the header:
   1. If there exists a origin in the /initiating origin list/ is not a member of the /origin white list/ for this server, return "MUST NOT modify state" and abort these steps.
4. Return "MAY modify state".

Example: A Web server could modify state in response to POST requests that lack an Sec-From header (because these requests are sent by non-supporting user agents) and could modify state in response to POST requests that have an Sec-From header of "http://example.com", "https://example.com", "http://www.example.com", or "https://www.example.com".

7.  Privacy Considerations

This section is not normative.

The Sec-From header improves on the Referer header by respecting the user's privacy: The Sec-From header includes only the information required to identify the principal that initiated the request (typically the scheme, host, and port of initiating origin). In particular, the Sec-From header does not contain the path or query portions of the URI included in the Referer header that invade privacy without providing additional security.

The Sec-From header also improves on the Referer header by NOT leaking intranet host names to external Web sites when a user follows a hyperlink from an intranet host to an external site because hyperlinks generate privacy-sensitive requests.

8.  Security Considerations

This section is not normative.

Because a supporting user agent will always include the Sec-From header when making HTTP requests, HTTP servers can detect that a request was initiated by a supporting user agent by observing the presence of the header. This design prevents an attacker from making a supporting user agent appear to be a non-supporting user agent. Unlike the Referer header, which is absent when suppressed by the user agent, the Sec-From header takes on the value "null" when suppressed by the user agent.

In some legacy user agents, The Sec-From header can be spoofed for same-site XMLHttpRequests. Sites that rely only on network connectivity for authentication should use a DNS rebinding defense, such as validating the HTTP Host header, in addition to CSRF protection.

9.  IANA Considerations

TODO: The "Sec-From" header should be registered.

10.  TODO

Think about how this interacts with proxies.

Think about how this interacts with caches.

Authors' Addresses