User Interface Security Directives for Content Security Policy

2.1 Terminology

This section defines several terms used throughout the document.

The term security policy, or simply policy, for the purposes of this specification refers to either:

1. a set of security preferences for restricting the behavior of content within a given resource, or
2. a fragment of text that codifies these preferences.

The security policies defined by this document are applied by a user agent on a per-resource representation basis. Specifically, when a user agent receives a policy along with the representation of a given resource, that policy applies to that resource representation only. This document often refers to that resource representation as the protected resource.

A server transmits its security policy for a particular resource as a collection of directives, such as default-src 'self', each of which controls a specific set of privileges for a document rendered by the user agent. More details are provided in the directives section.

A directive consists of a directive name, which indicates the privileges controlled by the directive, and a directive value, which specifies the restrictions the policy imposes on those privileges.

An ancestor is any resource between the protected resource and the top of the window frame tree; for example, if A embeds B which embeds C, both A and B are ancestors of C. If A embeds both B and C, B is not an ancestor of C, but A still is.

The term origin is defined in the Origin specification. [ORIGIN]

The term URI is defined in the URI specification. [URI]

The <iframe>, <object>, <embed>, and <frame> elements are defined in the HTML5 standard. [HTML5].

The <applet> element is defined in the HTML 4.01 standard. [HTML401].

The Augmented Backus-Naur Form (ABNF) notation used in this document is specified in RFC 5234. [ABNF]

The following core rules are included by reference, as defined in [ABNF Appendix B.1]: ALPHA (letters), DIGIT (decimal 0-9), WSP (white space) and VCHAR (printing characters).

The OWS rule is used where zero or more linear whitespace octets might appear. OWS SHOULD either not be produced or be produced as a single SP. Multiple OWS octets that occur within field-content SHOULD either be replaced with a single SP or transformed to all SP octets (each octet other than SP replaced with SP) before interpreting the field value or forwarding the message downstream.

OWS            = *( SP / HTAB / obs-fold )
               ; "optional" whitespace
obs-fold       = CRLF ( SP / HTAB )
               ; obsolete line folding

A selector string is a list of one or more complex selectors(see [SELECTORS4], section 3.1) that may be surrounded by whitespace and matches the dom_selectors_group production.

dom_selectors_group
  : S* [ selectors_group ] S*
  ;

A embedding source list follows the ABNF and parsing rules defined for source-list (see [CSP] section 3.22) with the following new productions:

embedding-keyword-source = "'self'" / "'deny'"
embedding-source-expression = host-source / embedding-keyword-source
embedding-source-list = *WSP [ embedding-source-expression *( 1*WSP embedding-source-expression ) *WSP ]

3.1 frame-options

The frame-options directive indicates whether the user-agent should embed the resource using a frame, iframe, object, embed or applet tag, or equivalent functionality in non-HTML resources. Resources can use this directive to avoid many UI Redressing attacks by ensuring they are not embedded into potentially hostile contexts.

The syntax for the name and value of the directive are described by the following ABNF grammar:

directive-name  = "frame-options"
directive-value = embedding-source-list

Unlike policies defined in Content Security Policy, the frame-options directive is not subject to the default-src directive. If this directive is not explicitly stated in the policy its value is assumed to be "*".

A host-source value in the embedding-source-list indicates an origin that is a valid ancestor for the resource.

If the directive-value contains the embedding-keyword-source 'deny', the resource cannot be displayed in an embedded context, regardless of the origin attempting to do so, and all other values in the directive are ignored.

If the directive-value contains the embedding-keyword-source 'self', ancestors are in the same origin as the protected resource.

Content-Security-Policy: frame-options https://alice https://bob

3.2 input-protection

The input-protection directive, if present or implied, instructs the user agent to apply the heuristic UI redressing protections described in the Input Protection Heuristic section to user input events, such as click, keypress, touch, and drag, before they are delivered to the resource.

The screenshot comparison heuristic, in particular, uses the body-bounding rectangle of the document triggering the event as its default reference area, or the rectangle defined by the input-protection-clip and by the input-protection-selectors directives if any of those is explicitly set.

If the input-protection-clip directive is set as part of a Content-Security-Policy, triggering of the heuristic should cancel delivery of the UI event to the target and cause a violation report to be sent. If set as part of a Content-Security-Policy-Report-Only, triggering of the heuristic should result in the event being delivered with the unsafe attribute on the UIEvent set to true and cause a violation report to be sent.

The optional directive value allows resource authors to provide options for heuristic tuning in the form of space-separated option-name=option-value pairs.

directive-name    = "input-protection"
directive-value   = ["display-time=" num-val] ["tolerance=" num-val]

If the policy does not contain a value for this directive or any of the hint name=value pairs are absent, the user agent SHOULD apply default values for hints as described in the following.

display-time

is a numeric value from 0 to 10000 that specifies how long, in milliseconds, the screen area containing the protected user interface must have been displayed continuously unchanged when the event is processed. If not specified, it defaults to 800. If a value out of the range stated above is specified, it defaults to the nearest value between the lower and the higher bounds.

tolerance

is a numeric value from 0 to 99 that defines the difference threshold at which the screenshot comparison procedure of the input protection heuristic triggers a violation. A value of 0 indicates that no difference between the two images is permitted. A value of 99 provides little to no practical protection. If not specified, it defaults to 0.

3.3 input-protection-clip

The input-protection-clip directive defines a rectangular screen area whose intersection with the bounding rectangle of the whole document's body should be used as the reference area in the screenshot comparison check explained in the Input Protection Heuristic section.

If the input-protection-clip directive is not explicitly set in a policy which includes the input-protection directive and no input-protection-selectors directive is set either, the bounding rectangle of the whole document's body should be used for screenshot comparisons.

If explicitly set as part of a policy where no input-protection directive is explicitly set, the input-protection-clip directive implies the input-protection directive as if it was set in the same policy with its default value.

directive-name  = "input-protection-clip"
directive-value = ["before=" num-val] ["above=" num-val] ["after=" num-val] ["below=" num-val]

The optional directive value can include up to four non-negative numeric labeled offsets, expressed in CSS pixels and relative to the screen coordinates of the UI event being processed (event.screenX and event.screenY for mouse, touch or pointer events) or, if not applicable (e.g. for keyboard events), to the geometrical center of the event target in screen coordinates. These offsets define a rectangle with

x = eX - left, y = eY - top, width = left + right, height = top + bottom

The default value for this directive is before=250 above=250 after=50 below=50. If a partial value is provided (i.e. any offset has been omitted) the default values should be implied for the missing offsets.

The intersection of the computed rectangle with the bounding rectangle of the document's body should be used as the reference area for the screenshot comparison check explained in the Input Protection Heuristic section, unless the UI event's target or one of its DOM ancestors match a input-protection-selector directive set in the same policy.

If the input-protection-clip directive is not set or provides an invalid value, the whole bounding rectangle of the document's body must be used as the reference area for the screenshot comparison, unless an input-protection-selectors directive is set in the same policy.

3.4 input-protection-selectors

The input-protection-selectors directive overrides the implicit or explicit input-protection-clip value when the processed UI event target or one of its DOM ancestors match the dom_selectors_group selector string provided as the mandatory directive's value: in this case, the reference area used for screenshot comparison is the bounding box of the event target itself, if it matches the selectors, or the bounding box of its nearest matching DOM ancestor, if any, augmented by the margins given by the leading optional labeled offsets, if any. UI events whose target and ancestors don't match any of the specified selectors should be ignored (not blocked) unless an input-protection-clip directive is explicitly included in the policy: if this is the case, the UI event must be checked and the screenshot reference area should be computed using the input-protection-clip directive.

If set as part of a policy where no input-protection directive is explicitly set, the input-protection-selectors directive implies the input-protection directive as if it was set in the same policy with its default value.

directive-name  = "input-protection-selectors"
directive-value = ["before=" num-value] ["after=" num-value] ["above=" num-value] ["below=" num-value] dom_selectors_group

Any of the four non-negative numeric labeled offsets, which represent margins expressed in CSS pixels, may be omitted, taking 0 (zero) as their default values.

The reference screenshot area is computed as the rectangle having

x = match.x - left, y = match.y - top, width = left + match.width + right, height = top + match.height + bottom

3.5 report-uri

The report-uri directive specifies a URI to which the user agent sends reports about policy violation.

The syntax for the name and value of this directive and the algorithm to prepare a report are described by Content Security Policy. [CSP]

The core Content Security Policy specification provides directives to restrict from where external content may be loaded. As such, violation reports include a blocked-uri key/value pair that specifies the attempted resource load that was blocked by the policy.

As this is not applicable to the directives in this document, the following additional steps MUST be added to the algorithm defined in Content Security Policy to prepare a violation report:

In step 1, when preparing the JSON object violation-object, add the following keys and values to the csp-report: [CSP]

If the violation is of the frame-options directive, no additional processing is required.

If the violation is of the input-protection directive, add the following keys and values. If a value is not set or applicable for the violation (e.g. pointer-height, if the violating event type is not a Pointer Event) the key SHOULD be omitted.

blocked-event-type

The type attribute of the UIEvent that was blocked by policy.

touch-event

A boolean indicating whether the event blocked by policy was a Touch Event [TOUCH-EVENTS].

pointer-type

The pointerType value of a Pointer Event [POINTER-EVENTS].

pointer-height

The height value of a Pointer Event [POINTER-EVENTS].

pointer-width

The width value of a Pointer Event [POINTER-EVENTS].

device-height

The device-height property as defined in [CSS3-MEDIAQUERIES].

device-width

The device-width property as defined in [CSS3-MEDIAQUERIES].

blocked-event-client-x

The clientX attribute of the UIEvent [DOM-LEVEL-2-EVENTS] that was blocked by policy, if set.

blocked-event-client-y

The clientY attribute of the UIEvent [DOM-LEVEL-2-EVENTS] that was blocked by policy, if set.

If the target of an UIEvent which triggers an input-protection violation has an explictly-set id attribute:

blocked-target-id

The id attribute of the DOM Element that a violating UIEvent targeted.

Otherwise, if the target element does not have an explicit id attribute:

blocked-target-xpath

An XPath [XPATH] expression that returns the target Element of the UIEvent that was blocked by policy.

function getXPathFor(e) {
 
    var xpath = '';
    
    while(e.nodeType == e.ELEMENT_NODE) {
      
      var child = e;
      var siblingIndex = 0;
      while( (child = child.previousSibling) != null ) {
        if(child.tagName == e.tagName) {
          siblingIndex++;  
        }
      }
        
      xpath = e.tagName + 
              '[' + siblingIndex + ']' + 
              (xpath == '' ? '' : '/') +
              xpath;
        
      e = e.parentNode;
   }
   xpath = '/' + xpath; 
   return(xpath);
}

partial interface UIEvent {
    readonly    attribute bool unsafe;
};

The unsafe attribute allows web applications to monitor and immediately respond to suspect violations in the report-only mode. Applications may also use this interface for capability detection. For example, a web application may monitor user inputs on a payment button element like this:

document.getElementById('payment-button').addEventListener("click", function(eventObj) {
  if ("unsafe" in eventObj) {
    if (eventObj.unsafe == true) {
      return reportUnsafeOrShowDialog();
    }
  }
  makePayment();
};

5.1 SecurityPolicyViolationEvent Events

[Constructor(DOMString type, optional SecurityPolicyViolationEventInit eventInitDict)]
partial interface SecurityPolicyViolationEvent : Event {
    readonly    attribute DOMString blockedEventType;
    readonly    attribute bool      touchEvent;
    readonly    attribute DOMString pointerType;
    readonly    attribute long      pointerHeight;
    readonly    attribute long      pointerWidth;
    readonly    attribute long      deviceHeight;
    readonly    attribute long      deviceWidth;
    readonly    attribute long      blockedEventClientX;
    readonly    attribute long      blockedEventClientY;
    readonly    attribute DOMString blockedTargetID;
    readonly    attribute DOMString blockedTargetXPath;
};

partial dictionary SecurityPolicyViolationEventInit {
    DOMString blockedEventType;
    bool      touchEvent;
    DOMString pointerType;
    long      pointerHeight;
    long      pointerWidth;
    long      deviceHeight;
    long      deviceWidth;
    long      blockedEventClientX;
    long      blockedEventClientY;
    DOMString blockedTargetID;
    DOMString blockedTargetXPath;
};

5.2 SecurityPolicy

Let the active CSP policies be the set of CSP policies the user agent is currently enforcing for the associated document.

partial interface Security {
    readonly    attribute bool inputProtection;
};