rendering_html.diviner

@title Rendering HTML
@group developer

Rendering HTML in the Phabricator environment.

= Overview =

Phabricator attempts to prevent XSS by treating strings as default-unsafe when
rendering. This means that if you try to build HTML through string
concatenation, it won't work: the string will be escaped by the rendering
pipeline, and the browser will treat it as plain text, not HTML.

This document describes the right way to build HTML components so they are safe
from XSS and render correctly. Broadly:

  - Use @{function@arcanist:phutil_tag} (and @{function:javelin_tag}) to build
    tags.
  - Use @{function@arcanist:hsprintf} where @{function@arcanist:phutil_tag}
    is awkward.
  - Combine elements with arrays, not string concatenation.
  - @{class:AphrontView} subclasses should return a
    @{class@arcanist:PhutilSafeHTML} object from their `render()` method.
  - @{class:AphrontView} subclasses act like tags when rendering.
  - @{function:pht} has some special rules.
  - There are some other things that you should be aware of.

See below for discussion.

= Building Tags: phutil_tag() =

Build HTML tags with @{function@arcanist:phutil_tag}. For example:

  phutil_tag(
    'div',
    array(
      'class' => 'some-class',
    ),
    $content);

@{function@arcanist:phutil_tag} will properly escape the content and all the
attributes, and return a @{class@arcanist:PhutilSafeHTML} object. The rendering
pipeline knows that this object represents a properly escaped HTML tag. This
allows @{function@arcanist:phutil_tag} to render tags with other tags as
content correctly (without double-escaping):

  phutil_tag(
    'div',
    array(),
    phutil_tag(
      'strong',
      array(),
      $content));

In Phabricator, the @{function:javelin_tag} function is similar to
@{function@arcanist:phutil_tag}, but provides special handling for the
`sigil` and `meta` attributes.

= Building Blocks: hsprintf() =

Sometimes, @{function@arcanist:phutil_tag} can be particularly awkward to
use. You can use @{function@arcanist:hsprintf} to build larger and more
complex blocks of HTML, when @{function@arcanist:phutil_tag} is a poor fit.
@{function:hsprintf} has `sprintf()` semantics, but `%s` escapes HTML:

  // Safely build fragments or unwieldy blocks.
  hsprintf(
    '<div id="%s">',
    $div_id);

@{function:hsprintf} can be especially useful when:

  - You need to build a block with a lot of tags, like a table with rows and
    cells.
  - You need to build part of a tag (usually you should avoid this, but if you
    do need to, @{function@arcanist:phutil_tag} can not do it).

Note that it is unsafe to provide any user-controlled data to the first
parameter of @{function@arcanist:hsprintf} (the `sprintf()`-style pattern).

Like @{function@arcanist:phutil_tag}, this function returns a
@{class@arcanist:PhutilSafeHTML} object.

= Composing Tags =

When you are building a view which combines smaller components, like a section
with a header and a body:

  $header = phutil_tag('h1', ...);
  $body = phutil_tag('p', ...);

...you should NOT use string concatenation:

  COUNTEREXAMPLE
  // Not dangerous, but does the wrong thing.
  phutil_tag('div', array(), $header.$body);

Instead, use an array:

  // Render a tag containing other tags safely.
  phutil_tag('div', array(), array($header, $body));

If you concatenate @{class@arcanist:PhutilSafeHTML} objects, they revert to
normal strings and are no longer marked as properly escaped tags.

(In the future, these objects may stop converting to strings, but for now they
must to maintain backward compatibility.)

If you need to build a list of items with some element in between each of them
(like a middot, comma, or vertical bar) you can use
@{function:phutil_implode_html}:

  // Render links with commas between them.
  phutil_tag(
    'div',
    array(),
    phutil_implode_html(', ', $list_of_links));

= AphrontView Classes =

Subclasses of @{class:AphrontView} in Phabricator should return a
@{class@arcanist:PhutilSafeHTML} object. The easiest way to do this is to
return `phutil_tag()` or `javelin_tag()`:

  return phutil_tag('div', ...);

You can use an @{class:AphrontView} subclass like you would a tag:

  phutil_tag('div', array(), $view);

= Internationalization: pht() =

The @{function:pht} function has some special rules. If any input to
@{function:pht} is a @{class@arcanist:PhutilSafeHTML} object, @{function:pht}
returns a @{class@arcanist:PhutilSafeHTML} object itself. Otherwise, it returns
normal text.

This is generally safe because translations are not permitted to have more tags
than the original text did (so if the original text had no tags, translations
can not add any).

Normally, this just means that @{function:pht} does the right thing and behaves
like you would expect, but it is worth being aware of.

= Special Cases =

NOTE: This section describes dangerous methods which can bypass XSS protections.
If possible, do not use them.

You can build @{class@arcanist:PhutilSafeHTML} out of a string explicitly by
calling @{function:phutil_safe_html} on it. This is **dangerous**, because if
you are wrong and the string is not actually safe, you have introduced an XSS
vulnerability. Consequently, you should avoid calling this if possible.

You can use @{function@arcanist:phutil_escape_html_newlines} to escape HTML
while converting newlines to `<br />`. You should not need to explicitly use
@{function@arcanist:phutil_escape_html} anywhere.

If you need to apply a string function (such as `trim()`) to safe HTML, use
@{method@arcanist:PhutilSafeHTML::applyFunction}.

If you need to extract the content of a @{class@arcanist:PhutilSafeHTML}
object, you should call `getHTMLContent()`, not cast it to a string. Eventually,
we would like to remove the string cast entirely.

Functions @{function@arcanist:phutil_tag} and @{function@arcanist:hsprintf}
are not safe if you pass the user input for the tag or attribute name. All the
following examples are dangerous:

  counterexample
  phutil_tag($evil);

  phutil_tag('span', array($evil => $evil2));

  phutil_tag('span', array('onmouseover' => $evil));

  // Use PhutilURI to check if $evil is valid HTTP link.
  hsprintf('<a href="%s">', $evil);

  hsprintf('<%s>%s</%s>', $evil, $evil2, $evil);

  // We have a lint rule disallowing this.
  hsprintf($evil);