sanitizer.py

from __future__ import unicode_literals

from itertools import chain
import re

import six
from six.moves.urllib.parse import urlparse
from xml.sax.saxutils import unescape

from bleach import html5lib_shim
from bleach.utils import alphabetize_attributes, force_unicode


#: List of allowed tags
ALLOWED_TAGS = [
    'a',
    'abbr',
    'acronym',
    'b',
    'blockquote',
    'code',
    'em',
    'i',
    'li',
    'ol',
    'strong',
    'ul',
]


#: Map of allowed attributes by tag
ALLOWED_ATTRIBUTES = {
    'a': ['href', 'title'],
    'abbr': ['title'],
    'acronym': ['title'],
}

#: List of allowed styles
ALLOWED_STYLES = []

#: List of allowed protocols
ALLOWED_PROTOCOLS = ['http', 'https', 'mailto']

#: Invisible characters--0 to and including 31 except 9 (tab), 10 (lf), and 13 (cr)
INVISIBLE_CHARACTERS = ''.join([chr(c) for c in chain(range(0, 9), range(11, 13), range(14, 32))])

#: Regexp for characters that are invisible
INVISIBLE_CHARACTERS_RE = re.compile(
    '[' + INVISIBLE_CHARACTERS + ']',
    re.UNICODE
)

#: String to replace invisible characters with. This can be a character, a
#: string, or even a function that takes a Python re matchobj
INVISIBLE_REPLACEMENT_CHAR = '?'


class Cleaner(object):
    """Cleaner for cleaning HTML fragments of malicious content

    This cleaner is a security-focused function whose sole purpose is to remove
    malicious content from a string such that it can be displayed as content in
    a web page.

    To use::

        from bleach.sanitizer import Cleaner

        cleaner = Cleaner()

        for text in all_the_yucky_things:
            sanitized = cleaner.clean(text)

    .. Note::

       This cleaner is not designed to use to transform content to be used in
       non-web-page contexts.

    .. Warning::

       This cleaner is not thread-safe--the html parser has internal state.
       Create a separate cleaner per thread!


    """

    def __init__(self, tags=ALLOWED_TAGS, attributes=ALLOWED_ATTRIBUTES,
                 styles=ALLOWED_STYLES, protocols=ALLOWED_PROTOCOLS, strip=False,
                 strip_comments=True, filters=None):
        """Initializes a Cleaner

        :arg list tags: allowed list of tags; defaults to
            ``bleach.sanitizer.ALLOWED_TAGS``

        :arg dict attributes: allowed attributes; can be a callable, list or dict;
            defaults to ``bleach.sanitizer.ALLOWED_ATTRIBUTES``

        :arg list styles: allowed list of css styles; defaults to
            ``bleach.sanitizer.ALLOWED_STYLES``

        :arg list protocols: allowed list of protocols for links; defaults
            to ``bleach.sanitizer.ALLOWED_PROTOCOLS``

        :arg bool strip: whether or not to strip disallowed elements

        :arg bool strip_comments: whether or not to strip HTML comments

        :arg list filters: list of html5lib Filter classes to pass streamed content through

            .. seealso:: http://html5lib.readthedocs.io/en/latest/movingparts.html#filters

            .. Warning::

               Using filters changes the output of ``bleach.Cleaner.clean``.
               Make sure the way the filters change the output are secure.

        """
        self.tags = tags
        self.attributes = attributes
        self.styles = styles
        self.protocols = protocols
        self.strip = strip
        self.strip_comments = strip_comments
        self.filters = filters or []

        self.parser = html5lib_shim.BleachHTMLParser(
            tags=self.tags,
            strip=self.strip,
            consume_entities=False,
            namespaceHTMLElements=False
        )
        self.walker = html5lib_shim.getTreeWalker('etree')
        self.serializer = html5lib_shim.BleachHTMLSerializer(
            quote_attr_values='always',
            omit_optional_tags=False,
            escape_lt_in_attrs=True,

            # We want to leave entities as they are without escaping or
            # resolving or expanding
            resolve_entities=False,

            # Bleach has its own sanitizer, so don't use the html5lib one
            sanitize=False,

            # Bleach sanitizer alphabetizes already, so don't use the html5lib one
            alphabetical_attributes=False,
        )

    def clean(self, text):
        """Cleans text and returns sanitized result as unicode

        :arg str text: text to be cleaned

        :returns: sanitized text as unicode

        :raises TypeError: if ``text`` is not a text type

        """
        if not isinstance(text, six.string_types):
            message = "argument cannot be of '{name}' type, must be of text type".format(
                name=text.__class__.__name__)
            raise TypeError(message)

        if not text:
            return ''

        text = force_unicode(text)

        dom = self.parser.parseFragment(text)
        filtered = BleachSanitizerFilter(
            source=self.walker(dom),

            # Bleach-sanitizer-specific things
            attributes=self.attributes,
            strip_disallowed_elements=self.strip,
            strip_html_comments=self.strip_comments,

            # html5lib-sanitizer things
            allowed_elements=self.tags,
            allowed_css_properties=self.styles,
            allowed_protocols=self.protocols,
            allowed_svg_properties=[],
        )

        # Apply any filters after the BleachSanitizerFilter
        for filter_class in self.filters:
            filtered = filter_class(source=filtered)

        return self.serializer.render(filtered)


def attribute_filter_factory(attributes):
    """Generates attribute filter function for the given attributes value

    The attributes value can take one of several shapes. This returns a filter
    function appropriate to the attributes value. One nice thing about this is
    that there's less if/then shenanigans in the ``allow_token`` method.

    """
    if callable(attributes):
        return attributes

    if isinstance(attributes, dict):
        def _attr_filter(tag, attr, value):
            if tag in attributes:
                attr_val = attributes[tag]
                if callable(attr_val):
                    return attr_val(tag, attr, value)

                if attr in attr_val:
                    return True

            if '*' in attributes:
                attr_val = attributes['*']
                if callable(attr_val):
                    return attr_val(tag, attr, value)

                return attr in attr_val

            return False

        return _attr_filter

    if isinstance(attributes, list):
        def _attr_filter(tag, attr, value):
            return attr in attributes

        return _attr_filter

    raise ValueError('attributes needs to be a callable, a list or a dict')


class BleachSanitizerFilter(html5lib_shim.SanitizerFilter):
    """html5lib Filter that sanitizes text

    This filter can be used anywhere html5lib filters can be used.

    """
    def __init__(self, source, attributes=ALLOWED_ATTRIBUTES,
                 strip_disallowed_elements=False, strip_html_comments=True,
                 **kwargs):
        """Creates a BleachSanitizerFilter instance

        :arg Treewalker source: stream

        :arg list tags: allowed list of tags; defaults to
            ``bleach.sanitizer.ALLOWED_TAGS``

        :arg dict attributes: allowed attributes; can be a callable, list or dict;
            defaults to ``bleach.sanitizer.ALLOWED_ATTRIBUTES``

        :arg list styles: allowed list of css styles; defaults to
            ``bleach.sanitizer.ALLOWED_STYLES``

        :arg list protocols: allowed list of protocols for links; defaults
            to ``bleach.sanitizer.ALLOWED_PROTOCOLS``

        :arg bool strip_disallowed_elements: whether or not to strip disallowed
            elements

        :arg bool strip_html_comments: whether or not to strip HTML comments

        """
        self.attr_filter = attribute_filter_factory(attributes)
        self.strip_disallowed_elements = strip_disallowed_elements
        self.strip_html_comments = strip_html_comments

        return super(BleachSanitizerFilter, self).__init__(source, **kwargs)

    def sanitize_stream(self, token_iterator):
        for token in token_iterator:
            ret = self.sanitize_token(token)

            if not ret:
                continue

            if isinstance(ret, list):
                for subtoken in ret:
                    yield subtoken
            else:
                yield ret

    def merge_characters(self, token_iterator):
        """Merge consecutive Characters tokens in a stream"""
        characters_buffer = []

        for token in token_iterator:
            if characters_buffer:
                if token['type'] == 'Characters':
                    characters_buffer.append(token)
                    continue
                else:
                    # Merge all the characters tokens together into one and then
                    # operate on it.
                    new_token = {
                        'data': ''.join([char_token['data'] for char_token in characters_buffer]),
                        'type': 'Characters'
                    }
                    characters_buffer = []
                    yield new_token

            elif token['type'] == 'Characters':
                characters_buffer.append(token)
                continue

            yield token

        new_token = {
            'data': ''.join([char_token['data'] for char_token in characters_buffer]),
            'type': 'Characters'
        }
        yield new_token

    def __iter__(self):
        return self.merge_characters(self.sanitize_stream(html5lib_shim.Filter.__iter__(self)))

    def sanitize_token(self, token):
        """Sanitize a token either by HTML-encoding or dropping.

        Unlike sanitizer.Filter, allowed_attributes can be a dict of {'tag':
        ['attribute', 'pairs'], 'tag': callable}.

        Here callable is a function with two arguments of attribute name and
        value. It should return true of false.

        Also gives the option to strip tags instead of encoding.

        :arg dict token: token to sanitize

        :returns: token or list of tokens

        """
        token_type = token['type']
        if token_type in ['StartTag', 'EndTag', 'EmptyTag']:
            if token['name'] in self.allowed_elements:
                return self.allow_token(token)

            elif self.strip_disallowed_elements:
                return None

            else:
                if 'data' in token:
                    # Alphabetize the attributes before calling .disallowed_token()
                    # so that the resulting string is stable
                    token['data'] = alphabetize_attributes(token['data'])
                return self.disallowed_token(token)

        elif token_type == 'Comment':
            if not self.strip_html_comments:
                return token
            else:
                return None

        elif token_type == 'Characters':
            return self.sanitize_characters(token)

        else:
            return token

    def sanitize_characters(self, token):
        """Handles Characters tokens

        Our overridden tokenizer doesn't do anything with entities. However,
        that means that the serializer will convert all ``&`` in Characters
        tokens to ``&``.

        Since we don't want that, we extract entities here and convert them to
        Entity tokens so the serializer will let them be.

        :arg token: the Characters token to work on

        :returns: a list of tokens

        """
        data = token.get('data', '')

        if not data:
            return token

        data = INVISIBLE_CHARACTERS_RE.sub(INVISIBLE_REPLACEMENT_CHAR, data)
        token['data'] = data

        # If there isn't a & in the data, we can return now
        if '&' not in data:
            return token

        new_tokens = []

        # For each possible entity that starts with a "&", we try to extract an
        # actual entity and re-tokenize accordingly
        for part in html5lib_shim.next_possible_entity(data):
            if not part:
                continue

            if part.startswith('&'):
                entity = html5lib_shim.match_entity(part)
                if entity is not None:
                    if entity == 'amp':
                        # LinkifyFilter can't match urls across token boundaries
                        # which is problematic with & since that shows up in
                        # querystrings all the time. This special-cases &
                        # and converts it to a & and sticks it in as a
                        # Characters token. It'll get merged with surrounding
                        # tokens in the BleachSanitizerfilter.__iter__ and
                        # escaped in the serializer.
                        new_tokens.append({'type': 'Characters', 'data': '&'})
                    else:
                        new_tokens.append({'type': 'Entity', 'name': entity})

                    # Length of the entity plus 2--one for & at the beginning
                    # and one for ; at the end
                    remainder = part[len(entity) + 2:]
                    if remainder:
                        new_tokens.append({'type': 'Characters', 'data': remainder})
                    continue

            new_tokens.append({'type': 'Characters', 'data': part})

        return new_tokens

    def sanitize_uri_value(self, value, allowed_protocols):
        """Checks a uri value to see if it's allowed

        :arg value: the uri value to sanitize
        :arg allowed_protocols: list of allowed protocols

        :returns: allowed value or None

        """
        # NOTE(willkg): This transforms the value into one that's easier to
        # match and verify, but shouldn't get returned since it's vastly
        # different than the original value.

        # Convert all character entities in the value
        new_value = html5lib_shim.convert_entities(value)

        # Nix backtick, space characters, and control characters
        new_value = re.sub(
            r"[`\000-\040\177-\240\s]+",
            '',
            new_value
        )

        # Remove REPLACEMENT characters
        new_value = new_value.replace('\ufffd', '')

        # Lowercase it--this breaks the value, but makes it easier to match
        # against
        new_value = new_value.lower()

        try:
            # Drop attributes with uri values that have protocols that aren't
            # allowed
            parsed = urlparse(new_value)
        except ValueError:
            # URI is impossible to parse, therefore it's not allowed
            return None

        if parsed.scheme:
            # If urlparse found a scheme, check that
            if parsed.scheme in allowed_protocols:
                return value

        else:
            # Allow uris that are just an anchor
            if new_value.startswith('#'):
                return value

            # Handle protocols that urlparse doesn't recognize like "myprotocol"
            if ':' in new_value and new_value.split(':')[0] in allowed_protocols:
                return value

            # If there's no protocol/scheme specified, then assume it's "http"
            # and see if that's allowed
            if 'http' in allowed_protocols:
                return value

        return None

    def allow_token(self, token):
        """Handles the case where we're allowing the tag"""
        if 'data' in token:
            # Loop through all the attributes and drop the ones that are not
            # allowed, are unsafe or break other rules. Additionally, fix
            # attribute values that need fixing.
            #
            # At the end of this loop, we have the final set of attributes
            # we're keeping.
            attrs = {}
            for namespaced_name, val in token['data'].items():
                namespace, name = namespaced_name

                # Drop attributes that are not explicitly allowed
                #
                # NOTE(willkg): We pass in the attribute name--not a namespaced
                # name.
                if not self.attr_filter(token['name'], name, val):
                    continue

                # Drop attributes with uri values that use a disallowed protocol
                # Sanitize attributes with uri values
                if namespaced_name in self.attr_val_is_uri:
                    new_value = self.sanitize_uri_value(val, self.allowed_protocols)
                    if new_value is None:
                        continue
                    val = new_value

                # Drop values in svg attrs with non-local IRIs
                if namespaced_name in self.svg_attr_val_allows_ref:
                    new_val = re.sub(r'url\s*\(\s*[^#\s][^)]+?\)',
                                     ' ',
                                     unescape(val))
                    new_val = new_val.strip()
                    if not new_val:
                        continue

                    else:
                        # Replace the val with the unescaped version because
                        # it's a iri
                        val = new_val

                # Drop href and xlink:href attr for svg elements with non-local IRIs
                if (None, token['name']) in self.svg_allow_local_href:
                    if namespaced_name in [
                            (None, 'href'), (html5lib_shim.namespaces['xlink'], 'href')
                    ]:
                        if re.search(r'^\s*[^#\s]', val):
                            continue

                # If it's a style attribute, sanitize it
                if namespaced_name == (None, 'style'):
                    val = self.sanitize_css(val)

                # At this point, we want to keep the attribute, so add it in
                attrs[namespaced_name] = val

            token['data'] = alphabetize_attributes(attrs)

        return token

    def disallowed_token(self, token):
        token_type = token["type"]
        if token_type == "EndTag":
            token["data"] = "</%s>" % token["name"]

        elif token["data"]:
            assert token_type in ("StartTag", "EmptyTag")
            attrs = []
            for (ns, name), v in token["data"].items():
                # If we end up with a namespace, but no name, switch them so we
                # have a valid name to use.
                if ns and not name:
                    ns, name = name, ns

                # Figure out namespaced name if the namespace is appropriate
                # and exists; if the ns isn't in prefixes, then drop it.
                if ns is None or ns not in html5lib_shim.prefixes:
                    namespaced_name = name
                else:
                    namespaced_name = '%s:%s' % (html5lib_shim.prefixes[ns], name)

                attrs.append(' %s="%s"' % (
                    namespaced_name,
                    # NOTE(willkg): HTMLSerializer escapes attribute values
                    # already, so if we do it here (like HTMLSerializer does),
                    # then we end up double-escaping.
                    v)
                )
            token["data"] = "<%s%s>" % (token["name"], ''.join(attrs))

        else:
            token["data"] = "<%s>" % token["name"]

        if token.get("selfClosing"):
            token["data"] = token["data"][:-1] + "/>"

        token["type"] = "Characters"

        del token["name"]
        return token

    def sanitize_css(self, style):
        """Sanitizes css in style tags"""
        # Convert entities in the style so that it can be parsed as CSS
        style = html5lib_shim.convert_entities(style)

        # Drop any url values before we do anything else
        style = re.compile(r'url\s*\(\s*[^\s)]+?\s*\)\s*').sub(' ', style)

        # The gauntlet of sanitization

        # Validate the css in the style tag and if it's not valid, then drop
        # the whole thing.
        parts = style.split(';')
        gauntlet = re.compile(
            r"""^([-/:,#%.'"\s!\w]|\w-\w|'[\s\w]+'\s*|"[\s\w]+"|\([\d,%\.\s]+\))*$""",
            flags=re.U
        )

        for part in parts:
            if not gauntlet.match(part):
                return ''

        if not re.match(r"^\s*([-\w]+\s*:[^:;]*(;\s*|$))*$", style):
            return ''

        clean = []
        for prop, value in re.findall(r'([-\w]+)\s*:\s*([^:;]*)', style):
            if not value:
                continue

            if prop.lower() in self.allowed_css_properties:
                clean.append(prop + ': ' + value + ';')

            elif prop.lower() in self.allowed_svg_properties:
                clean.append(prop + ': ' + value + ';')

        return ' '.join(clean)