Whitelist.java

package com.smartandroid.sa.tag.safety;

/*
 Thank you to Ryan Grove (wonko.com) for the Ruby HTML cleaner http://github.com/rgrove/sanitize/, which inspired
 this whitelist configuration, and the initial defaults.
 */

import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;

import com.smartandroid.sa.tag.helper.Validate;
import com.smartandroid.sa.tag.nodes.Attribute;
import com.smartandroid.sa.tag.nodes.Attributes;
import com.smartandroid.sa.tag.nodes.Element;

/**
 * Whitelists define what HTML (elements and attributes) to allow through the
 * cleaner. Everything else is removed.
 * <p/>
 * Start with one of the defaults:
 * <ul>
 * <li>{@link #none}
 * <li>{@link #simpleText}
 * <li>{@link #basic}
 * <li>{@link #basicWithImages}
 * <li>{@link #relaxed}
 * </ul>
 * <p/>
 * If you need to allow more through (please be careful!), tweak a base
 * whitelist with:
 * <ul>
 * <li>{@link #addTags}
 * <li>{@link #addAttributes}
 * <li>{@link #addEnforcedAttribute}
 * <li>{@link #addProtocols}
 * </ul>
 * <p/>
 * The cleaner and these whitelists assume that you want to clean a
 * <code>body</code> fragment of HTML (to add user supplied HTML into a
 * templated page), and not to clean a full HTML document. If the latter is the
 * case, either wrap the document HTML around the cleaned body HTML, or create a
 * whitelist that allows <code>html</code> and <code>head</code> elements as
 * appropriate.
 * <p/>
 * If you are going to extend a whitelist, please be very careful. Make sure you
 * understand what attributes may lead to XSS attack vectors. URL attributes are
 * particularly vulnerable and require careful validation. See
 * http://ha.ckers.org/xss.html for some XSS attack examples.
 * 
 * @author Jonathan Hedley
 */
public class Whitelist {
	private Set<TagName> tagNames; // tags allowed, lower case. e.g. [p, br,
									// span]
	private Map<TagName, Set<AttributeKey>> attributes; // tag -> attribute[].
														// allowed attributes
														// [href] for a tag.
	private Map<TagName, Map<AttributeKey, AttributeValue>> enforcedAttributes; // always
																				// set
																				// these
																				// attribute
																				// values
	private Map<TagName, Map<AttributeKey, Set<Protocol>>> protocols; // allowed
																		// URL
																		// protocols
																		// for
																		// attributes
	private boolean preserveRelativeLinks; // option to preserve relative links

	/**
	 * This whitelist allows only text nodes: all HTML will be stripped.
	 * 
	 * @return whitelist
	 */
	public static Whitelist none() {
		return new Whitelist();
	}

	/**
	 * This whitelist allows only simple text formatting:
	 * <code>b, em, i, strong, u</code>. All other HTML (tags and attributes)
	 * will be removed.
	 * 
	 * @return whitelist
	 */
	public static Whitelist simpleText() {
		return new Whitelist().addTags("b", "em", "i", "strong", "u");
	}

	/**
	 * This whitelist allows a fuller range of text nodes:
	 * <code>a, b, blockquote, br, cite, code, dd, dl, dt, em, i, li,
     ol, p, pre, q, small, strike, strong, sub, sup, u, ul</code>, and
	 * appropriate attributes.
	 * <p/>
	 * Links (<code>a</code> elements) can point to
	 * <code>http, https, ftp, mailto</code>, and have an enforced
	 * <code>rel=nofollow</code> attribute.
	 * <p/>
	 * Does not allow images.
	 * 
	 * @return whitelist
	 */
	public static Whitelist basic() {
		return new Whitelist()
				.addTags("a", "b", "blockquote", "br", "cite", "code", "dd",
						"dl", "dt", "em", "i", "li", "ol", "p", "pre", "q",
						"small", "strike", "strong", "sub", "sup", "u", "ul")

				.addAttributes("a", "href").addAttributes("blockquote", "cite")
				.addAttributes("q", "cite")

				.addProtocols("a", "href", "ftp", "http", "https", "mailto")
				.addProtocols("blockquote", "cite", "http", "https")
				.addProtocols("cite", "cite", "http", "https")

				.addEnforcedAttribute("a", "rel", "nofollow");

	}

	/**
	 * This whitelist allows the same text tags as {@link #basic}, and also
	 * allows <code>img</code> tags, with appropriate attributes, with
	 * <code>src</code> pointing to <code>http</code> or <code>https</code>.
	 * 
	 * @return whitelist
	 */
	public static Whitelist basicWithImages() {
		return basic()
				.addTags("img")
				.addAttributes("img", "align", "alt", "height", "src", "title",
						"width").addProtocols("img", "src", "http", "https");
	}

	/**
	 * This whitelist allows a full range of text and structural body HTML:
	 * <code>a, b, blockquote, br, caption, cite,
     code, col, colgroup, dd, dl, dt, em, h1, h2, h3, h4, h5, h6, i, img, li, ol, p, pre, q, small, strike, strong, sub,
     sup, table, tbody, td, tfoot, th, thead, tr, u, ul</code>
	 * <p/>
	 * Links do not have an enforced <code>rel=nofollow</code> attribute, but
	 * you can add that if desired.
	 * 
	 * @return whitelist
	 */
	public static Whitelist relaxed() {
		return new Whitelist()
				.addTags("a", "b", "blockquote", "br", "caption", "cite",
						"code", "col", "colgroup", "dd", "div", "dl", "dt",
						"em", "h1", "h2", "h3", "h4", "h5", "h6", "i", "img",
						"li", "ol", "p", "pre", "q", "small", "strike",
						"strong", "sub", "sup", "table", "tbody", "td",
						"tfoot", "th", "thead", "tr", "u", "ul")

				.addAttributes("a", "href", "title")
				.addAttributes("blockquote", "cite")
				.addAttributes("col", "span", "width")
				.addAttributes("colgroup", "span", "width")
				.addAttributes("img", "align", "alt", "height", "src", "title",
						"width")
				.addAttributes("ol", "start", "type")
				.addAttributes("q", "cite")
				.addAttributes("table", "summary", "width")
				.addAttributes("td", "abbr", "axis", "colspan", "rowspan",
						"width")
				.addAttributes("th", "abbr", "axis", "colspan", "rowspan",
						"scope", "width").addAttributes("ul", "type")

				.addProtocols("a", "href", "ftp", "http", "https", "mailto")
				.addProtocols("blockquote", "cite", "http", "https")
				.addProtocols("cite", "cite", "http", "https")
				.addProtocols("img", "src", "http", "https")
				.addProtocols("q", "cite", "http", "https");
	}

	/**
	 * Create a new, empty whitelist. Generally it will be better to start with
	 * a default prepared whitelist instead.
	 * 
	 * @see #basic()
	 * @see #basicWithImages()
	 * @see #simpleText()
	 * @see #relaxed()
	 */
	public Whitelist() {
		tagNames = new HashSet<TagName>();
		attributes = new HashMap<TagName, Set<AttributeKey>>();
		enforcedAttributes = new HashMap<TagName, Map<AttributeKey, AttributeValue>>();
		protocols = new HashMap<TagName, Map<AttributeKey, Set<Protocol>>>();
		preserveRelativeLinks = false;
	}

	/**
	 * Add a list of allowed elements to a whitelist. (If a tag is not allowed,
	 * it will be removed from the HTML.)
	 * 
	 * @param tags
	 *            tag names to allow
	 * @return this (for chaining)
	 */
	public Whitelist addTags(String... tags) {
		Validate.notNull(tags);

		for (String tagName : tags) {
			Validate.notEmpty(tagName);
			tagNames.add(TagName.valueOf(tagName));
		}
		return this;
	}

	/**
	 * Add a list of allowed attributes to a tag. (If an attribute is not
	 * allowed on an element, it will be removed.)
	 * <p/>
	 * E.g.: <code>addAttributes("a", "href", "class")</code> allows
	 * <code>href</code> and <code>class</code> attributes on <code>a</code>
	 * tags.
	 * <p/>
	 * To make an attribute valid for <b>all tags</b>, use the pseudo tag
	 * <code>:all</code>, e.g. <code>addAttributes(":all", "class")</code>.
	 * 
	 * @param tag
	 *            The tag the attributes are for. The tag will be added to the
	 *            allowed tag list if necessary.
	 * @param keys
	 *            List of valid attributes for the tag
	 * @return this (for chaining)
	 */
	public Whitelist addAttributes(String tag, String... keys) {
		Validate.notEmpty(tag);
		Validate.notNull(keys);
		Validate.isTrue(keys.length > 0, "No attributes supplied.");

		TagName tagName = TagName.valueOf(tag);
		if (!tagNames.contains(tagName))
			tagNames.add(tagName);
		Set<AttributeKey> attributeSet = new HashSet<AttributeKey>();
		for (String key : keys) {
			Validate.notEmpty(key);
			attributeSet.add(AttributeKey.valueOf(key));
		}
		if (attributes.containsKey(tagName)) {
			Set<AttributeKey> currentSet = attributes.get(tagName);
			currentSet.addAll(attributeSet);
		} else {
			attributes.put(tagName, attributeSet);
		}
		return this;
	}

	/**
	 * Add an enforced attribute to a tag. An enforced attribute will always be
	 * added to the element. If the element already has the attribute set, it
	 * will be overridden.
	 * <p/>
	 * E.g.: <code>addEnforcedAttribute("a", "rel", "nofollow")</code> will make
	 * all <code>a</code> tags output as
	 * <code><a href="..." rel="nofollow"></code>
	 * 
	 * @param tag
	 *            The tag the enforced attribute is for. The tag will be added
	 *            to the allowed tag list if necessary.
	 * @param key
	 *            The attribute key
	 * @param value
	 *            The enforced attribute value
	 * @return this (for chaining)
	 */
	public Whitelist addEnforcedAttribute(String tag, String key, String value) {
		Validate.notEmpty(tag);
		Validate.notEmpty(key);
		Validate.notEmpty(value);

		TagName tagName = TagName.valueOf(tag);
		if (!tagNames.contains(tagName))
			tagNames.add(tagName);
		AttributeKey attrKey = AttributeKey.valueOf(key);
		AttributeValue attrVal = AttributeValue.valueOf(value);

		if (enforcedAttributes.containsKey(tagName)) {
			enforcedAttributes.get(tagName).put(attrKey, attrVal);
		} else {
			Map<AttributeKey, AttributeValue> attrMap = new HashMap<AttributeKey, AttributeValue>();
			attrMap.put(attrKey, attrVal);
			enforcedAttributes.put(tagName, attrMap);
		}
		return this;
	}

	/**
	 * Configure this Whitelist to preserve relative links in an element's URL
	 * attribute, or convert them to absolute links. By default, this is
	 * <b>false</b>: URLs will be made absolute (e.g. start with an allowed
	 * protocol, like e.g. {@code http://}.
	 * <p />
	 * Note that when handling relative links, the input document must have an
	 * appropriate {@code base URI} set when parsing, so that the link's
	 * protocol can be confirmed. Regardless of the setting of the
	 * {@code preserve relative
	 * links} option, the link must be resolvable against the base URI to an
	 * allowed protocol; otherwise the attribute will be removed.
	 * 
	 * @param preserve
	 *            {@code true} to allow relative links, {@code false} (default)
	 *            to deny
	 * @return this Whitelist, for chaining.
	 * @see #addProtocols
	 */
	public Whitelist preserveRelativeLinks(boolean preserve) {
		preserveRelativeLinks = preserve;
		return this;
	}

	/**
	 * Add allowed URL protocols for an element's URL attribute. This restricts
	 * the possible values of the attribute to URLs with the defined protocol.
	 * <p/>
	 * E.g.: <code>addProtocols("a", "href", "ftp", "http", "https")</code>
	 * 
	 * @param tag
	 *            Tag the URL protocol is for
	 * @param key
	 *            Attribute key
	 * @param protocols
	 *            List of valid protocols
	 * @return this, for chaining
	 */
	public Whitelist addProtocols(String tag, String key, String... protocols) {
		Validate.notEmpty(tag);
		Validate.notEmpty(key);
		Validate.notNull(protocols);

		TagName tagName = TagName.valueOf(tag);
		AttributeKey attrKey = AttributeKey.valueOf(key);
		Map<AttributeKey, Set<Protocol>> attrMap;
		Set<Protocol> protSet;

		if (this.protocols.containsKey(tagName)) {
			attrMap = this.protocols.get(tagName);
		} else {
			attrMap = new HashMap<AttributeKey, Set<Protocol>>();
			this.protocols.put(tagName, attrMap);
		}
		if (attrMap.containsKey(attrKey)) {
			protSet = attrMap.get(attrKey);
		} else {
			protSet = new HashSet<Protocol>();
			attrMap.put(attrKey, protSet);
		}
		for (String protocol : protocols) {
			Validate.notEmpty(protocol);
			Protocol prot = Protocol.valueOf(protocol);
			protSet.add(prot);
		}
		return this;
	}

	/**
	 * Test if the supplied tag is allowed by this whitelist
	 * 
	 * @param tag
	 *            test tag
	 * @return true if allowed
	 */
	protected boolean isSafeTag(String tag) {
		return tagNames.contains(TagName.valueOf(tag));
	}

	/**
	 * Test if the supplied attribute is allowed by this whitelist for this tag
	 * 
	 * @param tagName
	 *            tag to consider allowing the attribute in
	 * @param el
	 *            element under test, to confirm protocol
	 * @param attr
	 *            attribute under test
	 * @return true if allowed
	 */
	protected boolean isSafeAttribute(String tagName, Element el, Attribute attr) {
		TagName tag = TagName.valueOf(tagName);
		AttributeKey key = AttributeKey.valueOf(attr.getKey());

		if (attributes.containsKey(tag)) {
			if (attributes.get(tag).contains(key)) {
				if (protocols.containsKey(tag)) {
					Map<AttributeKey, Set<Protocol>> attrProts = protocols
							.get(tag);
					// ok if not defined protocol; otherwise test
					return !attrProts.containsKey(key)
							|| testValidProtocol(el, attr, attrProts.get(key));
				} else { // attribute found, no protocols defined, so OK
					return true;
				}
			}
		}
		// no attributes defined for tag, try :all tag
		return !tagName.equals(":all") && isSafeAttribute(":all", el, attr);
	}

	private boolean testValidProtocol(Element el, Attribute attr,
			Set<Protocol> protocols) {
		// try to resolve relative urls to abs, and optionally update the
		// attribute so output html has abs.
		// rels without a baseuri get removed
		String value = el.absUrl(attr.getKey());
		if (value.length() == 0)
			value = attr.getValue(); // if it could not be made abs, run as-is
										// to allow custom unknown protocols
		if (!preserveRelativeLinks)
			attr.setValue(value);

		for (Protocol protocol : protocols) {
			String prot = protocol.toString() + ":";
			if (value.toLowerCase().startsWith(prot)) {
				return true;
			}
		}
		return false;
	}

	Attributes getEnforcedAttributes(String tagName) {
		Attributes attrs = new Attributes();
		TagName tag = TagName.valueOf(tagName);
		if (enforcedAttributes.containsKey(tag)) {
			Map<AttributeKey, AttributeValue> keyVals = enforcedAttributes
					.get(tag);
			for (Map.Entry<AttributeKey, AttributeValue> entry : keyVals
					.entrySet()) {
				attrs.put(entry.getKey().toString(), entry.getValue()
						.toString());
			}
		}
		return attrs;
	}

	// named types for config. All just hold strings, but here for my sanity.

	static class TagName extends TypedValue {
		TagName(String value) {
			super(value);
		}

		static TagName valueOf(String value) {
			return new TagName(value);
		}
	}

	static class AttributeKey extends TypedValue {
		AttributeKey(String value) {
			super(value);
		}

		static AttributeKey valueOf(String value) {
			return new AttributeKey(value);
		}
	}

	static class AttributeValue extends TypedValue {
		AttributeValue(String value) {
			super(value);
		}

		static AttributeValue valueOf(String value) {
			return new AttributeValue(value);
		}
	}

	static class Protocol extends TypedValue {
		Protocol(String value) {
			super(value);
		}

		static Protocol valueOf(String value) {
			return new Protocol(value);
		}
	}

	abstract static class TypedValue {
		private String value;

		TypedValue(String value) {
			Validate.notNull(value);
			this.value = value;
		}

		@Override
		public int hashCode() {
			final int prime = 31;
			int result = 1;
			result = prime * result + ((value == null) ? 0 : value.hashCode());
			return result;
		}

		@Override
		public boolean equals(Object obj) {
			if (this == obj)
				return true;
			if (obj == null)
				return false;
			if (getClass() != obj.getClass())
				return false;
			TypedValue other = (TypedValue) obj;
			if (value == null) {
				if (other.value != null)
					return false;
			} else if (!value.equals(other.value))
				return false;
			return true;
		}

		@Override
		public String toString() {
			return value;
		}
	}
}