I'm trying to figure out what the the minimum amount of encoding would be that would protect a site from XSS.
I know for sure I'll need to encode < (<) and > (>) inside of tags, " (") and ' (') inside attributes.
Do I also need to encode & (&)? I was having trouble with double encoding when the user was saving data (because & would become &). Are there any security vulnerabilities or downsides that would happy if I didn't encode the ampersands? This would mean they'd be able to input any HTML entities they wanted.
By HTML entities I specifically mean ampersand-prefixed sequences that correspond to entities (like © ™).
This question is language-agnostic (except for the HTML part, of course).
Edit: heh, stack-overflow lets me keep my html encoded entities :) That might be telling.
You only need to encode these entities if you are displaying them on a page (and & needs to be escaped just as much as > and < because it is the escape sequence identifier).
If you're having trouble with double encoding of & signs, it sounds like you're doing it before you insert the data into your storage mechanism (database?) Stop that. You should only escape the data for the page when it comes to display on the page.
Related
Should I be encoding characters contained within a url?
Example:
Some link using &
or
Some link using &
Yes.
In HTML (including XHTML and HTML5, as far as I know), all attribute values and tag content should be encoded:
Authors should also use "&" in attribute values since character references are allowed within CDATA attribute values.
There are two different kinds of encoding which are needed for different purposes in web programming, and it is easy to get confused.
Special characters in text which is to be displayed as HTML need to be encoded as HTML entities. This is particularly characters such as '<' which are part of HTML markup, but it may also be useful for other special characters if there is any doubt about the character encoding to be used.
Special characters in a URL need to be URL-encoded (replaced by %nn codes).
There is no harm in putting an HTML entity into a URL if it is going to be treated as HTML text by whatever receives it; but if it is part of an instruction to a program (such as the & used to separate arguments in a CGI query string) you should not encode it.
Depends how your files are being served up and identified.
For XHTML, yes and it's required.
For HTML, no and it's incorrect to do it.
I have implemented a search engine in C for my html website. My entire web is programmed in C.
I understand that html input sanitization is necessary because an attacker can input these 2 html snippets into my search page to trick my search page into downloading and displaying foreign images/scripts (XSS):
<img src="path-to-attack-site"/>
<script>...xss-code-here...</script>
Wouldn't these attacks be prevented simply by searching for '<' and '>' and stripping them from the search query ? Wouldn't that render both scripts useless since they would not be considered html ? I've seen html filtering that goes way beyond this where they filter absolutely all the JavaScript commands and html markup !
Input sanitisation is not inherently ‘necessary’.
It is a good idea to remove things like control characters that you never want in your input, and certainly for specific fields you'll want specific type-checking (so that eg. a phone number contains digits).
But running escaping/stripping functions across all form input for the purpose of defeating cross-site-scripting attacks is absolutely the wrong thing to do. It is sadly common, but it is neither necessary nor in many cases sufficient to protect against XSS.
HTML-escaping is an output issue which must be tackled at the output stage: that is, usually at the point you are templating strings into the output HTML page. Escape < to <, & to &, and in attribute values escape the quote you're using as an attribute delimiter, and that's it. No HTML-injection is possible.
If you try to HTML-escape or filter at the form input stage, you're going to have difficulty whenever you output data that has come from a different source, and you're going to be mangling user input that happens to include <, & and " characters.
And there are other forms of escaping. If you try to create an SQL query with the user value in, you need to do SQL string literal escaping at that point, which is completely different to HTML escaping. If you want to put a submitted value in a JavaScript string literal you would have to do JSON-style escaping, which is again completely different. If you wanted to put a value in a URL query string parameter you need URL-escaping, not HTML-escaping. The only sensible way to cope with this is to keep your strings as plain text and escape them only at the point you output them into a different context like HTML.
Wouldn't these attacks be prevented simply by searching for '<' and '>' and stripping them from the search query ?
Well yes, if you also stripped ampersands and quotes. But then users wouldn't be able to use those characters in their content. Imagine us trying to have this conversation on SO without being able to use <, & or "! And if you wanted to strip out every character that might be special when used in some context (HTML, JavaScript, CSS...) you'd have to disallow almost all punctuation!
< is a valid character, which the user should be permitted to type, and which should come out on the page as a literal less-than sign.
My entire web is programmed in C.
I'm so sorry.
Encoding brackets is indeed sufficient in most cases to prevent XSS, as anything between tags will then display as plain-text.
When outputting HTML, there are several different places where text can be interpreted as control characters rather than as text literals. For example, in "regular" text (that is, outside any element markup):
<div>This is regular text</div>
As well as within the values of attributes:
<input value="this is value text">
And, I believe, within HTML comments:
<!-- This text here might be programmatically generated
and could, in theory, contain the double-hyphen character
sequence, which is verboten inside comments -->
Each of these three kinds of text has different rules for how it must be escaped in order to be treated as non-markup. So my first question is, are there any other contexts in HTML in which characters can be interpreted as markup/control characters? The above contexts clearly have different rules about what needs to be escaped.
The second question is, what are the canonical, globally-safe lists of characters (for each context) that need to be escaped to ensure that any embedded text is treated as non-markup? For example, in theory you only need to escape ' and " in attribute values, since within an attribute value only the closing-delimiter character (' or " depending on which delimiter the attribute value started with) would have control meaning. Similarly, within "regular" text only < and & have control meaning. (I realize that not all HTML parsers are identical. I'm mostly interested in what is the minimum set of characters that need escaping in order to appease a spec-conforming parser.)
Tangentially: The following text will throw errors as HTML 4.01 Strict:
foo
Specifically, it says that it doesn't know what the entity "&y" is supposed to be. If you put a space after the &, however, it validates just fine. But if you're generating this on the fly, you're probably not going to want to check whether each use of & will cause a validation error, and instead just escape all & inside attribute values.
<div>This is regular text</div>
Text content: & must be escaped. < must be escaped.
If producing a document in a non-UTF encoding, characters that do not fit inside the chosen encoding must be escaped.
In XHTML (and XML in general), the sequence ]]> must not occur in text content, so in that specific case one of the characters in that sequence must be escaped, traditionally the >. For consistency, the Canonical XML specification chooses to escape > every time in text content, which is not a bad strategy for an escaping function, though you can certainly skip it for hand-authoring.
<input value="this is value text">
Attribute values: & must be escaped. The attribute value delimiter " or ' must be escaped. If no attribute value delimiter is used (don't do that) no escape is possible.
Canonical XML always chooses " as the delimiter and therefore escapes it. The > character does not need to be escaped in attribute values and Canonical XML does not. The HTML4 spec suggested encoding > anyway for backwards compatibility, but this affects only a few truly ancient and dreadful browsers that no-one remembers now; you can ignore that.
In XHTML < must be escaped. Whilst you can get away with not escaping it in HTML4, it's not a good idea.
To include tabs, CR or LF in attribute values (without them being turned into plain spaces by the attribute value normalisation algorithm) you must encode them as character references.
For both text content and attribute values: in XHTML under XML 1.1, you must escape the Restricted Characters, which are the Delete character and C0 and C1 control codes, minus tab, CR, LF and NEL. In total, [\x01-\x08\x0B\x0C\x0E-\x1F\x7F-\x84\x86-\x9F]. The null character may not be included at all even escaped in XML 1.1. Outside XML 1.1 you can't use any of these characters at all, nor is there a good reason you'd ever want to.
<!-- This text here might be programmatically generated
and could, in theory, contain the double-hyphen character
sequence, which is verboten inside comments -->
Yes, but since there is no escaping possible inside comments, there is nothing you can do about it. If you write <!-- < -->, it literally means a comment containing “ampersand-letter l-letter t-semicolon” and will be reflected as such in the DOM or other infoset. A comment containing -- simply cannot be serialised at all.
<![CDATA[ sections and <?pis in XML also cannot use escaping. The traditional solution to serialise a CDATA section including a ]]> sequence is to split that sequence over two CDATA sections so it doesn't occur together. You can't serialise it in a single CDATA section, and you can't serialise a PI with ?> in the data.
CDATA-elements like <script> and <style> in HTML (not XHTML) may not contain the </ (ETAGO) sequence as this would end the element early and then error if not followed by the end-tag-name. Since no escaping is possible within CDATA-elements, this sequence must be avoided and worked around (eg. by turning document.write('</p>') into document.write('<\/p>');. (You see a lot of more complicated silly strategies to get around this one, like calling unescape on a JS-%-encoded string; even often '</scr'+'ipt>' which is still quite invalid.)
There is one more context in HTML and XML where different rules apply, and that's in the DTD (including the internal subset in the DOCTYPE declaration, if you have one), where the % character has Special Powers and would need to be escaped to be used literally. But as an HTML document author it is highly unlikely you would ever need to go anywhere near that whole mess.
The following text will throw errors as HTML 4.01 Strict:
foo
Yes, and it's just as much an error in Transitional.
If you put a space after the &, however, it validates just fine.
Yes, under SGML rules anything but [A-Za-z] and # doesn't start parsing as a reference. Not a good idea to rely on this though. (Of course, it's not well-formed in XHTML.)
The above contexts clearly have different rules about what needs to be escaped.
I'm not sure that the different elements have different encoding rules like you say. All the examples you list require the HTML encoding.
E.g.
<h1>Fish & Chips</h1>
<img alt="Awesome picture of Meat Pie & Chips" />
Fish & Chips
The last example includes some URL Encoding for the ampersand too (&) and its at this point things get hairy (sending an ampersand as data, which is why it must be encoded).
So my first question is, are there any other contexts in HTML in which characters can be interpreted as markup/control characters?
Anywhere within the HTML document, if the control characters are not being used as control characters, you should encode them (as a good rule of thumb). Most of the time, its HTML Encoding, & or > etc. Othertimes, when trying to pass these characters via a URL, use URL Encoding %20, %26 etc.
The second question is, what are the canonical, globally-safe lists of characters (for each context) that need to be escaped to ensure that any embedded text is treated as non-markup?
I'd say that the Wikipedia article has a few good comments on it and might be worth a read - also the W3 Schools article I guess is a good point. Most languages have built in functions to prepare text as safe HTML, so it may be worth checking your language of choice (if you are indeed even using any scripting languages and not hand coding the HTML).
Specifically, Wikipedia says: "Characters <, >, " and & are used to delimit tags, attribute values, and character references. Character entity references <, >, " and &, which are predefined in HTML, XML, and SGML, can be used instead for literal representations of the characters."
For URL Encoding, this article seems a good starting point.
Closing thoughts as I've already rambled a bit: This is all excluding the thoughts of XML / XHTML which brings a whole other ballgame to the court and its requirement that pretty much the world and its dog needs to be encoded. If you are using a scripting language and writing out a variable via that, I'm pretty sure it'll be easier to find the built in function, or download a library that'll do this for you. :) I hope this answer was scoped ok and didn't miss the point or question or come across in the wrong tone. :)
If you are looking for the best practices to escape characters in web browsers (including HTML, JavaScript and style sheets), the XSS prevention cheat sheet by Michael Coates is probably what you're looking for. It includes a description of the different interpretation contexts, tables indicating how to encode characters in each context and code samples (using ESAPI).
http://www.owasp.org/index.php/XSS_(Cross_Site_Scripting)_Prevention_Cheat_Sheet
Beware that <script> followed by <!-- followed by <script> again, enters double-escaped state, in which you probably never want to be, so ideally you should escape < with "\u003C" within your script's strings (and regexps) to not trigger it accidentally.
You can read more about it here http://qbolec-memdump.blogspot.com/2013/11/script-tag-content-madness.html
If you are this concerned about the validity of the final HTML, you might consider constructing the HTML via a DOM, versus as text.
You don't say what environment you are targeting.
Within my HTML, can I use the character entity reference " " in place of "%20" in Web URLs?
They're both spaces, right?
The short answer is, they are both used to represent "spaces", but they represent different spaces.
%20 is the URL escaping for byte 32, which corresponds to plain old space in pretty much any encoding you're likely to use in a URL.
is an HTML character reference which actually refers to character 160 of Unicode (and also ISO-8859-1 aka Latin-1). It's a different space character entirely -- the "non-breaking space". Even though they look pretty much the same, they're different characters and it's unlikely that your server will treat them the same way.
No. Neither are spaces (technically). Both represent spaces in different ways though. Make every effort to NOT have spaces, or representatives of spaces, in your URLs. Many find it more elegant (me included) to replace spaces with _ or -
No. is an HTML non-breaking-space entity; this entity has no meaning when used in a filesystem or wherever else that a URL might point. URLs are not encoded in HTML.
No, not in the URLs. What you can do is replace spaces in the textual representation of the URL.
So instead of:
http://some.site/doc%20with%20spaces
you can have:
http://some.site/doc with spaces
%20 is what you get with URL encoding, so this is what you should use if you are going to use it in a URL.
is a HTML entity, which is what should be used for 'non breaking space' in an HTML document.
Most persons try to absolutely avoid spaces in their filenames in URLs. They will give you a serious headache every time so try to do so.
If you want to have spaces in an URL you have to encode them with %20.
  is used by the browser to know how to display the page. This information is only used for displaying. The %20 will be sent to the server that manages all the stuff needed to transfer the webpage to your visitors. The server doesn't speak html so the server would interpret   as a normal part of the filenname and search for a file called in the way foo bar. This file will not be found. Much worse the web server will think that the & begins the variable part of the url and only search for the page foo and then try to generate a variable nbsp and a variable bar but he want see any values for them. All in all the web server can't handle a URL with an in it.
Neither are spaces. You shouldnt be using spaces but if for what ever reason you can't avoid it you should just be able to do...
Hey there
...clicking on which will automatically navigate the user to
WebSite/Web%20Page.aspx
Why are HTML character entities necessary? What good are they? I don't see the point.
Two main things.
They let you use characters that are not defined in a current charset. E.g., you can legally use ASCII as the charset, and still include arbitrary Unicode characters thorugh entities.
They let you quote characters that HTML gives special meaning to, as Simon noted.
"1 < 2" lets you put "1 < 2" in your page.
Long answer:
Since HTML uses '<' to open tags, you can't just type '<' if you want that as text. Therefore, you have to have a way to say "I want the text < in my page". Whoever designed HTML (or, actually SGML, HTML's predecessor) decided to use '&something;', so you can also put things like non-breaking space: ' ' (spaces that are not collapsed or allow a line break). Of course, now you need to have a way to say '&', so you get '&'...
They aren't, apart from &, <, >, " and probably . For all other characters, just use UTF-8.
In SGML and XML they aren't just for characters. They are generic inclusion mechanism, and their use for special characters is just one of many cases.
<!ENTITY signature "<hr/><p>Regards, <i>&myname;</i></p>">
<!ENTITY myname "John Doe">
This kind of entities is not useful for web sites, because they work only in XML mode, and you can't use external DTD file without enabling "validating" parsing mode in browser configuration.
Entities can be expanded recursively. This allows use of XML for Denial of Serice attack called "Billion Laughs Attack".
Firefox uses entities internally (in XUL and such) for internationalization and brand-independent messages (to make life easier for Flock and IceWeasel):
<!ENTITY hidemac.label "Hide &brandShortName;">
<!ENTITY hidewin.label "Hide - &brandShortName;">
In HTML you just need <, & and " to avoid ambiguities between text and markup.
All other entities are basically obsoleted by Unicode encodings and remain only as covenience (but a good text editor should have macros/snippets that can replace them).
In XHTML all entities except the basic few are problematic, because won't work with stand-alone XML parsers (e.g. won't work).
To parse all XHTML entities you need validating XML parser (option's usually called "resolve externals") which is slower and needs DTD Catalog set up. If you ignore or screw up your DTD Catalog, you'll be participating in DDoS of W3C servers.
Character entities are used to represent character which are reserved to write HTML for.ex.
<, >, /, & etc, if you want to represent these characters in your content you should use character entities, this will help the parser to distinguish between the content and markup
You use entities to help the parser distinguish when a character should be represented as HTML, and what you really want to show the user, as HTML will reserve a special set of characters for itself.
Typing this literally in HTML
I don't mean it like that </sarcasm>
will cause the "</sarcasm>" tag to disappear,
e.g.
I don't mean it like that
as HTML does not have a tag defined as such. In this case, using entities will allow the text to display properly.
e.g.
No, really! </sarcasm>
gives
No, really! </sarcasm>
as desired.