I understand the need for &, <, etc. But is " necessary? I suppose it could be useful inside tag attributes, but inside the text, outside any tag, is it necessary?
No, it is not necessary when used in normal html content.
for escaping quotes in tag attributes you can use " or either \" or \' depending on which you want to escape.
Certainly not necessary, your HTML will validate just fine without it.
However, if your HTML is generated and includes user input in unknown text encodings, or if you’re supplying HTML to people who might not serve it with the correct text encoding, then you might want to err on the safe side and encode any slightly odd-looking character as an entity.
No, it's not. As an example, the specifications for HTML 4.01 (http://www.w3.org/TR/1999/REC-html401-19991224/charset.html#h-5.3.2) state:
[...] authors may use SGML character
references.
That 'may' implies you are not forced to use them.
Related
i have been working with HTML for some time, recently a question popped in my mind that when ever give tags
<input type = "text" name="c-id" />
<input type =text name=c-id />
with or without quotes there is no difference, when working with V-Studio, this IDE doesn't put quotes automatically, but while working with the Dreamweaver if i remember correctly it does put the quotes automatically...
what i want is to know is that at what point\s the absence of quotes makes a difference or creates a problem, and what is the best practice, always go quoted or quotes-less...
p.s. there is very good chance this question has been asked before but didn't show up in the search
It makes a difference when the attribute value contains a space, linefeed, formfeed, or tab character, because the attribute value will end at any one of those characters if the value is not wrapped in quotes.
From an IDE point of view, it's little more than a matter of preference whether to add the quotes or not when the attribute value does not contain one of those whitespace characters.
In XHTML syntax, the quotation marks are always required (but you can alternatively use single quotes, i.e. 'apostrophes'). This does not matter if the page is served with text/html content type, as it almost always is: browsers will parse it as sloppy HTML, not as real XHTML. But if the page is served with an XML content type, or if it is opened in a program that expects XHTML or other XML, then lack of quotation marks causes Draconian error processing: only an error message is shown, not the content at all.
In the example case, XHTML syntax is used otherwise: the “/” before closing “>” belongs to XHTML, not HTML. It makes little point to write XHTML in that respect but not consistently.
In HTML syntax, the formal rules depend on HTML version. HTML5 is much more permissive than HTML 4.01. For example, <a href=/foo/bar/ title=What???> is valid HTML5 but not valid HTML 4.01. This mostly matters in validation and depend on which version of HTML you wish to validate against (i.e., which one do you mostly try to comply with). In this issue, HTML5 reflects browser practices: browsers have long been more permissive than HTML 4.01.
By HTML5 rules, the quotes (though always allowed) are needed only when the attribute value contains any of the following characters: space, tab, line feed, form feed, carriage return, quotation mark ("), apostrophe ('), equals sign (=), less-than sign (<), greater-than sign (>), or grave (`), or is empty.
There are also opinions as well as coding style guides and other recommendations on the matter. Most people who think about such issues apparently favor the “safe” way of always putting quotes around attribute values. Some people think the quotes improve readability of code; others think they reduce it.
Not putting your values into quotes may cause some problems, ie you can't provide multiple css classes for your elements. Browser will only understand the first value, and the page won't pass validation.
You should always put your values into quotes. The markup is easier to read and you won't run into some unexpected browser behaviour.
For me, any practice that improves code/markup (re-)readability is worth making a habit, and having quotes on the attribute values visibly separates one property from another (readability again).
On the technical side, some attribute values have spaces and the whole tag might not work without the quotes on these. Those that come to mind: img's alt, multiple class values, title (for tooltips), an href with a url that has the equal sign (without quotes, the first equal in href= is doomed), etc.
And here is the almost same question in SO.
--
My thanks to Alohci for enlightening me. I can't up vote yet.
When encoding possibly unsafe data, is there a reason to encode >?
It validates either way.
The browser interprets the same either way, (In the cases of attr="data", attr='data', <tag>data</tag>)
I think the reasons somebody would do this are
To simplify regex based tag removal. <[^>]+>? (rare)
Non-quoted strings attr=data. :-o (not happening!)
Aesthetics in the code. (so what?)
Am I missing anything?
Strictly speaking, to prevent HTML injection, you need only encode < as <.
If user input is going to be put in an attribute, also encode " as ".
If you're doing things right and using properly quoted attributes, you don't need to worry about >. However, if you're not certain of this you should encode it just for peace of mind - it won't do any harm.
The HTML4 specification in its section 5.3.2 says that
authors should use ">" (ASCII decimal 62) in text instead of ">"
so I believe you should encode the greater > sign as > (because you should obey the standards).
Current browsers' HTML parsers have no problems with uquoted >s
However, unfortunately, using regular expressions to "parse" HTML in JS is pretty common. (example: Ext.util.Format.stripTags). Also poorly written command line tools, IDEs, or Java classes etc. may not be sophisticated enough to determine the limiter of an opening tag.
So, you may run into problems with code like this:
<script data-usercontent=">malicious();//"></script>
(Note how the syntax highlighter treats this snippet!)
Always
This is to prevent XSS injections (through users using any of your forms to submit raw HTML or javascript). By escaping your output, the browser knows not to parse or execute any of it - only display it as text.
This may feel like less of an issue if you're not dealing with dynamic output based on user input, however it's important to at least understand, if not to make a good habit.
Yes, because if signs were not encoded, this allows xss on forms social media and many other because a attacker can use <script> tag. If you parse the signs the browser would not execute it but instead show the sign.
Encoding html chars is always a delicate job. You should always encode what needs to be encoded and always use standards. Using double quotes is standard, and even quotes inside double quotes should be encoded. ENCODE always. Imagine something like this
<div> this is my text an img></div>
Probably the img> will be parsed from the browser as an image tag. Browsers always try to resolve unclosed tags or quotes. As basile says use standards, otherwise you could have unexpected results without understanding the source of errors.
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.
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.
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.