Say we have a form where the user types in various info. We validate the info, and find that something is wrong. A field is missing, invalid email, et cetera.
When displaying the form to the user again I of course don't want him to have to type in everything again so I want to populate the input fields. Is it safe to do this without sanitization? If not, what is the minimum sanitization that should be done first?
And to clearify: It would of course be sanitized before being for example added to a database or displayed elsewhere on the site.
No it isn't. The user might be directed to the form from a third party site, or simply enter data (innocently) that would break the HTML.
Convert any character with special meaning to its HTML entity.
i.e. & to &, < to <, > to > and " to " (assuming you delimit your attribute values using " and not '.
In Perl use HTML::Entities, in TT use the html filter, in PHP use htmlspecialchars. Otherwise look for something similar in the language you are using.
It is not safe, because, if someone can force the user to submit specific data to your form, you will output it and it will be "executed" by the browser. For instance, if the user is forced to submit '/><meta http-equiv="refresh" content="0;http://verybadsite.org" />, as a result an unwanted redirection will occur.
You cannot insert user-provided data into an HTML document without encoding it first. Your goal is to ensure that the structure of the document cannot be changed and that the data is always treated as data-values and never as HTML markup or Javascript code. Attacks against this mechanism are commonly known as "cross-site scripting", or simply "XSS".
If inserting into an HTML attribute value, then you must ensure that the string cannot cause the attribute value to end prematurely. You must also,of course, ensure that the tag itself cannot be ended. You can acheive this by HTML-encoding any chars that are not guaranteed to be safe.
If you write HTML so that the value of the tag's attribute appears inside a pair of double-quote or single-quote characters then you only need to ensure that you html-encode the quote character you chose to use. If you are not correctly quoting your attributes as described above, then you need to worry about many more characters including whitespace, symbols, punctuation and other ascii control chars. Although, to be honest, its arguably safest to encode these non-alphanumeric chars anyway.
Remember that an HTML attribute value may appear in 3 different syntactical contexts:
Double-quoted attribute value
<input type="text" value="**insert-here**" />
You only need to encode the double quote character to a suitable HTML-safe value such as "
Single-quoted attribute value
<input type='text' value='**insert-here**' />
You only need to encode the single quote character to a suitable HTML-safe value such as
Unquoted attribute value
<input type='text' value=**insert-here** />
You shouldn't ever have an html tag attribute value without quotes, but sometimes this is out of your control. In this case, we really need to worry about whitespace, punctuation and other control characters, as these will break us out of the attribute value.
Except for alphanumeric characters, escape all characters with ASCII values less than 256 with the &#xHH; format (or a named entity if available) to prevent switching out of the attribute. Unquoted attributes can be broken out of with many characters, including [space] % * + , - / ; < = > ^ and | (and more). [para lifted from OWASP]
Please remember that the above rules only apply to control injection when inserting into an HTML attribute value. Within other areas of the page, other rules apply.
Please see the XSS prevention cheat sheet at OWASP for more information
Yes, it's safe, provided of course that you encode the value properly.
A value that is placed inside an attribute in an HTML needs to be HTML encoded. The server side platform that you are using should have methods for this. In ASP.NET for example there is a Server.HtmlEncode method, and the TextBox control will automatically HTML encode the value that you put in the Text property.
Related
It is possible to do a XSS attack if my input does not allow < and > characters?
Example: I enter <script>alert('this');</script> text
But it if I delete < and > the script is not text:
I enter script alert('this'); script text
Yes, it could still be possible.
e.g. Say your site injects user input into the following location
<img src="http://example.com/img.jpg" alt="USER-INPUT" />
If USER-INPUT is " ONLOAD="alert('xss'), this will render
<img src="http://example.com/img.jpg" alt="" ONLOAD="alert('xss')" />
No angle brackets necessary.
Also, check out OWASP XSS Experimental Minimal Encoding Rules.
For HTML body:
HTML Entity encode < &
specify charset in metatag to avoid UTF7 XSS
For XHTML body:
HTML Entity encode < & >
limit input to charset http://www.w3.org/TR/2008/REC-xml-20081126/#charsets
So within the body you can get away with only encoding (or removing) a subset of the characters usually recommended to prevent XSS. However, you cannot do this within attributes - the full XSS (Cross Site Scripting) Prevention Cheat Sheet recommends the following, and they do not have a minimal alternative:
Except for alphanumeric characters, escape all characters with the HTML Entity &#xHH; format, including spaces. (HH = Hex Value)
The is mainly though to cover the three types of ways of specifying the attribute value:
Unquoted
Single quoted
Double quoted
Encoding in such a way will prevent XSS in attribute values in all three cases.
Also be wary that UTF-7 attacks do not need angle bracket characters. However, unless the charset is explicitly set to UTF-7, this type of attack isn't possible in modern browsers.
+ADw-script+AD4-alert(document.location)+ADw-/script+AD4-
Also beware of attributes that allow URLs like href and ensure any user input is a valid web URL. Using a reputable library to validate the URL is highly recommended using an allow-list approach (e.g. if protocol not HTTPS then reject). Attempting to block sequences like javascript: is not sufficient.
If the user-supplied input is printed inside an HTML attribute, you also need to escape quotation marks or you would be vulnerable inputs like this:
" onload="javascript-code" foobar="
You should also escape the ampersand character as it generally needs to be encoded inside HTML documents and might otherwise destroy your layout.
So you should take care of the following characters: < > & ' "
You should however not completely strip them but replace them with the correct HTML codes i.e. < > & " '
I am having trouble understanding how escaping works inside html tag attribute values that are javascript.
I was lead to believe that you should always escape & ' " < > . So for javascript as an attribute value I tried:
It doesn't work. However:
and
does work in all browsers!
Now I am totally confused. If all my attribute values are enclosed in double quotes, does this mean I do not have to escape single quotes? Or is apos and ascii 39 technically different characters? Such that javascript requires ascii 39, but not apos?
There are two types of “escapes” involved here, HTML and JavaScript. When interpreting an HTML document, the HTML escapes are parsed first.
As far as HTML is considered, the rules within an attribute value are the same as elsewhere plus one additional rule:
The less-than character < should be escaped. Usually < is used for this. Technically, depending on HTML version, escaping is not always required, but it has always been good practice.
The ampersand & should be escaped. Usually & is used for this. This, too, is not always obligatory, but it is simpler to do it always than to learn and remember when it is required.
The character that is used as delimiters around the attribute value must be escaped inside it. If you use the Ascii quotation mark " as delimiter, it is customary to escape its occurrences using " whereas for the Ascii apostrophe, the entity reference ' is defined in some HTML versions only, so it it safest to use the numeric reference ' (or ').
You can escape > (or any other data character) if you like, but it is never needed.
On the JavaScript side, there are some escape mechanisms (with \) in string literals. But these are a different issue, and not relevant in your case.
In your example, on a browser that conforms to current specifications, the JavaScript interpreter sees exactly the same code alert('Hello');. The browser has “unescaped” ' or ' to '. I was somewhat surprised to hear that ' is not universally supported these days, but it’s not an issue: there is seldom any need to escape the Ascii apostrophe in HTML (escaping is only needed within attribute values and only if you use the Ascii apostrophe as its delimiter), and when there is, you can use the ' reference.
' is not a valid HTML reference entity. You should escape using '
I.E. <img src="world.jpg" data-title="Hello World!<br/>What gives?"/>
As far as I understand the guidelines, it is basically valid, but it's better to use HTML entities.
From the HTML 4 reference:
You should also escape & within attribute values since entity references are allowed within cdata attribute values. In addition, you should escape > as > to avoid problems with older user agents that incorrectly perceive this as the end of a tag when coming across this character in quoted attribute values.
From the HTML 5 reference:
Except where otherwise specified, attributes on HTML elements may have any string value, including the empty string. Except where explicitly stated, there is no restriction on what text can be specified in such attributes.
So the best thing to do, as #tdammers already says, is to escape these characters (quoting the W3C reference)
& to represent the & sign.
< to represent the < sign.
> to represent the > sign.
" to represent the " mark.
and decoding them from their entity values if they are to be used as HTML.
Providing you're serving it as text/html, then yes it's valid.
Note that not only is it possible to include markup inside attributes, but the HTML5 srcdoc attribute on the iframe element positively encourages it. The HTML5 draft says:
In the HTML syntax, authors need only
remember to use U+0022 QUOTATION MARK
characters (") to wrap the attribute
contents and then to escape all U+0022
QUOTATION MARK (") and U+0026
AMPERSAND (&) characters, ....
Note, that when served with an XML content type (e.g. application/xhtml+xml), it is not valid, or even well-formed.
I'd say yes, as in it's still valid HTML5. Older browsers (which ones?) may not parse correctly.
Section 3.2.4.1 Attributes of the current HTML5 draft says this:
Except where otherwise specified, attributes on HTML elements may have any string value, including the empty string. Except where explicitly stated, there is no restriction on what text can be specified in such attributes.
HTML tags inside attributes also validates at http://html5.validator.nu
No. That would be invalid - HTML does not allow < or > inside attributes.
<img src="world.jpg" data-title="Hello World!<br/>What gives?"/> would be valid, but it would display the <br/> literally, not as a newline.
Is there any authoritative reference about the syntax and encoding of an URL for the pseudo-protocol javascript:? (I know it's not very well considered, but anyway it's useful for bookmarklets).
First, we know that standard URLs follow the syntax:
scheme://username:password#domain:port/path?query_string#anchor
but this format doesn't seem to apply here. Indeed, it seems, it would be more correct to speak of URI instead of URL : here is listed the "unofficial" format javascript:{body}.
Now, then, which are the valid characters for such a URI, (what are the escape/unescape rules) when embedding in a HTML?
Specifically, if I have the code of a javascript function and I want to embed it in a javascript: URI, which are the escape rules to apply?
Of course one could escape every non alfanumeric character, but that would be overkill and make the code unreadable. I want to escape only the necessary characters.
Further, it's clear that it would be bad to use some urlencode/urldecode routine pair (those are for query string values), we don't want to decode '+' to spaces, for example.
My findings, so far:
First, there are the rules for writing a valid HTML attribute value: but here the standard only requires (if the attribute value if enclosed in quotes) an arbitrary CDATA (actually a %URI, but HTML itself does not impose additional validation at its level: any CDATA will validate).
Some examples:
<a href="javascript:alert('Hi!')"> (1)
<a href="javascript:if(a > b && 1 < 0) alert( b ? 'hi' : 'bye')"> (2)
<a href="javascript:if(a>b &&& 1 < 0) alert( b ? 'hi' : 'bye')"> (3)
Example (1) is valid. But also example (2) is valid HTML 4.01 Strict. To make it valid XHTML we only need to escape the XML special characters < > & (example 3 is valid XHTML 1.0 Strict).
Now, is example (2) a valid javascript: URI ? I'm not sure, but I'd say it's not.
From RFC 2396: an URI is subject to some addition restrictions and, in particular, the escape/unescape via %xx sequences. And some characters are always prohibited:
among them spaces and {}# .
The RFC also defines a subset of opaque URIs: those that do not have hierarchical components, and for which the separating charactes have no special meaning (for example, they dont have a 'query string', so the ? can be used as any non special character). I assume javascript: URIs should be considered among them.
This would imply that the valid characters inside the 'body' of a javascript: URI are
a-zA-Z0-9
_|. !~*'();?:#&=+$,/-
%hh : (escape sequence, with two hexadecimal digits)
with the additional restriction that it can't begin with /.
This stills leaves out some "important" ASCII characters, for example
{}#[]<>^\
Also % (because it's used for escape sequences), double quotes " and (most important) all blanks.
In some respects, this seems quite permissive: it's important to note that + is valid (and hence it should not be 'unescaped' when decoding, as a space).
But in other respects, it seems too restrictive. Braces and brackets, specially: I understand that they are normally used unescaped and browsers have no problems.
And what about spaces? As braces, they are disallowed by the RFC, but I see no problem in this kind of URI. However, I see that in most bookmarklets they are escaped as "%20". Is there any (empirical or theorical) explanation for this?
I still don't know if there are some standard functions to make this escape/unescape (in mainstream languages) or some sample code.
javascript: URLs are currently part of the HTML spec and are specified at https://html.spec.whatwg.org/multipage/browsing-the-web.html#the-javascript:-url-special-case
I need to post multi-line data via a hidden field. The data will be viewed in a textarea after post. How can I post a newline/carriage return in the html form?
I've tried \r\n but that just posts the actual "\r\n" data
<input type="hidden" name="multiline_data" value="line one\r\nline two" />
Is there a way to do this?
Instead of using
<input type="hidden">
Try using
<textarea style="visibility:hidden;position:absolute;">
While new lines (Carriage Return & Line Feed) are technically allowed in <input>'s hidden state, they should be escaped for compatibility with older browsers. You can do this by replacing all Carriage Returns (\u000D or \r) and all Line Feeds (\u000A or \n) with proprietary strings that are recognized by your application to be a Carriage Return or New Line (and also escaped, if present in the original string).
Simply character entities don't work here, due to non-conforming browsers possibly knowing
and
are new lines and stripping them from the value.
Example
For example, in PHP, if you were to echo the passed value to a textarea, you would include the newlines (and unescaped string).
<textarea>Some text with a \ included
and a new line with \r\n as submitted value</textarea>
However, in PHP, if you were to echo the value to the value attribute of an <input> tag, you would escape the new lines with your proprietary strings (e.g. \r and \n), and escape any instances of your proprietary strings in the submitted value.
<input type="hidden" value="Some text with a \\ included\r\nand a new line\\r\\n as submitted value">
Then, before using the value elsewhere (inserting into a database, emailing, etc), be sure to unescape the submitted value, if necessary.
Reassurance
As further reassurance, I asked the WHATWG, and Ian Hickson, editor of the HTML spec currently, replied:
bfrohs Question about <input type=hidden> -- Are Line Feeds and Carriage Returns allowed in the value? They are specifically disallowed in Text state and Search state, but no mention is made for Hidden state. And, if not, is there an acceptable HTML solution for storing form data from a textarea?
Hixie yes, they are allowed // iirc // for legacy reasons you may wish to escape them though as some browsers normalise them away // i forget if we fixed that or not // in the spec
Source
Depends on the character set really but
should be linefeed and
should be carriage return. You should be able to use those in the value attribute.
You don't say what this is for or what technology you're using, but you need to be aware that you can't trust the hidden field to remain with value="line one
line two", because a hostile user can tamper with it before it gets sent back in the POST. Since you're putting the value in a <textarea> later, you will definitely be subject to, for example, cross site scripting attacks unless you verify and/or sanitize your "multiline_data" field contents before you write it back out.
When writing a value into a hidden field and reading it back, it's usually better to just keep it on the server, as an attribute of the session, or pageflow, or whatever your environment provides to do this kind of thing.