Is it safe to display user input as input values without sanitization?

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开发者_Python百科 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.