Develop Reference

Escaping JavaScript special characters from ASP.NET

I have following C# code in my ASP.NET application:
string script = #"alert('Message head:\n\n" + CompoundErrStr + " message tail.');";
System.Web.UI.ScriptManager.RegisterClientScriptBlock(this, this.GetType(), "Test", script, true);
CompoundErrStr is an error message generated by SQL Server (exception text bubbled up from the stored procedure). If it contains any table column names they are enclosed in single quotes and JavaScript breaks during execution because single quotes are considered a string terminator.
As a fix for single quotes I changed my code to this:
CompoundErrStr = CompoundErrStr.Replace("'", #"\'");
string script = #"alert('Message head:\n\n" + CompoundErrStr + " message tail.');";
System.Web.UI.ScriptManager.RegisterClientScriptBlock(this, this.GetType(), "Test", script, true);
and it now works fine.
However, are there any other special characters that need to be escaped like this? Is there a .Net function that can be used for this purpose? Something similar to HttpServerUtility.HtmlEncode but for JavaScript.
EDIT I use .Net 3.5

Note: for this task you can't (and you shouldn't) use HTML encoders (like HttpServerUtility.HtmlEncode()) because rules for HTML and for JavaScript strings are pretty different. One example: string "Check your Windows folder c:\windows" will be encoded as "Check your Windows folder c:'windows" and it's obviously wrong. Moreover it follows HTML encoding rules then it won't perform any escaping for \, " and '. Simply it's for something else.
If you're targeting ASP.NET Core or .NET 5 then you should use System.Text.Encodings.Web.JavaScriptEncoder class.
If you're targeting .NET 4.x you can use HttpUtility.JavaScriptStringEncode() method.
If you're targeting .NET 3.x and 2.x:
What do you have to encode? Some characters must be escaped (\, " and ') because they have special meaning for JavaScript parser while others may interfere with HTML parsing so should escaped too (if JS is inside an HTML page). You have two options for escaping: JavaScript escape character </kbd> or \uxxxx Unicode code points (note that \uxxxx may be used for them all but it won't work for characters that interferes with HTML parser).
You may do it manually (with search and replace) like this:
string JavaScriptEscape(string text)
{
return text
.Replace("\\", #"\u005c") // Because it's JS string escape character
.Replace("\"", #"\u0022") // Because it may be string delimiter
.Replace("'", #"\u0027") // Because it may be string delimiter
.Replace("&", #"\u0026") // Because it may interfere with HTML parsing
.Replace("<", #"\u003c") // Because it may interfere with HTML parsing
.Replace(">", #"\u003e"); // Because it may interfere with HTML parsing
}
Of course </kbd> should not be escaped if you're using it as escape character! This blind replacement is useful for unknown text (like input from users or text messages that may be translated). Note that if string is enclosed with double quotes then single quotes don't need to be escaped and vice-versa). Be careful to keep verbatim strings on C# code or Unicode replacement will be performed in C# and your client will receive unescaped strings. A note about interfere with HTML parsing: nowadays you seldom need to create a <script> node and to inject it in DOM but it was a pretty common technique and web is full of code like + "</s" + "cript>" to workaround this.
Note: I said blind escaping because if your string contains an escape sequence (like \uxxxx or \t) then it should not be escaped again. For this you have to do some tricks around this code.
If your text comes from user input and it may be multiline then you should also be ready for that or you'll have broken JavaScript code like this:
alert("This is a multiline
comment");
Simply add .Replace("\n", "\\n").Replace("\r", "") to previous JavaScriptEscape() function.
For completeness: there is also another method, if you encode your string Uri.EscapeDataString() then you can decode it in JavaScript with decodeURIComponent() but this is more a dirty trick than a solution.

While the original question mentions .NET 3.5, it should be known to users of 4.0+ that you can use HttpUtility.JavaScriptStringEncode("string")
A second bool parameter specifies whether to include quotation marks (true) or not (false) in the result.

All too easy:
#Html.Raw(myString)

JavaScript and XML and HTML tags

Using JavaScript to create an xml file (for later saving in PDF). When user enters some HTML characters, like < and >, these cause problems because the program thinks they are Beginning and End HTML tags. We’ve tried the Replace function but have not found the correct Syntax yet. Any ideas?

You might try :
'<foo><bar></foo></bar>'.replace(/>/g, '<').replace(/</g, '>')
The gat the end of the regex is really important b/c otherwise it will only replace the first occurence.

whay backaward slash in the parameter element of the javascript object?

I was inspecting this site in firebug. Inside the third <script/> tag in the head section of the page , I found an object variable declared in the following way ( truncated here however by me) :
var EM={
"ajaxurl":"http:\/\/ipsos.com.au\/wp-admin\/admin-ajax.php",
"bookingajaxurl":"http:\/\/ipsos.com.au\/wp-admin\/admin-ajax.php",
"locationajaxurl":"http:\/\/ipsos.com.au\/wp-admin\/admin-ajax.php?action=locations_search",
"firstDay":"1","locale":"en"};
The utility of the variable is unknown to me. What struck me is the 3 urls presented there. Why are the backward slashes present there? Couldn't it be something like :
"ajaxurl" : "http://ipsos.com.au/wp-admin/admin-ajax.php"
?

In a script element there are various character sequences (depending on the version of HTML) that will terminate the element. </script> will always do this.
<\/script> will not.
Escaping / characters will not change the meaning of the JS, but will prevent any such HTML from ending the script.

The \/\/ is to avoid the below scenario:
when the url looks something similar to "ajaxurl" : "http://google.com/search?q=</script>"
Try copy paste the url in browsers address bar. This is handled correctly. Otherwise, You might end up getting script errors and page might not work as you've expected.
imagine DOM manipulators replacing the value as it is in the src attribute of the script tag and then the javascript engine reporting multiple errors because that particular script referenced might not get loaded due to incorrectly defined src value
Hope this helps.
Life would be hectic without these lil things

It is used to escape the characters..
The backslash () can be used to insert apostrophes, new lines, quotes, and other special characters into a string.
var str = " Hello "World" !! ";
alert(str)
This won't work..
You have to escape them first
var str = " Hello \"World\" !! ";
alert(str) ; \\ This works
In terms of Javascript / and <\/ are identical inside a string. As far as HTML is concerned </ starts an end tag but <\/ does not.

Is it enough to use HTMLEncode to display uploaded text?

We're allowing users to upload pictures and provide a text description. Users can view this through a pop up box (actually a div ) via javascript. The uploaded text is a parameter to a javascript function. I 'm worried about XSS and also finding issues with HTMLEncode().
We're using HTMLEncode to guard against XSS. Unfortunately, we're finding that HTMLEncode() only replaces '<' and '>'. We also need to replace single and double quotes that people may include. Is there a single function that will do all these special type characters or must we do that manually via .NET string.Replace()?

Unfortunately, we're finding that HTMLEncode() only replaces '<' and '>'.
Assuming you are talking about HttpServerUtility.HtmlEncode, that does encode the double-quote character. It also encodes as character references the range U+0080 to U+00FF, for some reason.
What it doesn't encode is the single quote. Bit of a shame but you can usually work around it by using only double quotes as attribute value delimiters in your HTML/XML. In that case, HtmlEncode is enough to prevent HTML-injection.
However, javascript is in your tags, and HtmlEncode is decidedly not enough to escape content to go in a JavaScript string literal. JavaScript-encoding is a different thing to HTML-encoding, so if that's the reason you're worried about the single quote then you need to employ a JS string encoder instead.
(A JSON encoder is a good start for that, but you would want to ensure it encodes the U+2028 and U+2029 characters which are, annoyingly, valid in JSON but not in JavaScript. Also you might well need some variety of HTML-escaping on top of that, if you have JavaScript in an HTML context. This can get hairy; it's usually better to avoid these problems by hiding the content you want in plain HTML, for example in a hidden input or custom attribute, where you can use standard HTML-escaping, and then read that data from the DOM in JS.)

If the text description is embedded inside a JavaScript string literal, then to prevent XSS, you will need to escape special characters such as quotes, backslashes, and newlines. The HttpUtility.HtmlEncode method is not suitable for this task.
If the JavaScript string literal is in turn embedded inside HTML (for example, in an attribute), then you will need to apply HTML encoding as well, on top of the JavaScript escaping.
You can use Microsoft's Anti-Cross Site Scripting library to perform the necessary escaping and encoding, but I recommend that you try to avoid doing this yourself. For example, if you're using WebForms, consider using an <asp:HiddenField> control: Set its Value property (which will be HTML-encoded automatically) in your server-side code, and access its value property from client-side code.

how about you htmlencode all of the input with this extended function:
private string HtmlEncode(string text)
{
char[] chars = HttpUtility.HtmlEncode(text).ToCharArray();
StringBuilder result = new StringBuilder(text.Length + (int)(text.Length * 0.1));
foreach (char c in chars)
{
int value = Convert.ToInt32(c);
if (value > 127)
result.AppendFormat("&#{0};", value);
else
result.Append(c);
}
return result.ToString();
}
this function will convert all non-english characters, symbols, quotes, etc to html-entities..
try it out and let me know if this helps..

If you're using ASP.NET MVC2 or ASP.NET 4 you can replace <%= with <%: to encode your output. It's safe to use for everything it seems (like HTML Helpers).
There is a good write up of this here: New <%: %> Syntax for HTML Encoding Output in ASP.NET 4 (and ASP.NET MVC 2)

Error using Javascript and JSP, string with space gives `Unterminated string literal`

I have to pass a string value to JavaScript from JSP page.
I am using
display("<%=name%>")
It works fine but when i have string like 'sweet milk', JavaScript throws the error
Unterminated string literal
How to solve this?

Your string contains single quotes - you can escape single quotes as "\x27" and double quotes as "\x22" and then pass it to javascript.

You probably have characters in your String that should be escaped in Javascript. For example, if your string is My name is "John", your code will generate
var a = "My name is "John"";
which is invalid.
You should use StringEscapeUtils.escapeJavaScript from commons-lang to make sure everything is correctly escaped (single and double quotes, newlines, tabs, etc.).

I guess there's an error in the generated JavaScript code. Is there any way to look at you page? I suggest to look at the generated source code of that page.