We have all experienced our favorite browser asking us if it should save the data we just entered into a form on a web page so it can auto-complete / auto-populate that information the next time we visit the page.
Unfortunately, our browsers are not intelligent enough to think about security on their own and therefore they like to cache anything typed into a form including our passwords. This of course poses a security risk that not many web developers think about when building applications.
For obvious security reasons turning the auto-complete functionality off for all password input fields in a form is a great idea. In fact it is such a great idea it should be second nature to all developers creating forms with any input fields accepting sensitive information (passwords, SSN, account numbers, etc… ).
To turn the auto-complete functionality of on ASP.net TextBox controls you use the AutoCompleteType attribute. Simply add it to your TextBox markup in your aspx or ascx file like so.
Markup Method:

<asp:TextBox id="tbPassword" runat="server" TextMode="Password" Columns="35" AutoCompleteType="Disabled">
Or if you prefer to do it in code you can alter the AutoCompleteType property of the TextBox control in your C# code like so:
Code-Behind Method:

tbPassword.AutoCompleteType = AutoCompleteType.Disabled;
Now the above is what the MSDN documentation tells you to do to turn auto-complete off. But I must say in one of my applications, a rather complicated one, setting the AutoCompleteType property of the ASP.net control to Disabled did not cause the actual HTML attribute of “autocomplete” to be rendered with the off value. I spent time trying to figure out why it was never added to the final HTML output but could not find a reason. 
In this particular case I ended up giving in and just adding the attribute to the control manually as follows:
Code-Behind Method:

tbPassword.Attributes.Add("autocomplete", "off");
You can also turn auto-complete off for all input fields in an entire form by adding the “autocomplete” attribute to the From directly and setting its value to “off”.
Markup Method:

<form autocomplete="off">
Note: the “autocomplete” attribute we have been discussing here is not found in the actual HTML documentation for either the “input” or “form” tags as defined by the W3C which means that it is selectively implemented by browsers and therefore not universally functional across all browsers.