问题
Ok, yes, it's 2020 but don't laugh. I'm trying update some ASP.NET web forms. My goal is to lock them down, making them more secure by applying a more restrictive Content Security Policy (CSP). To that end, I'm using a nonce, rather than allowing unsafe-inline for scripting. For "simple" web forms, it's working fine. However, I hit a problem whenever there's an asp control that results in a post back. When I look in the page source, I see stuff like this:
<script type="text/javascript">
//<![CDATA[
var theForm = document.forms['form1'];
if (!theForm) {
theForm = document.form1;
}
function __doPostBack(eventTarget, eventArgument) {
if (!theForm.onsubmit || (theForm.onsubmit() != false)) {
theForm.__EVENTTARGET.value = eventTarget;
theForm.__EVENTARGUMENT.value = eventArgument;
theForm.submit();
}
}
and
<script type="text/javascript">
function previewFile() {
var preview = document.querySelector('#Body_Main_LogoImage');
var file = document.querySelector('#Body_Main_logoUpload').files[0];
var reader = new FileReader();
reader.onloadend = function () {
preview.src = reader.result;
}
if (file) {
reader.readAsDataURL(file);
} else {
preview.src = "";
}
}
</script>
This code is generated by some of the MS web form code, I believe. The problem is that neither of these script elements have a nonce, which I'd like to supply, so it is in violation of my CSP (which does not include unsafe-inline). Is there any thing I can override to customize this behaviour?
回答1:
Some of the offending code is defined in a private method (e.g. RenderPostBackScript) inside the Page class, which in turn is called by internal methods. So, overriding it the normal way isn't an option unless I replace a large bunch of code. As an alternative, I overrode CreateHtmlTextWriter in my page:
protected override System.Web.UI.HtmlTextWriter CreateHtmlTextWriter(TextWriter tw)
{
return new HtmlTextWriter(tw, this);
}
and then in my HtmlWriter class, I do this:
public override void Write(string s)
{
if (s != null && s.IndexOf("<script") > -1 && s.IndexOf("nonce") == -1 && s.IndexOf("src") == -1)
{
s = s.Replace("<script", "<script nonce=\"" + this._page.GetNonce() + "\"");
}
base.Write(s);
}
I used a similar approach in my HtmlTextWriter for AddAttribute to avoid inline javascript: inside href, which would required unsafe-inline for script-src (and preclude the use of a nonce).
public override void AddAttribute(HtmlTextWriterAttribute key, string value)
{
if (key == HtmlTextWriterAttribute.Href && value != null && value.IndexOf("javascript:") > -1)
{
base.AddAttribute(key, "#");
var newScript = value.Replace("javascript:", "");
newScript += "; return false;";
base.AddAttribute(HtmlTextWriterAttribute.Onclick, newScript);
}
else
{
base.AddAttribute(key, value);
}
}
回答2:
You could override __DoPostBack
and use your own function instead.
var __original= __doPostBack;
__doPostBack = myFunction();
some more ideas over here: How to intercept any postback in a page? - ASP.NET
回答3:
If you open up the dev tools in Chrome, you'll likely see a message like Refused to execute inline script because it violates the following Content Security Policy directive: "script-src 'self'. Either the 'unsafe-inline' keyword, a hash ('sha256-2NqnatcPqy5jjBXalTpZyJMO/0fUaYUb3ePlviUP4II='), or a nonce ('nonce-...') is required to enable inline execution.
If you look carefully at that message, it's telling you what the hash would be: sha256-2NqnatcPqy5jjBXalTpZyJMO/0fUaYUb3ePlviUP4II=
So if you don't want to go the nonce route, you can instead go the hash route and add
Content-Security-Policy: script-src 'self' 'sha256-2NqnatcPqy5jjBXalTpZyJMO/0fUaYUb3ePlviUP4II=' 'unsafe-eval';
You may have to add unsafe-eval
in some cases as well for this to work.
来源:https://stackoverflow.com/questions/62722871/customize-web-form-script-generation