OWASP HTML Sanitizer allow colon in HTML - jakarta-mail

Question

How can I allow : sign in sanitized HTML? I am using it to sanitize HTML code in generating java mail. This code has an inline image content id like <img src=\"cid:image\" height=\"70\" width=\"70\" />. Upon sanitizing, the src attribute is not included in the sanitized html.
PolicyFactory IMAGES = new HtmlPolicyBuilder().allowUrlProtocols("http", "https")
.allowElements("img")
.allowAttributes("src").matching(Pattern.compile("^cid[:][\\w]+$"))
.onElements("img")
.allowAttributes("border", "height", "width").onElements("img")
.toFactory();
String html = "<img src=\"cid:image\" height=\"70\" width=\"70\" />";
final String sanitized = IMAGES.sanitize(html);
System.out.println(sanitized);
The output of above code is:
<img height="70" width="70" />

Answer 1

Why it isn't working
Or rather, why it's working "too well"
By default, HtmlPolicyBuilder disallows URL protocols in src elements. This prevents injections such as
<img src="javascript:alert('xss')"/>
which could potentially lead to the execution of the script after javascript: (in this case, alert('xss'))
There are other protocols (on other elements) that can lead to similar issues:
Even though it doesn't use the javascript protocol, it's still possible to inject a base64-encoded XSS injection:
<object src="data:text/html;base64,PHNjcmlwdD5hbGVydCgneHNzJyk8L3NjcmlwdD4="/>
or
Click me
Because of this, HtmlPolicyBuilder assumes that any attribute value containing a colon (in certain attributes) should be treated as dangerous.
How to fix it:
You have to explicitly tell the HtmlPolicyBuilder to allow the cid "protocol", using the allowUrlProtocols method:
PolicyFactory IMAGES = new HtmlPolicyBuilder().allowUrlProtocols("http", "https")
.allowElements("img")
.allowUrlProtocols("cid") // Specifically allow "cid"
.allowAttributes("src").matching(Pattern.compile("^cid[:][\\w]+$"))
.onElements("img")
.allowAttributes("border", "height", "width").onElements("img")
.toFactory();
String html = "<img src=\"cid:image\" height=\"70\" width=\"70\" />";
final String sanitized = IMAGES.sanitize(html);
System.out.println(sanitized);
Output:
<img src="cid:image" height="70" width="70" />