CSS URLs longer than 1024 characters are not allowed

[sapio-dwelch]

When trying to sanitize large data URIs used as background images in CSS properties, there is a hard-coded URL limit of 1024 characters (this is in StylingPolicy.sanitizeAndAppendUrl). Any value larger than 1024 characters is removed.

public class HtmlSanitizer {
    public static String TOO_LONG = "<table style=\"width:100%;\">\r\n" + 
            "    <tr>\r\n" + 
            "        <td style=\"background-image:url('data:image/svg+xml;base64,PHN2ZyB2aWV3" + 
            "Qm94PSIwIDAgMjAwLjAgMjAwvoidIGZpbGwtb3BhY2l0eT0iMSIgeG1sbnM6eGxpbms9Imh0dHA6Ly93" + 
            "d3cudzMub3JnLzE5OTkveGxpbmsiIGNvbG9yLXJlbmRlcmluZz0iYXV0byIgY29sb3ItaW50ZXJwb2xh" + 
            "dGlvbj0iYXV0byIgdGV4dC1yZW5kZXJpbmc9ImF1dG8iIHN0cm9rZT0iYmxhY2siIHN0cm9rZS1saW5l" + 
            "Y2FwPSJzcXVhcmUiIHdpZHRoPSIyMDAiIHN0cm9rZS1taXRlcmxpbWl0PSIxMCIgc2hhcGUtcmVuZGVy" + 
            "aW5nPSJhdXRvIiBzdHJva2voidBhY2l0eT0iMSIgZmlsbD0iYmxhY2siIHN0cm9rZS1kYXNoYXJyYXk9" + 
            "Im5vbmUiIGZvbnQtd2VpZ2h0PSJub3JtYWwiIHN0cm9rZS13aWR0aD0iMSIgaGVpZ2h0PSIyMDAiIHht" + 
            "bG5zPSJodHRwOi8vd3d3LnczLm9yZy8yMDAwL3N2ZyIgZm9udC1mYW1pbHk9IidEaWFsb2cnIiBmb250" + 
            "LXN0eWxlPSJub3JtYWwiIHN0cm9rZS1saW5lam9pbj0ibWl0ZXIiIGZvbnQtc2l6ZT0iMTJweCIgc3Ry" + 
            "b2tlLWRhc2hvZmZzZXQ9IjAiIGltYWdlLXJlbmRlcmluZz0iYXV0byINCj48IS0tR2VuZXJhdGVkIGJ5" + 
            "IE1hcnZpbiB3aXRoIEJhdGlrIFNWRyBHZW5lcmF0b3ItLT48ZGVmcyBpZD0iZ2VuZXJpY0RlZnMiDQog" + 
            "IC8+PGcNCiAgPjxkZWZzIGlkPSIxNDQ4MjQ5ODEwMjEtZGVmczEiDQogICAgPjxjbGlwUGF0aCBjbGlw" + 
            "UGF0aFVuaXRzPSJ1c2VyU3BhY2VPblVzZSIgaWQ9voidNDgyNDk4MTA0OTgtY2xpcFBhdGgxIg0KICAg" + 
            "ICAgPjxwYXRoIGQ9Ik0wIDAgTDIwMCAwIEwyMDAgMjAwIEwwIDIwMCBMMCAwIFoiDQogICAgICAvPjwv" + 
            "Y2xpcFBhdGgNCiAgICAgID48Y2xpcFBhdGggY2xpcFBhdGhVbml0cz0idXNlclNwYWNlT25Vc2UiIGlk" + 
            "PSIxNDQ4MjQ5OD=='); background-size: contain; background-repeat: no-repeat; back" + 
            "ground-position:center; height:100px; margin:auto;\"/>\r\n" + 
            "    </tr>\r\n" + 
            "</table>";
    
    public static String NOT_TOO_LONG = "<table style=\"width:100%;\">\r\n" + 
            "    <tr>\r\n" + 
            "        <td style=\"background-image:url('data:image/svg+xml;base64,PHN2ZyB2aWV3" + 
            "Qm94PSIwIDAgMjAwLjAgMjAwvoidIGZpbGwtb3BhY2l0eT0iMSIgeG1sbnM6eGxpbms9Imh0dHA6Ly93" + 
            "d3cudzMub3JnLzE5OTkveGxpbmsiIGNvbG9yLXJlbmRlcmluZz0iYXV0byIgY29sb3ItaW50ZXJwb2xh" + 
            "dGlvbj0iYXV0byIgdGV4dC1yZW5kZXJpbmc9ImF1dG8ivoidcm9rZT0iYmxhY2siIHN0cm9rZS1saW5l" + 
            "Y2FwPSJzcXVhcmUiIHdpZHRoPSIyMDAiIHN0cm9rZS1taXRlcmxpbWl0PSIxMCIgc2hhcGUtcmVuZGVy" + 
            "IE1hcnZpbiB3aXRoIEJhdGlrIFNWRyBHZW5lcmF0b3ItLT48ZGVmcyBpZD0iZ2VuZXJpY0RlZnMiDQog" + 
            "IC8+PGcNCiAgPjxkZWZzIGlkPSIxNDQ4MjQ5ODEwMjEtZGVmczEiDQogICAgPjxjbGlwUGF0aCBjbGlw" + 
            "UGF0aFVuaXRzPSJ1c2VyU3BhY2VPblVzZSIgaWQ9IjE0NDgyNDk4MTA0OTgtY2xpcFBhdGgxIg0KICAg" + 
            "ICAgPjxwYXRoIGQ9Ik0wIDAgTDIwMCAwIEwyMDAgMjAwIEwwIDIwMCBMMCAwIFoiDQogICAgICAvPjwv" + 
            "Y2xpcFBhdGgNCiAgvoidID48Y2xpcFBhdGggY2xpcFBhdGhVbml0cz0idXNlclNwYWNlT25Vc2UiIGlk" + 
            "PSIxNDQ4MjQ5OD=='); background-size: contain; background-repeat: no-repeat; back" + 
            "ground-position:center; height:100px; margin:auto;\"/>\r\n" + 
            "    </tr>\r\n" + 
            "</table>";
    
    public static final PolicyFactory HTML_POLICY = new HtmlPolicyBuilder()
            // allow all elements to be styled
            .allowStyling()
            // allow urls in bg images.
            .allowUrlsInStyles(AttributePolicy.IDENTITY_ATTRIBUTE_POLICY)
            // and also allow "data" URLs
            .allowUrlProtocols("data")
            // allow some more "global" elements
            .allowElements("table", "tbody", "th", "tr", "td").toFactory();
    
    public static void main(String args[]) {
        System.out.println(HTML_POLICY.sanitize(TOO_LONG));
        System.out.println(HTML_POLICY.sanitize(NOT_TOO_LONG));
    }
}

The output for TOO_LONG will not include the background-image property, but the output for NOT_TOO_LONG will. Is there a reason for the limit?

[mikesamuel]

I believe the thought was that long URLs can be a denial-of-service vector.

That decision was made before data: URLs were widely used for images, so could be reconsidered.

Do you know of a good cutoff?

https://stackoverflow.com/questions/417142/what-is-the-maximum-length-of-a-url-in-different-browsers seems to conflate the address bar length and length limits to fetch.spec.whatwg.org.

[sapio-dwelch]

Thanks for the reply.

I am not aware of what a good cutoff would be.

I thought that maybe the justification for a limit of 1024 was long URL == attack, but, if so, why is it only being enforced for URLs found in style sheets and not for URLs found elsewhere? (I have tested an image with a data URL greater than 4000 bytes and there seems to be no such limit). If there is no enforcement of this for all URLs, then I would argue that it be removed for the URLs found in style sheets, as well.

Those who are concerned about the max lengths could still enforce a 1024 character limit (or any limit) using a custom AttributePolicy.

[ioleo]

Is there a workaround to allow any size data:image blocks? I have a size check on the whole payload of HTTP request, so I don't need to check individual properties length.

[mikesamuel]

Length limits avoid a lot of boundary problems in downstream code so seem in scope for sanitizers.

https://stackoverflow.com/a/417184/20394 suggests that maybe the limit should be 2000 instead of 1024