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js-api

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Description

We hired a new developer @sohomdatta1, dude coded something, we sent it for a security audit, it came back a sea of red :(

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Solution

This challenge had the same premise as Undocumented js-api. This time, the JavaScript source is different (and slightly more complex).

window.addEventListener('load', async () => {

    function NOTREACHED() {
        // destroy currently availiable data
        // Challenge Author (sohom): 
        // if you are hitting this codepath repeatedly
        // please use a incognito window, your ad-blocker
        // or other extensions might be sending spurious postMessages
        // to this page
        window.location.href = `https://www.youtube.com/watch?v=FtutLA63Cp8`
    }

    function escapeHtml(unsafe) {
        return unsafe
         .replace(/&/g, "&")
         .replace(/</g, "&lt;")
         .replace(/>/g, "&gt;")
         .replace(/"/g, "&quot;")
         .replace(/'/g, "&#039;");
    }



    class NotesManager {
        constructor() {
            this.noteData = window.localStorage.getItem( 'note' ) || '';
            this.noteTextArea = document.querySelector( '#note-text-area' );
            this.noteTextArea.value = this.noteData;
            this.previewNode = document.querySelector( '#output' );
            this.highlightNode = document.querySelector( '#note-search-highlights' )
            this.noteManager = this;
        }

        static getCurrentNoteManager() {
            if ( !this.noteManager ) {
                this.noteManager = new NotesManager();
            }
            return this.noteManager;
        }

        getNotesTextAreaValue() {
            return this.noteTextArea.value
        }
    
        get() {
            return this.noteData.toString();
        }
    
        set(text) {
            if ( typeof text !== 'string' ) return;
            const cleanedText = DOMPurify.sanitize(text);
            this.noteData = cleanedText;
            window.localStorage.setItem( 'note', cleanedText.toString() ); 
        }
    
        /**
         * Previews text, if text is null will preview existing note
         * @param {String} [text] String to preview
         */
        preview(text) {
            if ( typeof text !== 'string' && !!text ) return;
            if ( !text ) text = this.noteData;
            else text = DOMPurify.sanitize( text );
            this.previewNode.innerHTML = text;
        }
    
        /**
         * Search for the particular text
         * @param {String} text text to search for
         */
        search(text) {
            if ( typeof text !== 'string' ) return;
            if ( !window.enable_experimental_features ) return;
            // TODO(sohom): Address concerns raised by our internal security
            // team regarding this API at b/1337. Given that this API
            // is effectively a no-op and is not current exposed anywhere
            // as of version 0.0.1 it should be fine for now.
            // Since our internal bug tracker is well, "internal"
            // I have dumped relevant portion of the b/1337 at
            // https://github.com/sohomdatta1/jsapi-issues/issues/1
            text = DOMPurify.sanitize( text );
            const doesMatch = this.noteData.includes(text);
            if ( doesMatch ) {
                var lastIndex = 0, i = 0;
                for(var i = this.noteData.substring(i).indexOf(text); i < this.noteData.length; i = i + text.length + this.noteData.substring(i + text.length).indexOf(text)) {
                    if ( lastIndex > i ) break;
                    this.highlightNode.innerHTML += escapeHtml( this.noteData.substring(lastIndex,i) );
                    this.highlightNode.innerHTML += `<mark>${escapeHtml( text ) }</mark>`
                    lastIndex = i + text.length;
                }
                document.querySelector( '#note-text-highlight-wrapper' ).classList.remove( 'hidden' );
            }
        }
    }

    // initialize the document
    NotesManager.getCurrentNoteManager();
    NotesManager.getCurrentNoteManager().preview();

    window.document.querySelector( '#note-submit' ).addEventListener( 'click', (e) => {
        e.preventDefault();
        const nm = NotesManager.getCurrentNoteManager();

        nm.set( nm.getNotesTextAreaValue() );
        nm.preview();
    } );

    window.document.querySelector( '#note-save' ).addEventListener( 'click', (e) => {
        e.preventDefault();
        const nm = NotesManager.getCurrentNoteManager();

        nm.set( nm.getNotesTextAreaValue() );
    } );

    window.document.querySelector( '#note-render' ).addEventListener( 'click', (e) => {
        e.preventDefault();
        const nm = NotesManager.getCurrentNoteManager();

        nm.preview( nm.getNotesTextAreaValue() );
    } );

    /**
     * @experimental Added in 0.0.2
     */
    window.addEventListener( 'message', (e) => {
        if ( !e.origin.endsWith('jsapi.tech') ) return;
        const data = e.data;
        if ( typeof data !== 'object' && typeof data.op !== 'string' && typeof data.payload !== 'string' ) return;
        if ( data.op === 'preview' ) {
            NotesManager.getCurrentNoteManager().preview( data.payload );
        } else if ( data.op === 'set' ) {
            NotesManager.getCurrentNoteManager().set( data.payload );
        } else if ( data.op === 'search' ) {
            NotesManager.getCurrentNoteManager().search( data.payload );
        } else {
            NOTREACHED();
        }
    } );

});

The important part is, once again, the message event handler. Just like the previous challenge, we had to use a subdomain takeover to serve an exploit page from a .jsapi.tech subdomain.

One interesting feature in this version of the challenge is that we can "preview" our HTML without saving it. Everything is still sanitized through DOMPurify.

In the preview feature, we can insert sanitized HTML without changing this.noteData. When using the search feature, the original this.noteData is the one being searched for our input text.

The search feature checks for window.enable_experimental_features, which is a property that doesn't exist... or does it?

DOMPurify doesn't protect against DOM clobbering, so we can pollute this property by inserting the following HTML through the preview feature.

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Unintended Solution

When the text that we are searching is found in the victim's note, a new <div> is rendered with the search results (#note-text-highlight-wrapper has its hidden class removed).

For instance, the following shows a correct search (where the searched content is a substring of the flag).

And the following shows an incorrect search, where no matches are found. Notice how the extra <div> in the correct search was sufficient to push the previewed content out of the viewport.

We can make use of image lazy loadingarrow-up-right to only load an image if it is within the browser viewport. This way, we are able to tell if the results section was rendered.

If we do not receive a request for a particular character, that means that the results section was rendered, and therefore the search was a correct guess.

The following script implements this exploit.

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Intended Solution

The intended solution was to use the PerformanceLongTaskTiming APIarrow-up-right to identify if the search was taking more than 50ms.

It turns out, however, any timing attack with performance.now() would have worked as well.

Because the JavaScript event loop is single-threaded, we just need to use setTimeout to temporarily pass control to the next thing in the callback queue (which is the message handler taking care of the search request), then find out how long it took for control to be passed back to our exploit script.

Although the setTimeout is only for 1ms, it takes much longer in reality for execution to resume because the expensive search function blocks the event loop. By measuring this discrepancy, we can find out if our guess was correct.

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