63 lines
3.3 KiB
PHP
63 lines
3.3 KiB
PHP
<div class="body_padded">
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<h1>Help - Insecure CAPTCHA</h1>
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<div id="code">
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<table width='100%' bgcolor='white' style="border:2px #C0C0C0 solid">
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<tr>
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<td><div id="code">
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<h3>About</h3>
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<p>A <?php echo dvwaExternalLinkUrlGet( 'http://www.captcha.net/', 'CAPTCHA' ); ?> is a program that can tell whether its user is a human or a computer. You've probably seen
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them - colourful images with distorted text at the bottom of Web registration forms. CAPTCHAs are used by many websites to prevent abuse from
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"bots", or automated programs usually written to generate spam. No computer program can read distorted text as well as humans can, so bots
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cannot navigate sites protected by CAPTCHAs.</p>
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<p>CAPTCHAs are often used to protect sensitive functionality from automated bots. Such functionality typically includes user registration and changes,
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password changes, and posting content. In this example, the CAPTCHA is guarding the change password functionality for the user account. This provides
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limited protection from CSRF attacks as well as automated bot guessing.</p>
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<br /><hr /><br />
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<h3>Objective</h3>
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<p>Your aim, change the current user's password in a automated manner because of the poor CAPTCHA system.</p>
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<br /><hr /><br />
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<h3>Low Level</h3>
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<p>The issue with this CAPTCHA is that it is easily bypassed. The developer has made the assumption that all users will progress through screen 1, complete the CAPTCHA, and then
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move on to the next screen where the password is actually updated. By submitting the new password directly to the change page, the user may bypass the CAPTCHA system.</p>
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<p>The parameters required to complete this challenge in low security would be similar to the following:</p>
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<pre>Spoiler: <span class="spoiler">?step=2&password_new=password&password_conf=password&Change=Change</span>.</pre>
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<br />
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<h3>Medium Level</h3>
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<p>The developer has attempted to place state around the session and keep track of whether the user successfully completed the
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CAPTCHA prior to submitting data. Because the state variable (Spoiler: <span class="spoiler">passed_captcha</span>) is on the client side,
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it can also be manipulated by the attacker like so:</p>
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<pre>Spoiler: <span class="spoiler">?step=2&password_new=password&password_conf=password&passed_captcha=true&Change=Change</span>.</pre>
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<br />
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<h3>High Level</h3>
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<p>There has been development code left in, which was never removed in production. It is possible to mimic the development values, to allow
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invalid values in be placed into the CAPTCHA field.</p>
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<p>You will need to spoof your user-agent (Spoiler: <span class="spoiler">reCAPTCHA</span>) as well as use the CAPTCHA value of
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(Spoiler: <span class="spoiler">hidd3n_valu3</span>) to skip the check.</p>
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<br />
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<h3>Impossible Level</h3>
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<p>In the impossible level, the developer has removed all avenues of attack. The process has been simplified so that data and CAPTCHA verification occurs in one
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single step. Alternatively, the developer could have moved the state variable server side (from the medium level), so the user cannot alter it.</p>
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</div></td>
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</tr>
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</table>
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</div>
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<br />
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<p>Reference: <?php echo dvwaExternalLinkUrlGet( 'http://www.captcha.net/' ); ?></p>
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</div>
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