Sense of Security – Security Advisory – SOS-11-007
| Release Date. | 20-May-2011 |
| Last Update. | – |
| Vendor Notification Date. | 04-Apr-2011 |
| Product. | Securimage / PHPCaptcha |
| Platform. | PHP |
| Affected versions. | 1.0.4 – 2.0.2 |
| Severity Rating. | Medium |
| Impact. | Authentication bypass |
| Attack Vector. | Remote without authentication |
| Solution Status. | Vendor workaround (remove securimage_play.php) |
| CVE reference. | CVE – Not yet assigned |
Details.
PHPCaptcha, also known as Securimage, is a popular Open Source PHP CAPTCHA library. It is also used in popular WordPress plugins such as the ‘Fast Secure Contact Form’. Insufficient distortion in the audio version of the CAPTCHA allows an attacker to quickly decode the CAPTCHA by performing basic binary analysis of the generated audio file. The issue is compounded by the fact that even if the audio feature of the CAPTCHA has been disabled, it can still be accessed by forceful browsing to the /secure_play.php URI.
Proof of Concept.
Proof of concept code that works against the example_form.php page and the MP3 file format provided with the standard PHPCaptcha package available from www.phpcaptcha.org is available at:
/advisories/SOS-11-007.zip
Proof of concept code is only available for the MP3 version of the audio, however the WAV audio format is also affected by the same vulnerability.
Solution.
Remove the script securimage_play.php and disable the use of the Audio CAPTCHA.
Discovered by.
Phil Taylor from Sense of Security Labs.