This article describes how certain types of captchas (such as the ones used
by a German online-banking site) can be automatically recognized using
software. The attack does not recognize one particular captcha itself but
exploits a design error allowing to average multiple captchas containing
the same information. The result can be recognized by conventional OCR
programs thereby defeating the captcha.
Details
=======
The detailed article (including sample images) is online here:
http://www.cip.physik.uni-muenchen.de/~wwieser/misc/captcha/
Countermeasurements
===================
Website developers can easily defend against this attack by not
allowing the extraction of a series of different captcha images
with same content. Instead, the image should change only when the
text content changes.
Captcha designers can defend agaist averaging attacks by not using
noise-like distortions. For example, moving and rotaing individual
letters by a large enough distance/angle will spoil averaging by
reducing the contrast in averaged images.
Contact: wwieser (at) gmx -dot- de
PLEASE do not CC me when posting to the list; I am subscribed.
=======
This article describes how certain types of captchas (such as the ones used
by a German online-banking site) can be automatically recognized using
software. The attack does not recognize one particular captcha itself but
exploits a design error allowing to average multiple captchas containing
the same information. The result can be recognized by conventional OCR
programs thereby defeating the captcha.
Details
=======
The detailed article (including sample images) is online here:
http://www.cip.physik.uni-muenchen.de/~wwieser/misc/captcha/
Countermeasurements
===================
Website developers can easily defend against this attack by not
allowing the extraction of a series of different captcha images
with same content. Instead, the image should change only when the
text content changes.
Captcha designers can defend agaist averaging attacks by not using
noise-like distortions. For example, moving and rotaing individual
letters by a large enough distance/angle will spoil averaging by
reducing the contrast in averaged images.
Contact: wwieser (at) gmx -dot- de
PLEASE do not CC me when posting to the list; I am subscribed.
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