A new CAPTCHA in the trade system will block out malware trade attempts.
Steam's trading feature has been updated to incorporate a new CAPTCHA system as part of its security. Meaning that before you can trade, you need to enter the letters displayed on the screen, or tick a box, so that the system knows you're human.
Steam's John Cook said, "We’re updating trading to include a captcha as part of confirmation process. This is to prevent malware on users’ machines making trades on their behalf. We know it’s a bit of a hassle, and we don’t like making trading harder for users, but we do expect it to significantly help customers who are tricked into downloading and running malware from losing their items.
"We’ve excluded a few of the existing third-party trading services from this requirement so they can continue to function."
Some users are reporting that the update has cancelled all their pending trade offers.