Valve has patched a recently discovered exploit on Steam that could be used to gain unlimited funds.
This exploit was discovered by a user on the website, Hackerone. Valve has since awarded the user USD 7,500 for their discovery.
Hackerone is a website that connects companies like Valve with users who like to hack, test, and otherwise mess around with websites, apps, and other software. The users can submit the exploits they discover to companies privately and be awarded money for their efforts.
As for the exploit itself, it involves changing your email address and intercepting transactions that use any Smart2Pay payment method. You can read the full details of the exploit here.
One of the ways this exploit could have had a negative effect is by allowing someone to generate infinite money and break the Steam market by selling game keys at cheap prices.
Valve responded by thanking the user and stating that they were fixing the problem. They also gave him USD 7,500 as mentioned before. Seems a little small but, okay.
As of this writing, the problem has been fixed and a public report has been released by Valve and the user.