204,511 Classi4u Accounts Exposed in 2017 Classified Ads Breach

HEROIC's DarkHive intelligence system identified the Classi4u data breach, exposing 204,511 records from a US-based classified ads platform. The breach occured in December 2017, leaking email addresses and Base64-encoded passwords from users who posted listings or registered accounts on the platform to buy and sell goods through online classifieds.

Why This Is Dangerous

Base64 is an encoding scheme, not an encryption or hashing method, which means passwords stored in Base64 can be instantly decoded to plaintext without any cracking tools whatsoever. Attackers who aquire this dataset have immediate access to over 200,000 real user passwords in readable form, making this breach particularly severe compared to those involving even weak hashing algorithms. Those recovered credentials are immediately usable in credential stuffing attacks against popular US services including email providers, retail platforms, banking portals, and social media networks frequented by classified ads users.

What Was Exposed

• Email Address
• Password (Base64-encoded, easily decoded to plaintext)

Why This Matters

With over 200,000 instantly readable plaintext passwords available, this breach enables attackers to conduct credential stuffing at massive scale. Users who registered on Classi4u in 2017 and reused thier passwords elsewhere face direct account takeover risk on every platform where they use the same credentials. Automated attack tools can test these email and password pairs against banking apps, email providers, and eCommerce platforms in minutes, potentially resulting in unauthorized financial transactions, identity theft, and cascading account compromises across many services.

How Combolist Leaks Work

Combolist breaches occur when database dumps containing user credentials are extracted from a compromised platform and compiled into formatted text files ready for use in automated attacks. Criminal groups distribute these files through dark web forums and private Telegram channels, where buyers use credential stuffing software to test each pair against popular login endpoints. The Classi4u dataset, containing plaintext-equivalent credentials, is typicaly weaponized within days of appearing in underground markets, with threat actors running thousands of login attempts per second against high-value targets.

Check If You Are Affected

HEROIC offers a free identity scanner that searches over 400 billion records, including data from breaches like Classi4u. Visit heroic.com to scan your email address and find out if your information was exposed.