: Hackers use automated tools to "check" the credentials against mail servers (SMTP/IMAP). If the login works, it is added to a "valid base" file, which is much more valuable than raw, unverified data.
The phrase refers to a leaked database containing 530,000 email account credentials (usernames and passwords) that have been verified as "valid" or active by cybercriminals. Such files are typically distributed or sold on dark web forums to facilitate credential stuffing attacks, where bots automatically test these logins across other popular websites. The Lifecycle of a Leaked "Valid Base" 530k Mail Access valid base.txt
Credential Stuffing Prevention: Guide for Users & Businesses : Hackers use automated tools to "check" the
: Using personal identifiable information (PII) to open new credit lines or take out loans. Risks of Exposure 530k Mail Access valid base.txt