eBay Japan has closed a hole that allowed user accounts to be accessed by anyone.
The authentication flaw existed because eBay created passwords for accounts based on a combination of a username plus what should have been random data referred to as salt.
But eBay had used a static salt allowing anyone with knowledge of it to access any account, Romanian researcher Ionut Cernica said.
The salt used was the combination '123456', which was reported as last year's worst password.
"The salt was a static one for all users," Cernia said. "So all you have (sic) to do was to find random usernames."
The researcher reported the flaw last month to eBay and later posted a blog and proof of concept video detailing it.
eBay has been contacted for comment. It was unknown at the time of publishing if other regional eBay sites were affected.
Salt re-use is one of the most common errors when administrators apply the security mechanism, Defuse Security researcher Taylor Hornby said.
"Either the salt is hardcoded into the program, or is generated randomly once," Hornby wrote.
"This is ineffective because if two users have the same password, they'll still have the same hash.
"An attacker can still use a reverse lookup table attack to run a dictionary attack on every hash at the same time."
Hornby recommended administrators apply long salts which are the same size as the output of the hash function.
"For example, the output of SHA256 is 256 bits (32 bytes), so the salt should be at least 32 random bytes," he said.