Many of the LinkedIn e-mails alerts instructing users on how to reset passwords accessed by hackers were dumped into spam boxes, according to e-mail security vendor Cloudmark.
In a blog post last week, Andrew Conway, a Cloudmark researcher, said a substantial increase in spam reports last weekend were traced to LinkedIn password reset e-mail alerts
In many cases, the e-mail messages that users' marked as spam were legitimate alerts from LinkedIn, Conway said.
"Over 4 percent of the people receiving this e-mail thought it was spam and sent it straight to the bit bucket," Conway said. "If Linkedin sends out 6.5 million e-mails, then a quarter of a million people are congratulating themselves on avoiding spam -- and still have a compromised Linkedin password." (See also "Another Breach Reveals Weak Passwords; Will We Ever Learn?").
Conway said that LinkedIn did all the right things to ensure that users would not treat its e-mails with suspicion. All were addressed to the recipient by name, did not contain any links and were DomainKeys Identified Mail (DKIM) signed to validate their authenticity.
"Even so, it was taken for spam," Conway said. "Part of the problem is that people are used to getting e-mail that they don't want from LinkedIn, and rather than unsubscribe, some of them just mark it as spam and hope that it will go away."
Estimating the Overlooked Alerts
In an e-mail to Computerworld, Conway said that Cloudmark, a provider of messaging security services to Internet Service Providers, monitors messages for clients by assigning a number of digital signatures based on the content of the messages. Thus it can determine which signatures are present on e-mails that are manually flagged as spam by users.
"The Linkedin compromised e-mail message[s] generated several unique signatures, so we are able to measure the rate at which these are marked [by users] as spam," he said.
Cloudmark was able to confirm that LinkedIn, and not spammers, had sent the alerts because the e-mails were DKIM signed by Linkedin.com, he said.
The alerts were sent after hackers last week accessed about 6.5 million hashed passwords from a LinkedIn database and posted the stolen data on Russian hacker site.
By last weekend, most of the passwords were believed to have been decrypted by hackers and made available in plain text on many sites.
How LinkedIn Handled the Hack
LinkedIn has confirmed the password compromise but released few details about the incident.
In three separate LinkedIn communications about the incident, the company didn't say how the passwords were accessed or whether other data, such as e-mail IDs, were also compromised. LinkedIn said only that no e-mail IDs have yet been publicly posted.
The latest LinkedIn update, posted Tuesday, repeats much of the information that was included in previous notes.
The latest post did say that all passwords posted by the hackers, and that "we believed created risk for our members, based on our investigation," have been disabled.
Since the attack, LinkedIn has come in for some criticism for not better protecting passwords.
The company has said that it has completed a "long-planned transition" from merely hashing passwords to a system that both hashes and salts passwords. Salting is a process in which a random string of characters is appended to a password before it is hashed.
Jaikumar Vijayan covers data security and privacy issues, financial services security and e-voting for Computerworld. Follow Jaikumar on Twitter at @jaivijayan, or subscribe to Jaikumar's RSS feed . His e-mail address is firstname.lastname@example.org.
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This story, "Spam Filters Grabbed Many LinkedIn Break-In Warnings" was originally published by Computerworld.