Google Online Security Blog: New Research: Some Tough Questions for ‘Security Questions’

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New Research: Some Tough Questions for ‘Security Questions’

2015年5月21日

Posted by Elie Bursztein, Anti-Abuse Research Lead and Ilan Caron, Software Engineer
What was your first pet’s name?What is your favorite food?What is your mother’s maiden name?
What do these seemingly random questions have in common? They’re all familiar examples of ‘security questions’. Chances are you’ve had to answer one these before; many online services use them to help users recover access to accounts if they forget their passwords, or as an additional layer of security to protect against suspicious logins.
But, despite the prevalence of security questions, their safety and effectiveness have rarely been studied in depth. As part of our constant efforts to improve account security, we analyzed hundreds of millions of secret questions and answers that had been used for millions of account recovery claims at Google. We then worked to measure the likelihood that hackers could guess the answers.
Our findings, summarized in a paper that we recently presented at WWW 2015, led us to conclude that secret questions are neither secure nor reliable enough to be used as a standalone account recovery mechanism. That’s because they suffer from a fundamental flaw: their answers are either somewhat secure or easy to remember—but rarely both.

Click infographic for larger version
Easy Answers Aren’t Secure
Not surprisingly, easy-to-remember answers are less secure. Easy answers often contain commonly known or publicly available information, or are in a small set of possible answers for cultural reasons (ie, a common family name in certain countries).
Here are some specific insights:

With a single guess, an attacker would have a 19.7% chance of guessing English-speaking users’ answers to the question "What is your favorite food?" (it was ‘pizza’, by the way)

With ten guesses, an attacker would have a nearly 24% chance of guessing Arabic-speaking users’ answer to the question "What’s your first teacher’s name?"

With ten guesses, an attacker would have a 21% chance of guessing Spanish-speaking users’ answers to the question, "What is your father’s middle name?"

With ten guesses, an attacker would have a 39% chance of guessing Korean-speaking users’ answers to the question "What is your city of birth?" and a 43% chance of guessing their favorite food.

"What’s your phone number?""What’s your frequent flyer number?"Difficult Answers Aren’t Usable

40% of our English-speaking US users couldn’t recall their secret question answers when they needed to. These same users, meanwhile, could recall reset codes sent to them via SMS text message more than 80% of the time and via email nearly 75% of the time.

Some of the potentially safest questions—"What is your library card number?" and "What is your frequent flyer number?"—have only 22% and 9% recall rates, respectively.

For English-speaking users in the US the easier question, "What is your father’s middle name?" had a success rate of 76% while the potentially safer question "What is your first phone number?" had only a 55% success rate.

Why not just add more secret questions?"What city were you born in?""What is your father’s middle name?"The Next Question: What To Do?Security Checkup