According to the researchers, a 19-year old vulnerability was hiding in plain sight

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According to recent news reports, a 19-year old vulnerability that enabled attackers to decrypt encrypted data and sign communications sites' secret encryption keys has returned. The vulnerability was disclosed back in 1998 in the TLS predecessor known as secure sockets layer.

A recent study found that 27 of the 100 most-visited websites—including Facebook and PayPal—are vulnerable to what is essentially the same attack. About 2.8 percent of the top 1 million sites also tested positive. According to the researchers, it was hiding in plain sight.

Amit Sethi, principal consultant at Synopsys, said: "The attack is a variation of the Bleichenbacher padding oracle attack. The original attack relied on the server returning error messages explicitly stating whether decryption of a message failed due to a padding error. The original issue was fixed in most SSL/TLS implementations by not returning the error messages. However, some implementations still behaved slightly differently in response to different error conditions. By observing these differences instead of actual error messages, attackers can still perform the original attack.

"We see these types of issues regularly when assessing software that attempts to hide error conditions. For example, the server’s actual response may be the same regardless of whether an error occurred, but the amount of time that the server takes to respond may be noticeably different when errors occur.

"This attack illustrates why security issues are often difficult to find and mitigate properly. Even subtle information leakage can lead to significant problems."