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Accessing amazon.de via Firefox 57, I was recently blocked with the error code ssl_error_bad_mac_alert - see the attached image below:

Error code in Firefox 57

There's limited information regarding the error code in Firefox documentation and I am having difficulties finding the root of the problem. This is further complicated by the fact that I can not replicate the issue. Is this probably an attack or more likely to be a harmless connection issue?

  • Did this happen just once, did it persist across multiple refreshes? did you try any other browsers? Is you date/time correct on your machine? – iainpb Dec 14 '17 at 15:59
  • So far: once. Other browsers fine. All SSL settings otherwise fine. The only explanation (so far) is a momentary issue with the ISP (which runs over a 4G connection and may have switched towers). – pw_v Dec 14 '17 at 16:01
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    Found this "This usually indicates that the client and server have failed to come to agreement on the set of keys used to encrypt the application data and to check message integrity" (serverfault.com/questions/6653/ssl-error-ssl-error-bad-mac-read). So, if the connection was switching towers, it's feasible your connection could have dropped as it was negotiating the keys. – iainpb Dec 14 '17 at 16:12
  • It could literally have been a malformed packet during the handshake, leading to a failed MAC validation. They're very rare due to the TCP checksum, but it is possible. – Polynomial Dec 14 '17 at 17:39

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