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Fix DTLS replay protection
The DTLS implementation provides some protection against replay attacks in accordance with RFC6347 section 188.8.131.52. A sliding "window" of valid record sequence numbers is maintained with the "right" hand edge of the window set to the highest sequence number we have received so far. Records that arrive that are off the "left" hand edge of the window are rejected. Records within the window are checked against a list of records received so far. If we already received it then we also reject the new record. If we have not already received the record, or the sequence number is off the right hand edge of the window then we verify the MAC of the record. If MAC verification fails then we discard the record. Otherwise we mark the record as received. If the sequence number was off the right hand edge of the window, then we slide the window along so that the right hand edge is in line with the newly received sequence number. Records may arrive for future epochs, i.e. a record from after a CCS being sent, can arrive before the CCS does if the packets get re-ordered. As we have not yet received the CCS we are not yet in a position to decrypt or validate the MAC of those records. OpenSSL places those records on an unprocessed records queue. It additionally updates the window immediately, even though we have not yet verified the MAC. This will only occur if currently in a handshake/renegotiation. This could be exploited by an attacker by sending a record for the next epoch (which does not have to decrypt or have a valid MAC), with a very large sequence number. This means the right hand edge of the window is moved very far to the right, and all subsequent legitimate packets are dropped causing a denial of service. A similar effect can be achieved during the initial handshake. In this case there is no MAC key negotiated yet. Therefore an attacker can send a message for the current epoch with a very large sequence number. The code will process the record as normal. If the hanshake message sequence number (as opposed to the record sequence number that we have been talking about so far) is in the future then the injected message is bufferred to be handled later, but the window is still updated. Therefore all subsequent legitimate handshake records are dropped. This aspect is not considered a security issue because there are many ways for an attacker to disrupt the initial handshake and prevent it from completing successfully (e.g. injection of a handshake message will cause the Finished MAC to fail and the handshake to be aborted). This issue comes about as a result of trying to do replay protection, but having no integrity mechanism in place yet. Does it even make sense to have replay protection in epoch 0? That issue isn't addressed here though. This addressed an OCAP Audit issue. CVE-2016-2181 Reviewed-by: Richard Levitte <email@example.com>
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