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Fix illegal client float (CVE-2020-11810)
There is a time frame between allocating peer-id and initializing data channel key (which is performed on receiving push request or on async push-reply) in which the existing peer-id float checks do not work right. If a "rogue" data channel packet arrives during that time frame from another address and with same peer-id, this would cause client to float to that new address. This is because: - tls_pre_decrypt() sets packet length to zero if data channel key has not been initialized, which leads to - openvpn_decrypt() returns true if packet length is zero, which leads to - process_incoming_link_part1() returns true, which calls multi_process_float(), which commits float Note that problem doesn't happen when data channel key is initialized, since in this case openvpn_decrypt() returns false. The net effect of this behaviour is that the VPN session for the "victim client" is broken. Since the "attacker client" does not have suitable keys, it can not inject or steal VPN traffic from the other session. The time window is small and it can not be used to attack a specific client's session, unless some other way is found to make it disconnect and reconnect first. CVE-2020-11810 has been assigned to acknowledge this risk. Fix illegal float by adding buffer length check ("is this packet still considered valid") before calling multi_process_float(). Trac: #1272 CVE: 2020-11810 Signed-off-by: Lev Stipakov <lev@openvpn.net> Acked-by: Arne Schwabe <arne@rfc2549.org> Acked-by: Antonio Quartulli <antonio@openvpn.net> Acked-by: Gert Doering <gert@greenie.muc.de> Message-Id: <20200415073017.22839-1-lstipakov@gmail.com> URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg19720.html Signed-off-by: Gert Doering <gert@greenie.muc.de> (cherry picked from commit 37bc691)
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