Asterisk SIP Channel Authentication Bypass (AST-2016-009)
Critical Nessus Plugin ID 95927
SynopsisA telephony application running on the remote host is affected by an authentication bypass vulnerability.
DescriptionAccording to its SIP banner, the version of Asterisk running on the remote host is 11.x prior to 11.25.1, 13.x prior to 13.13.1, 14.x prior to 14.2.1, 11.6 prior to 11.6-cert16, or 13.8 prior to 13.8-cert4. Is it, therefore, affected by an authentication bypass vulnerability in the chan_sip channel driver when handling the content between the SIP header name and a colon character due to incorrect stripping of non-printable ASCII characters. An unauthenticated, remote attacker can exploit this issue, via a specially crafted combination of valid and invalid 'To' headers, to cause a proxy to allow an INVITE request into Asterisk without authentication. This is because, in situations where Asterisk is placed in tandem with an authenticating SIP proxy, the proxy will treats the request as an in-dialog request; however, due to this issue, the request will appear to be an out-of-dialog request to Asterisk, which will then be processed as a new call, thus allowing calls from unauthenticated sources.
Note that Nessus has not tested for this issue but has instead relied only on the application's self-reported version number.
SolutionUpgrade to Asterisk version 11.25.1 / 13.13.1 / 14.2.1 / 11.6-cert16 / 13.8-cert4 or later.