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Cisco Bug: CSCvk42668 - Cisco IOS XE Software ISDN Data Leak Vulnerability

Last Modified

Sep 21, 2020

Products (1)

  • Cisco 4000 Series Routers

Known Affected Releases


Description (partial)

A vulnerability in the Dialer interface feature for ISDN connections in Cisco IOS XE Software for Cisco 4000 Series Integrated Services Routers (ISR) could allow an unauthenticated, adjacent attacker to pass IPv4 traffic through an ISDN channel prior to successful PPP authentication.

The vulnerability is due to insufficient validation of the state of the PPP IP Control Protocol (IPCP). An attacker could exploit this vulnerability by making an ISDN call to an affected device and sending traffic through the ISDN channel prior to successful PPP authentication. Alternatively, an unauthenticated, remote attacker could exploit this vulnerability by sending traffic through an affected device that is configured to exit via an ISDN connection for which both the Dialer interface and the Basic Rate Interface (BRI) have been configured, but the Challenge Handshake Authentication Protocol (CHAP) password for PPP does not match the remote end. A successful exploit could allow the attacker to pass IPv4 traffic through an unauthenticated ISDN connection for a few seconds, from initial ISDN call setup until PPP authentication fails.

Cisco has published a security advisory containing additional details which is available at the following link:

For information on fixed versions of software consult the Cisco IOS Software checker: 

See Vulnerable Products Section of the advisory for full details:
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