CVE-2021-1616: Cisco IOS XE

Medium severity, CVSS 4.7. EPSS: 1.2% chance of exploitation in the next 30 days.

A vulnerability in the H.323 application level gateway (ALG) used by the Network Address Translation (NAT) feature of Cisco IOS XE Software could allow an unauthenticated, remote attacker to bypass the ALG. This vulnerability is due to insufficient data validation of traffic that is traversing the ALG. An attacker could exploit this vulnerability by sending crafted traffic to a targeted device. A successful exploit could allow the attacker to bypass the ALG and open connections that should not be allowed to a remote device located behind the ALG. Note: This vulnerability has been publicly discussed as NAT Slipstreaming.

Affected products

• Cisco IOS XE: before 17.6.1 (fixed in 17.6.1)

Published 2021-09-23. Last modified 2026-06-17.