CVE-2021-1625: Cisco IOS XE Software Zone-Based Policy Firewall ICMP and UDP Inspection Vulnerability

Severity: medium (CVSS 5.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.9% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2021-09-23 · updated 2026-06-17
Weakness: CWE-284

Description

A vulnerability in the Zone-Based Policy Firewall feature of Cisco IOS XE Software could allow an unauthenticated, remote attacker to prevent the Zone-Based Policy Firewall from correctly classifying traffic. This vulnerability exists because ICMP and UDP responder-to-initiator flows are not inspected when the Zone-Based Policy Firewall has either Unified Threat Defense (UTD) or Application Quality of Experience (AppQoE) configured. An attacker could exploit this vulnerability by attempting to send UDP or ICMP flows through the network. A successful exploit could allow the attacker to inject traffic through the Zone-Based Policy Firewall, resulting in traffic being dropped because it is incorrectly classified or in incorrect reporting figures being produced by high-speed logging (HSL).

Affected products

Cisco IOS XE: before 17.3.2 — fixed in 17.3.2

How to fix

Upgrade to a fixed version: Cisco IOS XE 17.3.2. Check the vendor advisory for mitigations.

References

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