---
id: CVE-2021-1625
title: >-
  Cisco IOS XE Software Zone Based Firewall ICMP and UDP Inspection
  Vulnerability
summary: "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.\r\n\r\nThis vulnerability exists b…"
severity: medium
cvss: 5.8
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:L/A:N'
cvssSource: vendor
vendor: Cisco
product: Cisco IOS XE Software
affected:
  - ios_xe_software 3.16.0S
  - ios_xe_software 3.17.0S
  - ios_xe_software 3.17.1S
  - ios_xe_software 3.16.1aS
  - ios_xe_software 3.16.2S
  - ios_xe_software 3.16.0cS
  - ios_xe_software 3.16.3S
  - ios_xe_software 16.2.1
  - ios_xe_software 16.3.1
  - ios_xe_software 16.4.1
  - ios_xe_software 3.17.2S
  - ios_xe_software 16.2.2
  - ios_xe_software 16.3.2
  - ios_xe_software 16.3.3
  - ios_xe_software 16.4.2
  - ios_xe_software 16.5.1
  - ios_xe_software 16.6.1
  - ios_xe_software 16.7.1
  - ios_xe_software 16.8.1
  - ios_xe_software 16.3.1a
  - ios_xe_software 3.16.4aS
  - ios_xe_software 3.16.4bS
  - ios_xe_software 3.16.4gS
  - ios_xe_software 3.16.5S
  - ios_xe_software 16.3.4
  - ios_xe_software 3.16.4cS
  - ios_xe_software 3.17.3S
  - ios_xe_software 3.16.4dS
  - ios_xe_software 3.16.4eS
  - ios_xe_software 3.16.6S
  - ios_xe_software 3.16.5aS
  - ios_xe_software 3.16.5bS
  - ios_xe_software 16.9.1
  - ios_xe_software 16.10.1
  - ios_xe_software 3.18.2aSP
  - ios_xe_software 16.5.1b
  - ios_xe_software 3.17.4S
  - ios_xe_software 16.11.1
  - ios_xe_software 16.12.1
  - ios_xe_software 3.16.7S
published: '2021-09-22'
updated: '2021-09-22'
sourceUpdated: '2021-09-22T16:00:00+00:00'
source: CSAF
sourceUrl: >-
  https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-zbfw-pP9jfzwL
references:
  - url: >-
      https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-zbfw-pP9jfzwL
  - url: 'https://software.cisco.com'
tags:
  - csaf
  - vendor-advisory
  - cisco
epss: 0.00909
epssPercentile: 0.58286
ingestedAt: '2026-09-25T07:41:37.793Z'
---

## Overview

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).

Cisco has released software updates that address this vulnerability. There are no workarounds that address this vulnerability.



This advisory is part of the September 2021 release of the Cisco IOS and IOS XE Software Security Advisory Bundled Publication. For a complete list of the advisories and links to them, see Cisco Event Response: September 2021 Semiannual Cisco IOS and IOS XE Software Security Advisory Bundled Publication. ["https://sec.cloudapps.cisco.com/security/center/viewErp.x?alertId=ERP-74581"]

## Vendor advisories

- **cisco-sa-zbfw-pP9jfzwL** · Cisco · affected: Cisco IOS XE Software (118 versions), Cisco 1000 Series Integrated Services Routers, Cisco ASR 1000 Series Aggregation Services Routers, Cisco 4000 Series Integrated Services Routers, Cisco Integrated Services Virtual Router, Cisco Cloud Services Router 1000V Series, … · updated 2021-09-22 · [advisory](https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-zbfw-pP9jfzwL)

**Cisco IOS XE Software Zone-Based Policy Firewall ICMP and UDP Inspection Vulnerability**. Released 2021-09-22.

Affected:

- Cisco IOS XE Software 3.16.0S
- Cisco IOS XE Software 3.17.0S
- Cisco IOS XE Software 3.17.1S
- Cisco IOS XE Software 3.16.1aS
- Cisco IOS XE Software 3.16.2S
- Cisco IOS XE Software 3.16.0cS
- Cisco IOS XE Software 3.16.3S
- Cisco IOS XE Software 16.2.1
- Cisco IOS XE Software 16.3.1
- Cisco IOS XE Software 16.4.1
- Cisco IOS XE Software 3.17.2S
- Cisco IOS XE Software 16.2.2
- Cisco IOS XE Software 16.3.2
- Cisco IOS XE Software 16.3.3
- Cisco IOS XE Software 16.4.2
- Cisco IOS XE Software 16.5.1
- Cisco IOS XE Software 16.6.1
- Cisco IOS XE Software 16.7.1
- Cisco IOS XE Software 16.8.1
- Cisco IOS XE Software 16.3.1a
- Cisco IOS XE Software 3.16.4aS
- Cisco IOS XE Software 3.16.4bS
- Cisco IOS XE Software 3.16.4gS
- Cisco IOS XE Software 3.16.5S
- Cisco IOS XE Software 16.3.4
- Cisco IOS XE Software 3.16.4cS
- Cisco IOS XE Software 3.17.3S
- Cisco IOS XE Software 3.16.4dS
- Cisco IOS XE Software 3.16.4eS
- Cisco IOS XE Software 3.16.6S
- Cisco IOS XE Software 3.16.5aS
- Cisco IOS XE Software 3.16.5bS
- Cisco IOS XE Software 16.9.1
- Cisco IOS XE Software 16.10.1
- Cisco IOS XE Software 3.18.2aSP
- Cisco IOS XE Software 16.5.1b
- Cisco IOS XE Software 3.17.4S
- Cisco IOS XE Software 16.11.1
- Cisco IOS XE Software 16.12.1
- Cisco IOS XE Software 3.16.7S

## Remediation

Cisco has released software updates that address this vulnerability. https://software.cisco.com