Is CVE-2021-1616 being actively exploited?

CVE-2021-1616 is not currently listed in the CISA Known Exploited Vulnerabilities (KEV) catalog.

What is the CVSS score for CVE-2021-1616?

CVE-2021-1616 has a CVSS 3.1 base score of 4.7 (MEDIUM).

CVE-2021-1616

MEDIUM · CVSS 4.7EPSS 1.2%EU EUVD

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.

Key facts

CVSS score: 4.7 (v3.1)
Severity: MEDIUM
CVSS vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:N/I:L/A:N
Weakness (CWE): CWE-693
EPSS score: 1.2% (67th percentile)
In CISA KEV: No
In EU EUVD: Yes (EUVD-2021-7083)
Published: September 23, 2021
NVD last modified: June 17, 2026
Status: Modified

EU Vulnerability Database (EUVD)

This vulnerability is also tracked by ENISA as EUVD-2021-7083 in the EU Vulnerability Database. Last updated in EUVD on November 7, 2024.

View on the EU Vulnerability Database

Affected products

cisco / ios xe

Certified products that may be affected

21 FIPS 140 or Common Criteria certified products, from automated matching by sec-certs.org.

Cisco Catalyst 9800 (40/80/L) Wireless Controllers running IOS-XE 16.12 - Cisco Systems, Inc. - FIPS 140 4606 - historical
Cisco Aironet 1562e/i/d/ps, 2802e/i, 3802e/i/p, 4800 and IW6300H- AC/DC/DCW Wireless LAN Access Points, AireOS Version 8.10MR3 and IOS-XE 17.3 - Cisco Systems, Inc. - FIPS 140 4129 - historical
Cisco Aggregation Services Router 1000 Series (ASR1K), Cisco Cloud Services Router 1000V (CSR1000V), Cisco Integrated Services Router 1100 Series (ISR1100), Cisco Integrated Services Router 4200 Series (ISR4K) running IOS-XE 17.3 - Cisco Systems, Inc. - Common Criteria CCEVS-VR-VID-11186-2021 - archived
Cisco Catalyst Industrial Ethernet 3x00 Rugged Series (IE3200, IE3300, IE3400, IE3400H) Switches running IOS-XE 17.3 - Cisco Systems, Inc. - Common Criteria CCEVS-VR-VID-11177-2021 - archived
Cisco Catalyst 9800 (40/80/L) Wireless Controllers running IOS-XE 16.12 - Cisco Systems, Inc. - FIPS 140 3883 - historical

See all 21 in the Security Certifications Explorer.

References

https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-iosxe-h323alg-bypass-4vy2MP2Q(Vendor Advisory)

Last updated September 17, 2026 · Source: NVD, CISA KEV, FIRST.org EPSS

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