CVE-2021-1616

mediumpriority · track · 19EPSS 0.012 · 68 %ileEURUUS
grimoire priorityP3 · Track
Exploitable?
—
no known exploitation
Likely / severe?
1.19% (68th) EPSS
CVSS 4.3 · medium
Is it mine?
285 products
Check your exposure
Who says so?
4 sources
cnnvd · enisa_euvd · fstec_bdu · cisco_csaf

EPSS · scored 2026-10-27

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.

01

remediations

Vendor fix, mitigation, or workaround guidance, as published by each source.

A vendor fix is available.

02

cvss

Technical severity, 0–10. One row per scoring source.

4.7
010

base score 0–10 · higher = more severe · hover a vector to decode

v2.04.3mediumAV:N/AC:M/Au:N/C:N/I:P/A:Nprimary · NVD / NIST USexploitability 8.6 · impact 2.9
v3.14.7mediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:N/I:L/A:Nprimary · NVD / NIST USexploitability 2.8 · impact 1.4
v3.14.7mediumAV:N/AC:L/PR:N/UI:R/S:C/C:N/I:L/A:Nsecondary · FSTEC (BDU) RU
v3.14.7mediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:N/I:L/A:Nsecondary · ENISA EU
v3.14.7mediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:N/I:L/A:Nsecondary · psirt@cisco.com USexploitability 2.8 · impact 1.4
v3.14.3mediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:N/I:L/A:N/E:Usecondary · CVSS-BT (t0sche)
v3.14.7mediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:N/I:L/A:Nsecondary · Cisco PSIRT US
03

ssvc

A stakeholder decision — act, attend, or track — from exploitation and impact.

Trackcoordinatorcisa_ssvcUS
04

weaknesses

The CWE weakness classes this flaw belongs to.

CWE-693USCWE-693US
05

affected products (285)

Products and version ranges known to be affected.

showing 20 of 285
06

advisories (1)

Vendor and ecosystem advisories describing this.

  • cisco-sa-iosxe-h323alg-bypass-4vy2MP2QCisco IOS XE Software H.323 Application Level Gateway Bypass Vulnerability2021-09-22csaf · cisco_csafUS

    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 ["https://samy.pl/slipstream/"]. 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"]

07

also known as (4)

Other names and identifiers for this entity.

08

sources

Which sources assert this record, and when.

cnnvdenisa_euvdfstec_bducisco_csafNVDCVE.orgMITRE
published 2021-09-23modified 2026-06-17
09

references (7)

External citations — patches, advisories, analysis.

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