CVE-2021-34791Public exploitNetwork attack vector

Unauthorized connections via insufficient FTP traffic validation

Published Oct 27, 2021 · Updated Nov 7, 2024

FTP ALG bypass in Cisco Adaptive Security Appliance Software allows remote attackers to open connections to hosts behind the firewall. The FTP ALG inadequately validates crafted FTP traffic and creates a NAT/firewall pinhole for an attacker-selected connection. The FTP ALG is enabled by default, and exploitation requires crafted traffic to traverse the device after a user behind it visits attacker-controlled web content.

CVSS severity4.7
Medium
EPSS probability1.14%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

FTP ALG bypass in Cisco Adaptive Security Appliance Software allows remote attackers to open connections to hosts behind the firewall. The FTP ALG inadequately validates crafted FTP traffic and creates a NAT/firewall pinhole for an attacker-selected connection. The FTP ALG is enabled by default, and exploitation requires crafted traffic to traverse the device after a user behind it visits attacker-controlled web content.

The record

CVE
CVE-2021-34791
Published
Oct 27, 2021
Updated
Nov 7, 2024
Vendor
ClamAV
Product
Cisco Adaptive Security Appliance (ASA) Software
Classifications
CWE-20, CWE-358, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Oct 27, 2021CVE publishedPublication date reported by the CVE source.
  2. Nov 7, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=n/a

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

ClamAV · Cisco Adaptive Security Appliance (ASA) Softwareversion=n/a

Published CVSS scores

5.3NIST National Vulnerability DatabaseCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
4.7Cisco Systems, Inc.CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:N/I:L/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-20Improper Input Validation
CWE-358Improperly Implemented Security Check for Standard
T1190Exploit Public-Facing Application

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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