Adaptive Security ApplianceApplication · Cisco

CVE-2021-34791

MEDIUM · 5.3 CVSS v3.1 Published 2021-10-27

Fix available

A fix is available. Upgrade to 6.4.0.12 / 6.6.5 or later.

See remediation →

62/100

Remediation priority · Elevated

Remotely reachable No privileges Zero-click

Official description

NVD · unedited

Multiple vulnerabilities in the Application Level Gateway (ALG) for the Network Address Translation (NAT) feature of Cisco Adaptive Security Appliance (ASA) Software and Firepower Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to bypass the ALG and open unauthorized connections with a host located behind the ALG. For more information about these vulnerabilities, see the Details section of this advisory. Note: These vulnerabilities have been publicly discussed as NAT Slipstreaming.

Technical summary

dbcve analysis · high confidence

This vulnerability in Cisco ASA and FTD's NAT ALG allows remote attackers to bypass the Application Level Gateway and establish unauthorized connections to internal hosts behind the NAT. Attackers exploit ALG parsing weaknesses to inject malicious payloads that trick NAT into opening inbound connections to victim systems, a technique publicly documented as NAT Slipstreaming.

MitigationApply the relevant Cisco security updates for ASA and FTD software to patch the ALG vulnerabilities; verify NAT ALG rules and consider restricting inbound ALG traffic at perimeter controls until patches are deployed.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions

NVD · CPE data

Adaptive Security ApplianceApplication

Affected:< 9.8.4.40

Secure Firewall Threat DefenseApplication

Affected:< 6.4.0.12>= 6.5.0, < 6.6.5>= 6.7.0, < 6.7.0.2

Adaptive Security Appliance SoftwareOperating system

Affected:>= 9.12.0, < 9.12.4.18>= 9.13.0, < 9.14.2.15>= 9.15.0, < 9.15.1.15

Asa 5512 X FirmwareOperating system

Affected:= 009.008= 009.015

Asa 5505 FirmwareOperating system

Affected:= 009.008= 009.015

Asa 5515 X FirmwareOperating system

Affected:= 009.008= 009.015

Asa 5525 X FirmwareOperating system

Affected:= 009.008= 009.015

Asa 5545 X FirmwareOperating system

Affected:= 009.008= 009.015

CVSS breakdown

From the vector

Attack vector: Network
Complexity: Low
Privileges: None
User interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: Low
Availability: None

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

Am I affected?

dbcve checks

Work through these to decide whether this CVE applies to you.

Identify the Cisco product model

Access the device CLI and run 'show version' or check the device management interface to confirm whether the device is a Cisco Adaptive Security Appliance (ASA) or Firepower Threat Defense (FTD)

Affected if Device is an ASA or FTD model listed in the affected products
Check the installed ASA software version

Run 'show version' in the ASA CLI. Look for the version number in the output (format: 9.x.x). Compare against affected ranges: < 9.8.4.40, >= 9.12.0 < 9.12.4.18, >= 9.13.0 < 9.14.2.15, >= 9.15.0 < 9.15.1.15

Affected if Version falls within any of the affected version ranges
Check the installed FTD software version

Run 'show version' in the FTD CLI or check via FMC. Compare against affected ranges: < 6.4.0.12, >= 6.5.0 < 6.6.5, >= 6.7.0 < 6.7.0.2

Affected if FTD version falls within any of the affected version ranges
Verify NAT ALG is enabled

Run 'show nat' and 'show service-policy global' commands in the ASA/FTD CLI to check if NAT and Application Layer Gateway (ALG) services are configured and active. ALG handles protocols like FTP, HTTP, and others that require deep packet inspection

Affected if NAT is configured with ALG services enabled (the vulnerability requires active NAT ALG processing to be exploitable)
Check for firmware version on legacy ASA models

For ASA 5505, 5512X, 5515X, 5525X, or 5545X devices, run 'show version' and check the firmware version. Affected firmware versions are exactly 009.008 and 009.015

Affected if Firmware version equals 009.008 or 009.015 on these legacy models

The device is affected if it is a Cisco ASA or FTD running an affected software/firmware version AND has NAT ALG functionality enabled and configured.

Generated from the published advisory. Verify against your own configuration.

Remediation

dbcve · scoped

Upgrade available Upgrade to 6.4.0.12 / 6.6.5 / 6.7.0.2 or later

Fixed in 6.4.0.126.6.56.7.0.2

Interim mitigation

Apply the relevant Cisco security updates for ASA and FTD software to patch the ALG vulnerabilities; verify NAT ALG rules and consider restricting inbound ALG traffic at perimeter controls until patches are deployed.

Recommended fix High confidence

Minimum fixed versions: ASA 9.8.4.40+ / 9.12.4.18+ / 9.14.2.15+ / 9.15.1.15+; FTD 6.4.0.12+ / 6.6.5+ / 6.7.0.2+ (or latest available release in each branch)

Identify the current installed version of Cisco ASA Software or FTD Software using 'show version' command
Determine which software branch (9.8, 9.12, 9.13, 9.14, 9.15 for ASA or 6.4, 6.5, 6.6, 6.7 for FTD) is currently in use
For ASA 9.8.x branch: upgrade to version 9.8.4.40 or later
For ASA 9.12.x branch: upgrade to version 9.12.4.18 or later
For ASA 9.13.x branch: upgrade to version 9.14.2.15 or later
For ASA 9.15.x branch: upgrade to version 9.15.1.15 or later
For FTD 6.4.x branch: upgrade to version 6.4.0.12 or later
For FTD 6.5.x/6.6.x branch: upgrade to version 6.6.5 or later

Caveat Review Cisco release notes for any compatibility or configuration changes between versions; some upgrades may require downtime and configuration adjustments

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Adaptive Security Appliance Scoped from the published advisory

Consultation2.0 h
Implementation4.0 h
Testing3.0 h
Review / QA2.0 h

11.0 hours of engineering $1,930

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References

Primary sources

tools.cisco.comVendor Advisory
nvd.nist.govNVD record

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-34791 in production — separate from our analysis above.

Notes0 Guidelines

Know something about CVE-2021-34791?

The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.

The version that genuinely resolved it — not the one the vendor claimed
A config change or rule that shut the vector down
A gotcha in the upgrade path that cost you an afternoon

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here

Verified mitigations, workarounds, and config changes
Version or environment caveats, and links to real fixes
No weaponised exploit code, or anything meant to cause harm
No spam, self-promotion, credentials, or personal data

At a glance

A fix existsUpgrade to 6.4.0.12 / 6.6.5 or later

Severity 5.3 Medium

Exploit likelihood EPSS, next 30 days 1.1%

Affects: Adaptive Security Appliance

Remediation

Need this fixed?

Fix this in Adaptive Security Appliance 11 hrs · Moderate

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Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

Check your environment

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

Scan for this in your stack

References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Practitioner notes

No notes yet

Official description

Technical summary

Affected products & versions

CVSS breakdown

Am I affected?

Remediation

References