dbcve.org
AsyncosOperating system · Cisco

CVE-2021-34741

HIGH · 7.5 CVSS v3.1 Published 2021-11-04
Fix available
A fix is available. Upgrade to 13.0.4 or later.
See remediation →
84/100
Remediation priority · High
Remotely reachable No privileges Zero-click

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.

NVD · unedited
A vulnerability in the email scanning algorithm of Cisco AsyncOS software for Cisco Email Security Appliance (ESA) could allow an unauthenticated, remote attacker to perform a denial of service (DoS) attack against an affected device. This vulnerability is due to insufficient input validation of incoming emails. An attacker could exploit this vulnerability by sending a crafted email through Cisco ESA. A successful exploit could allow the attacker to exhaust all the available CPU resources on an affected device for an extended period of time, preventing other emails from being processed and resulting in a DoS condition.

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.

dbcve analysis · high confidence

A vulnerability in the email scanning algorithm of Cisco AsyncOS software for Cisco Email Security Appliance (ESA) allows unauthenticated remote attackers to send crafted emails with insufficient input validation, causing excessive CPU consumption and preventing legitimate email processing.

MitigationApply the Cisco security update/patch for CVE-2021-34741 to the AsyncOS software on the affected ESA devices.

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

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.

NVD · CPE data
AsyncosOperating system
Affected:< 13.0.4= 13.5.3-010= 13.7.0-093

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.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

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

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.

dbcve checks

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

  1. Identify Cisco ESA device
    Log into the AsyncOS CLI and run the 'version' or 'show version' command to confirm the device model is an Email Security Appliance (ESA)
    Affected if The device is not a Cisco ESA (the vulnerability only affects ESA devices)
  2. Check AsyncOS version
    Run 'version' or 'show version' in the AsyncOS CLI to display the installed AsyncOS software version number
    Affected if The displayed version is < 13.0.4, or exactly matches 13.5.3-010 or 13.7.0-093
  3. Verify email scanning is active
    Run 'uscan' or 'scan' status command in the CLI, or check the IronPort Text UI (if accessible) to confirm email scanning/filtering services are enabled
    Affected if Email scanning is disabled, the vulnerability cannot be triggered (the flaw exists in the scanning algorithm itself)
  4. Check specific build string for exact versions
    Run 'version' and look for the full build string (e.g., '13.5.3-010' or '13.7.0-093') to confirm exact version match
    Affected if The build string exactly shows 13.5.3-010 or 13.7.0-093 (these specific builds are vulnerable)

The environment is affected if the device is a Cisco ESA running AsyncOS version < 13.0.4, or exactly build 13.5.3-010 or 13.7.0-093, with email scanning services enabled.

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

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

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.

dbcve · scoped
Upgrade available Upgrade to 13.0.4 or later
Fixed in 13.0.4
Interim mitigation

Apply the Cisco security update/patch for CVE-2021-34741 to the AsyncOS software on the affected ESA devices.

Recommended fix High confidence

AsyncOS 13.0.4 or later stable release (13.5.x or 13.7.x series beyond the specific vulnerable builds)

  1. 1. Identify the current AsyncOS version on the Cisco Email Security Appliance using the CLI command: version
  2. 2. If running a version < 13.0.4, or exactly 13.5.3-010, or exactly 13.7.0-093, plan for an upgrade
  3. 3. Download the fixed AsyncOS version from Cisco (13.0.4 or later stable release) from tools.cisco.com or the Cisco Software Download center
  4. 4. Back up the ESA configuration using the saveconfig command
  5. 5. Upload the AsyncOS upgrade file to the ESA via the GUI (System Administration > Software Update) or CLI
  6. 6. Install the upgrade and restart the device
  7. 7. Verify the new version is installed using the version command
  8. 8. Confirm the email scanning functionality is operational
Caveat ESA upgrades may require downtime; verify compatibility with existing mail flow and any integrated systems before scheduling

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

Fix this in Asyncos Scoped from the published advisory
  • Consultation3.0 h
  • Implementation4.0 h
  • Testing3.0 h
  • Review / QA2.0 h
12.0 hours of engineering $2,130
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $3,408.

Scan for this in your stack

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

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-34741 in production — separate from our analysis above.

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