dbcve.org
AsyncosOperating system · Cisco

CVE-2021-1534

MEDIUM · 5.3 CVSS v3.1 Published 2021-10-06
Fix available
A fix is available. Upgrade to 14.0.1 or later.
See remediation →
62/100
Remediation priority · Elevated
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 antispam protection mechanisms of Cisco AsyncOS Software for Cisco Email Security Appliance (ESA) could allow an unauthenticated, remote attacker to bypass the URL reputation filters on an affected device. This vulnerability is due to improper processing of URLs. An attacker could exploit this vulnerability by crafting a URL in a particular way. A successful exploit could allow the attacker to bypass the URL reputation filters that are configured for an affected device, which could allow malicious URLs to pass through the device.

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 · moderate confidence

This vulnerability in Cisco Email Security Appliance allows unauthenticated remote attackers to bypass URL reputation filters by crafting URLs in a specific way that the antispam mechanism doesn't properly process. The improper URL handling enables malicious URLs to pass through the device unchecked.

MitigationApply Cisco's patch for this vulnerability when available. In the interim, implement supplementary URL filtering or email content inspection controls to catch malicious links that bypass the reputation filters.

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:< 14.0.1

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
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? 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. Check Cisco AsyncOS version
    Access the Cisco Email Security Appliance CLI and run the 'version' command, or log into the web interface and navigate to System Administration > Software Updates to view the installed AsyncOS version.
    Affected if The installed AsyncOS version is below 14.0.1 (for example, 13.x.x or earlier releases).
  2. Verify URL reputation filtering is enabled
    Log into the Cisco Email Security Appliance web interface and navigate to Mail Policies > URL Filtering, or use the CLI command 'urlfilterconfig' to check if URL reputation filtering is configured and active.
    Affected if URL reputation filtering is enabled and processing incoming/outgoing emails.
  3. Confirm the appliance is exposed to external email traffic
    Check network accessibility by reviewing the interface configurations (Network > Interfaces) and verifying the appliance accepts external SMTP connections on port 25 from untrusted networks.
    Affected if The appliance accepts external email connections from untrusted networks, allowing remote attackers to send malicious emails through the device.
  4. Review antispam URL processing logs
    Use the CLI command 'grep -i url /var/log/ironport.ini' or navigate to Monitor > Message Tracking in the web interface to examine recent messages with URLs and check if URL reputation scoring is being applied.
    Affected if URL reputation filters are present but URLs may not be properly scored or processed in recent message logs.

The environment is affected if the installed Cisco AsyncOS version is below 14.0.1 and URL reputation filtering is enabled for external email traffic.

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 14.0.1 or later
Fixed in 14.0.1
Interim mitigation

Apply Cisco's patch for this vulnerability when available. In the interim, implement supplementary URL filtering or email content inspection controls to catch malicious links that bypass the reputation filters.

Recommended fix High confidence

AsyncOS 14.0.1 or later

  1. Contact Cisco Technical Support (TAC) or access the Cisco Software Download center to obtain the AsyncOS 14.0.1 or later upgrade image for your Cisco Email Security Appliance (ESA) model.
  2. Review the Cisco AsyncOS 14.0.1 release notes for any special upgrade instructions, prerequisites, or configuration changes required.
  3. Back up the current ESA configuration before performing the upgrade.
  4. Upload the AsyncOS upgrade image to the ESA using the CLI command 'update' or through the web interface (System Administration > Software Update).
  5. Reboot the ESA to complete the upgrade process.
  6. Verify the upgrade was successful by checking the AsyncOS version with the 'version' command in the CLI.
  7. Confirm that URL reputation filters are functioning correctly after the upgrade.
Caveat Review Cisco release notes for 14.0.1 for any configuration or behavioral changes; some feature deprecations may occur between major versions

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

Fix this in Asyncos Scoped from the published advisory
  • Consultation4.0 h
  • Implementation4.0 h
  • Testing3.0 h
  • Review / QA2.0 h
13.0 hours of engineering $2,330
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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-2021-1534 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