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EosOperating system · Arista

CVE-2021-28504

HIGH · 7.5 CVSS v3.1 Published 2022-04-01
Fix available
A fix is available. Upgrade to 4.26.4m / 4.27.1f 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
On Arista Strata family products which have “TCAM profile” feature enabled when Port IPv4 access-list has a rule which matches on “vxlan” as protocol then that rule and subsequent rules ( rules declared after it in ACL ) do not match on IP protocol field as expected.

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

On Arista Strata switches with TCAM profile enabled, a bug exists in IPv4 Port ACL processing where a rule matching on 'vxlan' protocol causes that rule and all subsequent rules in the ACL to fail matching on the IP protocol field as intended, effectively breaking ACL enforcement for affected rules.

MitigationContact Arista for the appropriate EOS software update/patch that addresses this TCAM profile ACL bug; in the meantime, verify ACLs do not use vxlan protocol matching or implement compensating controls.

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
EosOperating system
Affected:>= 4.26, < 4.26.4m>= 4.27, < 4.27.1f

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
High
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/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 EOS version
    Run 'show version' and locate the EOS software version string (e.g., 4.26.x or 4.27.x)
    Affected if Version is 4.26.0 through 4.26.3m, or 4.27.0 through 4.27.1e (before 4.26.4m or 4.27.1f)
  2. Verify TCAM profile is enabled
    Run 'show hardware tcam profile' to see if a TCAM profile is active
    Affected if A TCAM profile is loaded and active (default TCAM profiles such as 'default', 'border-router', or 'datacenter' are affected)
  3. Identify IPv4 Port ACLs with vxlan protocol rules
    Run 'show ip access-lists' and grep for 'vxlan' or inspect ACL configurations with 'show running-config | include ip access-list'
    Affected if Any IPv4 Port ACL contains a rule matching on 'vxlan' protocol (e.g., 'permit vxlan any any')
  4. Check ACL rule behavior after vxlan rule
    Run 'show ip access-lists <acl-name>' with traffic matching rules after the vxlan entry; compare against expected match counts
    Affected if Rules positioned after the vxlan protocol rule do not match traffic as expected, indicating the protocol field matching is broken

You are affected if running a vulnerable EOS version (4.26.0-4.26.3m or 4.27.0-4.27.1e) with TCAM profile enabled AND you have IPv4 Port ACLs containing vxlan protocol matching rules, with subsequent rules failing to match on IP protocol.

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 4.26.4m / 4.27.1f or later
Fixed in 4.26.4m4.27.1f
Interim mitigation

Contact Arista for the appropriate EOS software update/patch that addresses this TCAM profile ACL bug; in the meantime, verify ACLs do not use vxlan protocol matching or implement compensating controls.

Recommended fix High confidence

EOS 4.26.4m (or 4.27.1f, depending on your release train)

  1. 1. Identify the current EOS version running on the affected Arista Strata device using 'show version'
  2. 2. If running EOS version >= 4.26 and < 4.26.4m, plan upgrade to EOS 4.26.4m or later stable release
  3. 3. If running EOS version >= 4.27 and < 4.27.1f, plan upgrade to EOS 4.27.1f or later stable release
  4. 4. Review the current Port IPv4 access-list configurations to identify any rules matching on 'vxlan' protocol
  5. 5. Download the appropriate fixed EOS image from Arista software downloads (requires valid support contract)
  6. 6. Follow standard Arista EOS upgrade procedure: upload the image, verify checksum, reboot or switchover to the new image
  7. 7. After upgrade, verify the access-list rules matching on 'vxlan' now work correctly and subsequent rules match as expected
Caveat Standard EOS upgrade risks apply - ensure you have configuration backup and plan for brief downtime; consider hitless upgrade procedure if redundant hardware is available

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

Fix this in Eos Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing6.0 h
  • Review / QA2.0 h
20.0 hours of engineering $3,500
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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-2021-28504 in production — separate from our analysis above.

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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