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Big Ip Access Policy ManagerApplication · F5

CVE-2020-5941

HIGH · 7.5 CVSS v3.1 Published 2020-11-05
Fix available
A fix is available. Upgrade to 15.1.1 / 16.0.1 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 BIG-IP versions 16.0.0-16.0.0.1 and 15.1.0-15.1.0.5, using the RESOLV::lookup command within an iRule may cause the Traffic Management Microkernel (TMM) to generate a core file and restart. This issue occurs when data exceeding the maximum limit of a hostname passes to the RESOLV::lookup command.

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 denial of service vulnerability exists in F5 BIG-IP where the RESOLV::lookup iRule command fails to properly validate hostname length, causing the Traffic Management Microkernel (TMM) to crash, generate a core file, and restart when data exceeding the maximum hostname limit is passed to the command.

MitigationUpgrade BIG-IP to a version beyond 16.0.0.1 and 15.1.0.5 that contains the fix. As a workaround, audit and modify or disable iRules using RESOLV::lookup to ensure they do not pass oversized hostname data.

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
Big Ip Access Policy ManagerApplication
Affected:>= 15.1.0, < 15.1.1>= 16.0.0, < 16.0.1
Big Ip Advanced Firewall ManagerApplication
Affected:>= 15.1.0, < 15.1.1>= 16.0.0, < 16.0.1
Big Ip AnalyticsApplication
Affected:>= 15.1.0, < 15.1.1>= 16.0.0, < 16.0.1
Big Ip Application Acceleration ManagerApplication
Affected:>= 15.1.0, < 15.1.1>= 16.0.0, < 16.0.1
Big Ip Application Security ManagerApplication
Affected:>= 15.1.0, < 15.1.1>= 16.0.0, < 16.0.1
Big Ip Domain Name SystemApplication
Affected:>= 15.1.0, < 15.1.1>= 16.0.0, < 16.0.1
Big Ip Fraud Protection ServiceApplication
Affected:>= 15.1.0, < 15.1.1>= 16.0.0, < 16.0.1
Big Ip Global Traffic ManagerApplication
Affected:>= 15.1.0, < 15.1.1>= 16.0.0, < 16.0.1

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
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. Check BIG-IP version
    Run 'tmsh show sys version' or access the web UI under Diagnostics > General > Version Info to determine the installed version
    Affected if The version falls within 15.1.0 to 15.1.0.4 or 16.0.0 to 16.0.0.3 (the unpatched ranges)
  2. Identify iRules using RESOLV::lookup
    Run 'tmsh list ltm rule all' and search for 'RESOLV::lookup' in the output, or use the web UI to review iRule definitions under Local Traffic > iRules
    Affected if Any iRule contains the RESOLV::lookup command
  3. Examine RESOLV::lookup usage for hostname handling
    Review each iRule found with RESOLV::lookup and examine how hostname or domain name data is passed to the command - look for dynamic data sources such as HTTP headers, URI components, or variable data
    Affected if The iRule passes dynamically constructed or potentially oversized hostname data to RESOLV::lookup without length validation
  4. Check for TMM crash history
    Review /var/log/ltm for TMM crash entries, or run 'tmsh show ltm core' to list core dump files
    Affected if Recent TMM crashes or core dumps exist and coincide with RESOLV::lookup iRule execution

You are affected if your BIG-IP version is 15.1.0.x before 15.1.1 or 16.0.0.x before 16.0.1 AND you have iRules using RESOLV::lookup that could pass oversized hostname data to the command.

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.

From vendor data
Upgrade available Upgrade to 15.1.1 / 16.0.1 or later
Fixed in 15.1.116.0.1
Interim mitigation

Upgrade BIG-IP to a version beyond 16.0.0.1 and 15.1.0.5 that contains the fix. As a workaround, audit and modify or disable iRules using RESOLV::lookup to ensure they do not pass oversized hostname data.

Fix this in Big Ip Access Policy Manager Scoped from the published advisory
  • Consultation3.0 h
  • Implementation6.0 h
  • Testing6.0 h
  • Review / QA3.0 h
18.0 hours of engineering $3,120
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $4,992.

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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-2020-5941 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