CVE-2021-31999
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 · uneditedA Reliance on Untrusted Inputs in a Security Decision vulnerability in Rancher allows users in the cluster to act as others users in the cluster by forging the "Impersonate-User" or "Impersonate-Group" headers. This issue affects: Rancher versions prior to 2.5.9. Rancher versions prior to 2.4.16.
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 confidenceRancher fails to properly validate Impersonate-User and Impersonate-Group HTTP headers, allowing authenticated cluster users to forge these headers and impersonate other users or groups. This is a classic input validation flaw where untrusted client-supplied headers are trusted without sufficient verification, enabling privilege escalation within the cluster.
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< 2.4.16>= 2.5.0, < 2.5.9CVSS 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
- Low
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- High
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/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 checksWork through these to decide whether this CVE applies to you.
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Identify installed Rancher versionRun 'rancher --version' or check the Rancher UI under 'Support' > 'About Rancher', or inspect the Rancher container/pod image tagAffected if The version is below 2.4.16 or is 2.5.0 through 2.5.8 (inclusive)
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Confirm Kubernetes API accessibilityVerify that the Rancher-managed Kubernetes cluster API server is accessible to authenticated users. Check if users have kubeconfig files or API access via 'kubectl get nodes' with user credentialsAffected if Cluster users have direct or indirect access to the Kubernetes API server (this is the attack vector for the impersonation headers)
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Review authentication configurationCheck the authentication providers configured in Rancher under 'Users & Authentication' > 'Auth' to see which external identity providers are enabled (e.g., Active Directory, LDAP, OAuth)Affected if External authentication is configured, because the flaw allows forging impersonation headers to hijack identities from these providers
You are affected if your Rancher version falls in the vulnerable ranges (<2.4.16 or >=2.5.0 to <2.5.9) AND your cluster has authenticated users who can access the Kubernetes API.
Generated from the published advisory. Verify against your own configuration.
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 · scoped2.4.162.5.9
Upgrade Rancher to version 2.5.9 or 2.4.16 or later to receive the patch that properly validates impersonation headers.
Rancher 2.4.16+ or 2.5.9+
- Backup your Rancher deployment and all cluster data before proceeding
- If running Rancher 2.4.x: Upgrade to Rancher version 2.4.16 or later
- If running Rancher 2.5.x: Upgrade to Rancher version 2.5.9 or later
- After upgrade, verify the Rancher version reflects the patched release
- Test that the Impersonate-User and Impersonate-Group headers are properly validated and cannot be forged
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation3.0 h
- Implementation6.0 h
- Testing6.0 h
- Review / QA2.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-31999 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2021-31999 in production — separate from our analysis above.
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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- Version or environment caveats, and links to real fixes
- No weaponised exploit code, or anything meant to cause harm
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