Rancher Privilege escalation vulnerability via malicious "Connection" header

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

8.7 · CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N · score source

EPSS

• CVE-2021-31999: 1.05% estimated exploitation activity in the next 30 days; percentile 64.39 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

A vulnerability was discovered in Rancher 2.0.0 through the aforementioned patched versions, where a malicious Rancher user could craft an API request directed at the proxy for the Kubernetes API of a managed cluster to gain access to information they do not have access to. This is done by passing the "Impersonate-User" or "Impersonate-Group" header in the Connection header, which is then correctly removed by the proxy. At this point, instead of impersonating the user and their permissions, the request will act as if it was from the Rancher management server and incorrectly return the information. The vulnerability is limited to valid Rancher users with some level of permissions on the cluster. There is not a direct mitigation besides upgrading to the patched Rancher versions.

Published by the source

24 Apr 2024, 21:01 UTC

Last modified by the source

7 Aug 2024, 05:01 UTC

Source last checked

26 Oct 2026, 15:04 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-pvxj-25m6-7vqr

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# Patch Next advisory brief: CVE-2021-31999

Advisory: Rancher Privilege escalation vulnerability via malicious "Connection" header
IDs: CVE-2021-31999, GHSA-pvxj-25m6-7vqr
Record: https://patchnext.com/advisories/cve-2021-31999 (revision 1, source checked 26 Oct 2026, 15:04 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.7)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: github.com/rancher/rancher 2.5.9, 2.4.16. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/rancher/rancher: affected >= 2.0.0, < 2.4.16; >= 2.5.0, < 2.5.9; fixed in 2.4.16, 2.5.9

## Suggested inspection (read-only)
- Go github.com/rancher/rancher: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on github.com/rancher/rancher.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-pvxj-25m6-7vqr (GitHub-reviewed advisory GHSA-pvxj-25m6-7vqr lists affected ranges: Go github.com/rancher/rancher >=2.0.0 =2.5.0 <2.5.9)

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