Grafana Loki Path Traversal - CVE-2021-36156 Bypass

Exploitation

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

CVSS

5.3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N

EPSS

• CVE-2026-21726: 0.41% estimated exploitation activity in the next 30 days; percentile 35.26 (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.

The CVE-2021-36156 fix validates the namespace parameter for path traversal sequences after a single URL decode, by double encoding, an attacker can read files at the Ruler API endpoint /loki/api/v1/rules/{namespace} Thanks to Prasanth Sundararajan for reporting this vulnerability.

Published by the source

15 Apr 2026, 21:30 UTC

Last modified by the source

27 Apr 2026, 16:38 UTC

Source last checked

26 Oct 2026, 15:19 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-497x-rrr9-68jp

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# Patch Next advisory brief: CVE-2026-21726

Advisory: Grafana Loki Path Traversal - CVE-2021-36156 Bypass
IDs: CVE-2026-21726, GHSA-497x-rrr9-68jp
Record: https://patchnext.com/advisories/cve-2026-21726 (revision 1, source checked 26 Oct 2026, 15:19 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: github.com/grafana/loki/v3 3.6.4. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/grafana/loki/v3: affected < 3.6.4; fixed in 3.6.4

## Suggested inspection (read-only)
- Go github.com/grafana/loki/v3: 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/grafana/loki/v3.
- 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-497x-rrr9-68jp (GitHub-reviewed advisory GHSA-497x-rrr9-68jp lists affected ranges: Go github.com/grafana/loki/v3 >=0 <3.6.4)

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