LokiApplication · Grafana
CVE-2026-21726
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 · uneditedThe 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.
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 · moderate confidenceThis is a path traversal vulnerability in Grafana Loki's Ruler API endpoint /loki/api/v1/rules/{namespace}. The CVE-2021-36156 fix validated namespace parameters for path traversal sequences after a single URL decode, but attackers can bypass this validation using double URL encoding (e.g., %252e%252e%252f for ../) to read arbitrary files from the filesystem.
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
LokiApplication
< 3.6.4CVSS 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
- Low
- Integrity
- None
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/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 checksWork through these to decide whether this CVE applies to you.
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Identify installed Grafana Loki versionRun 'loki -version' or check the Loki binary/ container version. Query the /ready or /health endpoint which may expose version information in response headers.Affected if Version is lower than 3.6.4
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Confirm Ruler API is enabledInspect Loki configuration file (usually loki-config.yaml or loki.yaml) for 'ruler' block with 'api: enabled' setting, or attempt a benign request to /loki/api/v1/rules endpoint.Affected if Ruler API is enabled and externally accessible
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Test double URL encoding bypassSend a GET request to /loki/api/v1/rules/%252e%252e%252f (double-encoded '../') and examine if the response differs from a 400 Bad Request or indicates path traversal succeeded.Affected if Double-encoded path traversal sequences are accepted and return unexpected data or file contents
User is affected if running Grafana Loki version below 3.6.4 with the Ruler API enabled and the namespace parameter accepts double-encoded path traversal sequences.
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 · scoped3.6.4
Apply the vendor patch that implements validation after double URL decoding, or implement input normalization that decodes all encoding layers before performing path traversal checks on the namespace parameter.
Loki 3.6.4
- Backup your current Loki configuration and data directory
- Stop the Loki service
- Upgrade Loki to version 3.6.4 or later using your package manager or deployment method (e.g., Helm chart, binary, Docker image)
- Verify the upgrade was successful by checking the Loki version endpoint
- Restart the Loki service
- Test that the Ruler API endpoint /loki/api/v1/rules/{namespace} properly validates and rejects path traversal attempts
Generated from the published advisory — verify against the referenced sources before acting.
Fix this in Loki
Scoped from the published advisory
- Consultation3.0 h
- Implementation6.0 h
- Testing3.0 h
- Review / QA2.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $3,984.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-21726 — 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-2026-21726 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
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