CVE-2026-21726 Loki Path Traversal - CVE-2021-36156 Bypass

Medium 5.3 Published 15 Apr 2026 Updated 27 Aug 2026

Attackers can bypass path traversal protections in Grafana Loki to read arbitrary files via the Ruler API endpoint.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: Available

What is CVE-2026-21726?

CVE-2026-21726 is a medium-severity path traversal vulnerability in Grafana Loki, published on 15 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

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.

Weakness: CWE-22

Which versions are affected by CVE-2026-21726?

4 open source packages ship an affected version, including github.com/grafana/loki/v3 and github.com/grafana/loki. It affects 1 product.

Package	Affected	Fixed in
`github.com/grafana/loki/v3` Go	All versions	`3.6.4`
`github.com/grafana/loki/v3` Go	All versions	`3.6.4`
`github.com/grafana/loki/v3` Go	All versions	`3.6.4`
`github.com/grafana/loki` Go	All versions	None

Products include Grafana Loki.

How do I fix CVE-2026-21726?

Upgrade github.com/grafana/loki/v3 to 3.6.4 or later.

Upgrade, and resolve every user-supplied path against an allowed base folder before opening it. More about Path traversal.

Is CVE-2026-21726 being exploited?

There is no known exploitation of CVE-2026-21726 in the wild and no public exploit code.

CVE-2021-36156 Path traversal in Grafana Loki Medium 5.3

Sources

Also known as CLEANSTART-2026-JO14647, CLEANSTART-2026-WN62977, CLEANSTART-2026-YM30518, CLEANSTART-2026-ZS27491, ECHO-d86c-cc77-aee6, GHSA-497x-rrr9-68jp and 28 more.