Grafana Loki up to 3.5.8 Ruler API Endpoint /loki/api/v1/rules/ namespace path traversal

Summaryinfo

A vulnerability classified as critical has been found in Grafana Loki up to 3.5.8. This impacts an unknown function of the file /loki/api/v1/rules/ of the component Ruler API Endpoint. The manipulation of the argument namespace leads to path traversal. This vulnerability is traded as CVE-2026-21726. It is possible to initiate the attack remotely. There is no exploit available. It is recommended to upgrade the affected component.

Detailsinfo

A vulnerability was found in Grafana Loki up to 3.5.8. It has been rated as critical. This issue affects some unknown processing of the file /loki/api/v1/rules/ of the component Ruler API Endpoint. The manipulation of the argument namespace with an unknown input leads to a path traversal vulnerability. Using CWE to declare the problem leads to CWE-22. The product uses external input to construct a pathname that is intended to identify a file or directory that is located underneath a restricted parent directory, but the product does not properly neutralize special elements within the pathname that can cause the pathname to resolve to a location that is outside of the restricted directory. Impacted is confidentiality. The summary by CVE is:

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.

The weakness was shared by Prasanth Sundararajan. It is possible to read the advisory at grafana.com. The identification of this vulnerability is CVE-2026-21726 since 01/05/2026. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. Technical details of the vulnerability are known, but there is no available exploit. The attack technique deployed by this issue is T1006 according to MITRE ATT&CK.

The vulnerability scanner Nessus provides a plugin with the ID 307378 (Linux Distros Unpatched Vulnerability : CVE-2026-21726), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 3.5.9 eliminates this vulnerability.

The vulnerability is also documented in the vulnerability database at Tenable (307378). Be aware that VulDB is the high quality source for vulnerability data.

Productinfo

Vendor

• Grafana

Name

• Loki

Version

• 3.5.0
• 3.5.1
• 3.5.2
• 3.5.3
• 3.5.4
• 3.5.5
• 3.5.6
• 3.5.7
• 3.5.8

License

• open-source

CPE 2.3info

• 🔒
• 🔒
• 🔒

CPE 2.2info

• 🔒
• 🔒
• 🔒

CVSSv4info

CVSSv3info

CVSSv2info

Exploitinginfo

Threat Intelligenceinfo

Countermeasuresinfo

Timelineinfo

Sourcesinfo

Entryinfo

Be aware that VulDB is the high quality source for vulnerability data.

Discussion