CVE-2026-21726: Path Traversal Vulnerability via Encoding

CVE-2026-21726 Overview

CVE-2026-21726 is a path traversal vulnerability in Grafana Loki that allows attackers to bypass the existing CVE-2021-36156 fix by using double URL encoding. The original security patch validates the namespace parameter for path traversal sequences after performing a single URL decode. By double encoding malicious path traversal sequences, an attacker can evade this validation and read arbitrary files through the Ruler API endpoint /loki/api/v1/rules/{namespace}.

Critical Impact

Unauthenticated attackers can read sensitive files from the server by bypassing path traversal protections through double URL encoding, potentially exposing configuration files, credentials, or other sensitive data.

Affected Products

• Grafana Loki (versions with incomplete CVE-2021-36156 fix)

Discovery Timeline

• 2026-04-15 - CVE CVE-2026-21726 published to NVD
• 2026-04-15 - Last updated in NVD database

Technical Details for CVE-2026-21726

Vulnerability Analysis

This vulnerability represents a bypass of the previous security fix implemented for CVE-2021-36156. The original patch introduced input validation to detect path traversal sequences (such as ../) in the namespace parameter of the Ruler API endpoint. However, this validation occurs only after a single URL decode operation.

The attack exploits the difference between how the validation logic and the underlying file system access handle encoded characters. When an attacker double-encodes path traversal sequences (e.g., %252e%252e%252f instead of ../), the first URL decode operation transforms this into the still-encoded form %2e%2e%2f, which does not match the path traversal patterns being filtered. The second decode operation, occurring later in the processing pipeline closer to file access, then converts this to the actual ../ sequence, allowing the traversal attack to succeed.

This is a network-accessible vulnerability requiring no authentication or user interaction, enabling remote attackers to read files from the affected system with the permissions of the Loki process.

Root Cause

The root cause is improper input validation that fails to account for multiple levels of URL encoding. The security fix for CVE-2021-36156 performs path traversal detection after only a single URL decode operation, while the application's processing pipeline applies additional decoding before accessing the file system. This mismatch between validation and actual use creates a window for exploitation through double-encoded payloads.

Attack Vector

The attack targets the Ruler API endpoint at /loki/api/v1/rules/{namespace}. An attacker crafts a malicious namespace parameter containing double-encoded path traversal sequences. For example, to traverse to /etc/passwd, an attacker would encode ../ as %252e%252e%252f (where %25 is the URL-encoded form of %).

The request bypasses the path traversal check because after the first decode, the string appears as %2e%2e%2f rather than ../. When the application subsequently accesses the file system, a second decode reveals the actual path traversal sequence, allowing the attacker to read files outside the intended directory structure.

The attack can be executed remotely over the network without requiring any privileges or user interaction, making it particularly dangerous for internet-exposed Loki instances.

Detection Methods for CVE-2026-21726

Indicators of Compromise

• HTTP requests to /loki/api/v1/rules/ containing double-encoded characters such as %252e, %252f, or %255c
• Access logs showing unusual namespace values with multiple layers of URL encoding
• File access attempts to sensitive system files (e.g., /etc/passwd, configuration files) from the Loki process
• Unexpected read operations outside the configured rules directory

Detection Strategies

• Implement web application firewall (WAF) rules to detect and block requests with double-encoded path traversal sequences
• Configure intrusion detection systems to alert on requests containing patterns like %252e%252e%252f or %252e%252e%255c
• Monitor Loki access logs for requests to the Ruler API with encoded special characters in the namespace parameter
• Enable file integrity monitoring on sensitive configuration files and system files

Monitoring Recommendations

• Review web server and application logs for requests to /loki/api/v1/rules/ endpoints with suspicious encoded values
• Set up alerting for any file access outside the expected rules storage directory by the Loki process
• Monitor network traffic for reconnaissance patterns targeting the Ruler API endpoint
• Implement logging of all file read operations performed by the Loki service for forensic analysis

How to Mitigate CVE-2026-21726

Immediate Actions Required

• Apply the vendor security patch addressing CVE-2026-21726 as soon as available
• Restrict network access to the Loki Ruler API endpoint to trusted sources only
• Implement authentication requirements for all API endpoints if not already configured
• Consider temporarily disabling the Ruler API if it is not required for operations

Patch Information

Grafana has released a security advisory for this vulnerability. Organizations should refer to the Grafana Security Advisory CVE-2026-21726 for specific patch versions and upgrade instructions. Apply the latest security update to ensure complete protection against both the original CVE-2021-36156 and this bypass technique.

Workarounds

• Deploy a reverse proxy or WAF in front of Loki to recursively decode and validate all URL parameters before forwarding requests
• Implement network-level access controls to limit access to the Ruler API endpoint to authorized IP addresses only
• Use network segmentation to isolate Loki instances from sensitive systems and limit the impact of potential file disclosure
• Configure application-level authentication to prevent unauthorized access to the Ruler API

# Example: Nginx configuration to block double-encoded path traversal attempts
location /loki/api/v1/rules/ {
    # Block requests containing double-encoded path traversal sequences
    if ($request_uri ~* "%252e%252e") {
        return 403;
    }
    if ($request_uri ~* "%252f") {
        return 403;
    }
    # Restrict access to trusted networks
    allow 10.0.0.0/8;
    deny all;
    
    proxy_pass http://loki-backend;
}