CVE-2026-42272

Heimdall handles URL-encoded slashes (%2F) in a case-sensitive manner, while percent-encoding is defined to be case-insensitive. As a result, the lowercase equivalent (%2f) is not recognized and therefore not processed as expected when allow_encoded_slashes is set to off (the default setting).

This discrepancy can lead to differences in how request paths are interpreted by heimdall and upstream components, which may result in authorization bypass.

Note: The issue can only lead to unintended access if heimdall is configured with an "allow all" default rule. Since v0.16.0, heimdall enforces secure defaults and refuses to start with such a configuration unless this enforcement is explicitly disabled (e.g. via --insecure-skip-secure-default-rule-enforcement or the broader --insecure flag).

Details

Consider the following rule configuration:

id: rule-1
match:
  routes:
    - path: /admin/**
execute: # configured to require authentication and authorization
  # ...

If an adversary sends a request such as /admin%2fsecret, neither is the above rule matched, nor is the request rejected (as would be expected when allow_encoded_slashes is set to off). Instead, the default rule (if configured) will be executed.

If the configured default rule is overly permissive (e.g. allowing anonymous access), and the upstream service interprets %2f as a path separator, the request may ultimately be processed as /admin/secret.

This results in the request being authorized based on a different path than the one processed by the upstream service, leading to authorization bypass.

Workarounds

• Developers should not use the --insecure or the --insecure-skip-secure-default-rule-enforcement flags and configure their default rule to implement "deny by default".
• Reject HTTP paths containing encoded slashes in the layers in front of heimdall. Some proxies, like e.g., Traefik, do that by default.
• Include the ID of the rule expected to be executed in the JWT issued by heimdall and verify that value in the project's service.

Bypass of access control policies enforced by heimdall may lead to the following consequences:

• Access to or modification of data that should be restricted
• Invocation of functionality that is expected to require authentication or authorization
• In certain configurations, escalation of privileges depending on the exposed functionality

Kodem intelligence

1. What is CVE-2026-42272? CVE-2026-42272 is a high-severity security vulnerability in github.com/dadrus/heimdall (go), affecting versions < 0.17.14. It is fixed in 0.17.14.
2. Which versions of github.com/dadrus/heimdall are affected by CVE-2026-42272? github.com/dadrus/heimdall (go) versions < 0.17.14 is affected.
3. Is there a fix for CVE-2026-42272? Yes. CVE-2026-42272 is fixed in 0.17.14. Upgrade to this version or later.
4. Is CVE-2026-42272 exploitable, and should I be worried? Whether CVE-2026-42272 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
5. What actually determines whether CVE-2026-42272 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
6. How do I fix CVE-2026-42272? Upgrade github.com/dadrus/heimdall to 0.17.14 or later.