Heimdall: Case-sensitive handling of URL-encoded slashes may lead to inconsistent path interpretation

Summary

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.

Impact

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

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.