CVE-2026-42272: Heimdall: Case-sensitive handling of URL-encoded slashes may lead to inconsistent path interpretation

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).