CVE-2026-42272 — heimdall
Fix: dadrus/heimdall@8b0de6aCVE-2026-42272 is a CWE-178 vulnerability in github.com/dadrus/heimdall. A fix is available for github.com/dadrus/heimdall — see the affected versions and patch details below.
Heimdall: Case-sensitive handling of URL-encoded slashes may lead to inconsistent path interpretation
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- A successful exploit gives an attacker total control of the affected component, not partial access.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-42272.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Real-World Exposure
github.com/dadrus/heimdallReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.
Description
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
--insecureor the--insecure-skip-secure-default-rule-enforcementflags 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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/dadrus/heimdall | all versions | 0.17.14go get github.com/dadrus/heimdall@v0.17.14 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/dadrus/heimdall, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/dadrus/heimdall to 0.17.14 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-42272 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-42272 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-42272. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-42272 in your dependencies?
O3 Security finds CVE-2026-42272 across Go dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.