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GHSA-72h4-mxfc-jx37

HighCVSS 7.8 / 10
Published Apr 25, 2026·Last modified May 12, 2026
Affected Components(0)

No affected components available

Description

Summary

Heimdall performs host matching in a case-sensitive manner, while HTTP hostnames are case-insensitive. This discrepancy can result in heimdall failing to match a rule for a request host that differs only in letter casing, potentially causing the request to be classified differently than intended.

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

This vulnerability can potentially be exploited by an adversary if rule matching relies on the request host.

For example, consider the following rule:

id: rule-1
match:
  hosts:
    - type: exact
      value: admin.example.com
execute: # configured to require authentication and authorization
  # ...

If an adversary now sends a request with the Host header set to Admin.Example.Com, rule-1 will not be matched, and the following will happen instead:

  • If no default rule is configured, the request will result in an error (404 Not Found)
  • If a default rule is configured, it will be executed. If the default rule is configured in an overly permissive way (e.g. allowing anonymous access), this results in a policy 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

  • Normalize request hosts to lowercase in the layers in front of heimdall.
  • Do not configure a permissive default rule. Respectively, do not make use of the --insecure or the --insecure-skip-secure-default-rule-enforcement flags.
  • When using regex type for host matching, expressions shall be defined in a case-insensitive manner (e.g. (?i)^admin\.example\.com$)
  • Include the ID of the rule expected to be executed in the JWT issued by heimdall and check that value in the consuming project's service.
Risk Scores
Base Score
7.8

The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. No user interaction is needed for the attacker to exploit this vulnerability.

Threat Intelligence
4.7

Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.

EPSS
0.30%

The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.

Exploit
Not available

We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.

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High severity
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