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The vulnerability arises from a logical inconsistency in how Heimdall processes HTTP requests. According to the HTTP standard, hostnames are case-insensitive; however, Heimdall’s internal rule engine performs host matching in a case-sensitive manner. This means when a request arrives with the `Host` header set to Admin.Example.Com, it will not match a rule defined for admin.example.com.
If an “allow all” default rule is configured, the request falls back to it, potentially bypassing intended authentication and authorization controls. The issue is only exploitable if the default rule is overly permissive. By default, versions after v0.16.0 include a secure configuration that blocks such setups unless explicitly overridden with `–insecure` flags.
The exact impact includes rule bypass, leading to unrestricted access to resources and functions that should be protected. Mitigations include normalizing hostnames to lowercase upstream, avoiding permissive default rules, or using regex patterns with case‑insensitive flags.
dailycve form:
Platform: Heimdall
Version: 0.7.0-alpha‑0.17.10
Vulnerability: Rule Bypass
Severity: Medium
Date: Not specified
Prediction: Fixed in v0.17.11
What Undercode Say:
Normalize Host header before passing to Heimdall !/bin/bash HOST="$(echo "$HOST" | tr '[:upper:]' '[:lower:]')" export HOST
Example of a case‑insensitive regex rule match: hosts: - type: regex value: (?i)^admin.example.com$
Exploit:
- Send a request with the `Host` header in mixed case (e.g.,
Admin.Example.Com) to a Heimdall instance configured with a permissive default rule. - The rule for `admin.example.com` is not triggered, and the request is handled by the default rule, resulting in unauthorized access.
Protection from this CVE
- Normalize `Host` headers to lowercase in reverse proxies or ingress controllers upstream of Heimdall.
- Avoid using `–insecure` or `–insecure-skip-secure-default-rule-enforcement` flags.
- Use regex‑based host matching with the `(?i)` flag for case‑insensitivity.
- Include the expected rule ID in JWT tokens issued by Heimdall and validate it in the consuming service.
Impact:
- Bypass of access control policies
- Unauthorized access to or modification of protected data
- Invocation of functionality that should require authentication
- Privilege escalation in certain configurations
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Sources:
Reported By: github.com
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