Heimdall has an authorization bypass via path normalization mismatch
Summary
Heimdall performs rule matching on the raw (non-normalized) request path, while downstream components may normalize dot-segments according to [RFC 3986, Section 6.2.2.3](https://www.rfc-editor.org/rfc/rfc3986#section-6.2.2.3). This discrepancy can result in heimdall authorizing a request for one path (e.g., /user/../admin, or URL-encoded variants such as /user/%2e%2e/admin or /user/%2e%2e%2fadmin. The latter would require the allow_encoded_slashes option to be set to on or no_decode.) while the downstream ultimately processes a different, normalized path (/admin).
Details
This vulnerability can be exploited by an adversary if rule matching is performed using free (named or unnamed) wildcards without further constraints, as shown in the example snippets below.
```yaml
id: rule-1
match:
routes:
execute: # configured to require authentication and authorization
# ...
```
```yaml
id: rule-2
match:
routes:
execute: # configured to allow anonymous access
# ...
```
If an adversary sends a request to /public/../user/whatever, rule-2 will be matched and executed. The downstream service may, however, normalize the request path and interpret it as /user/whatever.
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 HTTP paths or reject HTTP paths containing relative path expressions in the layers in front of Heimdall - this is good practice anyway. Some proxies do that by default, such as Traefik; others, such as Envoy, require additional configuration (for Envoy see [
normalize_path](https://www.envoyproxy.io/docs/envoy/latest/api-v3/extensions/filters/network/http_connection_manager/v3/http_connection_manager.proto#envoy-v3-api-field-extensions-filters-network-http-connection-manager-v3-httpconnectionmanager-normalize-path)). - 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.
