GHSA-fgv2-4q4g-wc35: 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.DetailsThe root cause is in `ManagedWebAccessUtils.getServer()` at `org.hl7.fhir.utilities/src/main/java/org/hl7/fhir/utilities/http/ManagedWebAccessUtils.java:26`:The configured production terminology server URL is defined without a trailing slash in `FhirSettingsPOJO.java:19`: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):**`SimpleHTTPClient.get()` (`SimpleHTTPClient.java:68-105`) makes a request to `http://tx.fhir.org/ValueSet/$expand`On each redirect, the loop calls `getHttpGetConnection(url, accept)` (line 84) → `setHeaders(connection)` (line 117)`setHeaders()` (line 122-133) calls `authProvider.canProvideHeaders(url)` and `authProvider.getHeaders(url)` on the **redirect target URL**`ServerDetailsPOJOHTTPAuthProvider.getServerDetails()` (line 83-84) delegates to `ManagedWebAccessUtils.getServer(url.toString(), servers)`The `startsWith()` check matches `http://tx.fhir.org.attacker.com` against `http://tx.fhir.org`Credentials are dispatched to the attacker's server via `ServerDetailsPOJOHTTPAuthProvider.getHeaders()` (lines 38-58):Bearer tokens: `Authorization: Bearer {token}`Basic auth: `Authorization: Basic {base64(user:pass)}`API keys: `Api-Key: {apikey}`Custom headers from server configNote: An earlier fix (commit `6b615880` "Strip headers on redirect") added an `isNotSameHost()` check, but this was **removed** in commit `3871cc69` ("Rework authorization providers in ManagedWebAccess"). The current code on master has no host validation during redirect following.**Exploit chain via ManagedFhirWebAccessor (OkHttp path):**`ManagedFhirWebAccessor.httpCall()` (line 81-112) sets auth headers via `requestWithAuthorizationHeaders()` before passing the request to OkHttpClient. OkHttpClient follows redirects by default (up to 20) and carries the pre-set auth headers to all redirect targets. The same `startsWith()` check in `canProvideHeaders()` applies.The same vulnerable pattern also exists in `ManagedWebAccess.isLocal()` (line 214), where `url.startsWith(server.getUrl())` is used to determine whether HTTP (non-TLS) access is allowed, potentially enabling TLS downgrade for attacker-controlled domains that match the prefix.PoC**Step 1: Verify the prefix match behavior****Step 2: Demonstrate credential dispatch to wrong host**Given a `fhir-settings.json` configuration at `~/.fhir/fhir-settings.json`:When `SimpleHTTPClient.get("http://tx.fhir.org/ValueSet/$expand")` follows a 302 redirect to `http://tx.fhir.org.attacker.com/capture`:`setHeaders()` is called with the redirect target URL`authProvider.canProvideHeaders(new URL("http://tx.fhir.org.attacker.com/capture"))` returns `true``authProvider.getHeaders(...)` returns `{"Authorization": "Bearer secret-bearer-token-12345"}`The `Authorization` header with the secret token is sent to `tx.fhir.org.attacker.com`**Step 3: Attacker captures the credential**Impact**Credential theft**: Bearer tokens, Basic authentication passwords, API keys, and custom authentication headers configured for FHIR terminology servers can be exfiltrated by an attacker who can inject a redirect (via MITM, compromised CDN, or DNS poisoning).**Impersonation**: Stolen credentials allow an attacker to make authenticated requests to the legitimate FHIR server, potentially accessing or modifying clinical terminology data.**Broad exposure**: The FHIR Validator is widely used in healthcare IT for validating FHIR resources. Any deployment that configures server authentication in `fhir-settings.json` and makes outbound HTTP requests to terminology servers is affected.**TLS downgrade**: The same `startsWith()` pattern in `ManagedWebAccess.isLocal()` could allow an attacker-controlled domain to be treated as "local," bypassing the HTTPS enforcement.Recommended FixReplace the `startsWith()` check in `ManagedWebAccessUtils.getServer()` with proper URL host boundary validation:Apply the same fix to `ManagedWebAccess.isLocal()` at line 214 and the three-argument `getServer()` overload at line 14.Additionally, consider re-introducing the host-equality check for redirects in `SimpleHTTPClient` (as was previously implemented in commit `6b615880` but removed in `3871cc69`) to provide defense-in-depth against credential leakage on cross-origin redirects.
CVSS v3.1: 7.4