{"id":"CVE-2026-34359","aliases":["GHSA-fgv2-4q4g-wc35"],"url":"https://o3.security/vulnerability/CVE-2026-34359","summary":"HAPI FHIR: Authentication Credential Leakage via Improper URL Prefix Matching on HTTP Redirect in HAPI FHIR Core","details":"## Summary\n\n`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.\n\n## Details\n\nThe root cause is in `ManagedWebAccessUtils.getServer()` at `org.hl7.fhir.utilities/src/main/java/org/hl7/fhir/utilities/http/ManagedWebAccessUtils.java:26`:\n\n```java\npublic static ServerDetailsPOJO getServer(String url, Iterable<ServerDetailsPOJO> serverAuthDetails) {\n    if (serverAuthDetails != null) {\n      for (ServerDetailsPOJO serverDetails : serverAuthDetails) {\n          if (url.startsWith(serverDetails.getUrl())) {  // <-- no host boundary check\n            return serverDetails;\n          }\n      }\n    }\n    return null;\n}\n```\n\nThe configured production terminology server URL is defined without a trailing slash in `FhirSettingsPOJO.java:19`:\n\n```java\nprotected static final String TX_SERVER_PROD = \"http://tx.fhir.org\";\n```\n\nThis means:\n- `\"http://tx.fhir.org.attacker.com/capture\".startsWith(\"http://tx.fhir.org\")` → **true**\n- `\"http://tx.fhir.org:8080/evil\".startsWith(\"http://tx.fhir.org\")` → **true**\n\n**Exploit chain via SimpleHTTPClient (redirect path):**\n\n1. `SimpleHTTPClient.get()` (`SimpleHTTPClient.java:68-105`) makes a request to `http://tx.fhir.org/ValueSet/$expand`\n2. On each redirect, the loop calls `getHttpGetConnection(url, accept)` (line 84) → `setHeaders(connection)` (line 117)\n3. `setHeaders()` (line 122-133) calls `authProvider.canProvideHeaders(url)` and `authProvider.getHeaders(url)` on the **redirect target URL**\n4. `ServerDetailsPOJOHTTPAuthProvider.getServerDetails()` (line 83-84) delegates to `ManagedWebAccessUtils.getServer(url.toString(), servers)`\n5. The `startsWith()` check matches `http://tx.fhir.org.attacker.com` against `http://tx.fhir.org`\n6. Credentials are dispatched to the attacker's server via `ServerDetailsPOJOHTTPAuthProvider.getHeaders()` (lines 38-58):\n   - Bearer tokens: `Authorization: Bearer {token}`\n   - Basic auth: `Authorization: Basic {base64(user:pass)}`\n   - API keys: `Api-Key: {apikey}`\n   - Custom headers from server config\n\nNote: 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.\n\n**Exploit chain via ManagedFhirWebAccessor (OkHttp path):**\n\n`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.\n\nThe 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.\n\n## PoC\n\n**Step 1: Verify the prefix match behavior**\n\n```java\n// This demonstrates the core vulnerability\nString configuredUrl = \"http://tx.fhir.org\";  // FhirSettingsPOJO.TX_SERVER_PROD\nString attackerUrl = \"http://tx.fhir.org.attacker.com/capture\";\n\nSystem.out.println(attackerUrl.startsWith(configuredUrl));\n// Output: true\n```\n\n**Step 2: Demonstrate credential dispatch to wrong host**\n\nGiven a `fhir-settings.json` configuration at `~/.fhir/fhir-settings.json`:\n```json\n{\n  \"servers\": [\n    {\n      \"url\": \"http://tx.fhir.org\",\n      \"authenticationType\": \"token\",\n      \"token\": \"secret-bearer-token-12345\"\n    }\n  ]\n}\n```\n\nWhen `SimpleHTTPClient.get(\"http://tx.fhir.org/ValueSet/$expand\")` follows a 302 redirect to `http://tx.fhir.org.attacker.com/capture`:\n\n1. `setHeaders()` is called with the redirect target URL\n2. `authProvider.canProvideHeaders(new URL(\"http://tx.fhir.org.attacker.com/capture\"))` returns `true`\n3. `authProvider.getHeaders(...)` returns `{\"Authorization\": \"Bearer secret-bearer-token-12345\"}`\n4. The `Authorization` header with the secret token is sent to `tx.fhir.org.attacker.com`\n\n**Step 3: Attacker captures the credential**\n\n```bash\n# On attacker-controlled server (tx.fhir.org.attacker.com)\nnc -l -p 80 | head -20\n# Output includes:\n# GET /capture HTTP/1.1\n# Host: tx.fhir.org.attacker.com\n# Authorization: Bearer secret-bearer-token-12345\n```\n\n## Impact\n\n- **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).\n- **Impersonation**: Stolen credentials allow an attacker to make authenticated requests to the legitimate FHIR server, potentially accessing or modifying clinical terminology data.\n- **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.\n- **TLS downgrade**: The same `startsWith()` pattern in `ManagedWebAccess.isLocal()` could allow an attacker-controlled domain to be treated as \"local,\" bypassing the HTTPS enforcement.\n\n## Recommended Fix\n\nReplace the `startsWith()` check in `ManagedWebAccessUtils.getServer()` with proper URL host boundary validation:\n\n```java\npublic static ServerDetailsPOJO getServer(String url, Iterable<ServerDetailsPOJO> serverAuthDetails) {\n    if (serverAuthDetails != null) {\n      for (ServerDetailsPOJO serverDetails : serverAuthDetails) {\n          if (urlMatchesServer(url, serverDetails.getUrl())) {\n            return serverDetails;\n          }\n      }\n    }\n    return null;\n}\n\n/**\n * Check if a URL matches a configured server URL with proper host boundary validation.\n * After the configured prefix, the next character must be '/', '?', '#', ':', or end-of-string.\n */\nprivate static boolean urlMatchesServer(String url, String serverUrl) {\n    if (url == null || serverUrl == null) return false;\n    if (!url.startsWith(serverUrl)) return false;\n    if (url.length() == serverUrl.length()) return true;\n    char nextChar = url.charAt(serverUrl.length());\n    return nextChar == '/' || nextChar == '?' || nextChar == '#' || nextChar == ':';\n}\n```\n\nApply the same fix to `ManagedWebAccess.isLocal()` at line 214 and the three-argument `getServer()` overload at line 14.\n\nAdditionally, 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.","published":"2026-03-31T16:56:01.235Z","modified":"2026-10-28T02:30:49.237474307Z","cvss":{"score":7.4,"severity":"HIGH","vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N"},"epss":{"score":0.00158,"percentile":0.05397,"asOf":"2026-09-17"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"Maven","name":"ca.uhn.hapi.fhir:org.hl7.fhir.core","fixedVersion":"6.9.4"},{"ecosystem":"Maven","name":"ca.uhn.hapi.fhir:org.hl7.fhir.utilities","fixedVersion":"6.9.4"}],"fix":null,"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/34xxx/CVE-2026-34359.json"},{"type":"ADVISORY","url":"https://github.com/hapifhir/org.hl7.fhir.core/security/advisories/GHSA-fgv2-4q4g-wc35"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-34359"},{"type":"PACKAGE","url":"https://github.com/hapifhir/org.hl7.fhir.core"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-28T02:30:49.237474307Z"}}