CVE-2026-81875: HAPI FHIR Denial of Service Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-3w98-rrpr-fprr",
  "modified" : "2026-09-17T20:32:16Z",
  "published" : "2026-09-17T20:32:16Z",
  "aliases" : [ "CVE-2026-81875" ],
  "summary" : "HAPI FHIR: SHCParser unbounded DEFLATE decompression causes denial of service",
  "details" : "### Summary\n`SHCParser` inflates compressed Smart Health Card JWT payloads into memory without a decompressed-size limit. An attacker who can submit SHC content for validation can craft a small compressed JWT payload that expands to a very large byte array, causing memory exhaustion or severe garbage collection pressure.\n\n### Details\nThe vulnerable code is in `org.hl7.fhir.r5/src/main/java/org/hl7/fhir/r5/elementmodel/SHCParser.java`.\n\n`decodeJWT()` checks `MAX_ALLOWED_SHC_LENGTH`, but this only logs an error and parsing continues:\n\n```java\n// SHCParser.java:282-284\nif (jwt.length() > MAX_ALLOWED_SHC_LENGTH) {\n  logError(...);\n}\n```\n\nIf the header contains `\"zip\":\"DEF\"`, the payload is inflated before JSON parsing:\n\n```java\n// SHCParser.java:300-304\nif (\"DEF\".equals(res.header.asString(\"zip\"))) {\n  payloadJson = inflate(payloadJson);\n}\nres.payload = JsonParser.parseObject(FileUtilities.bytesToString(payloadJson), true);\n```\n\n`inflate()` accumulates all decompressed output in a `ByteArrayOutputStream` and has no maximum output size:\n\n```java\n// SHCParser.java:455-468\nwhile (!inflater.finished()) {\n  final int count = inflater.inflate(buffer);\n  outputStream.write(buffer, 0, count);\n}\nreturn outputStream.toByteArray();\n```\n\nThe same unbounded decompression pattern exists in `decompress()` at `SHCParser.java:410-423`.\n\n### PoC\nCreate a highly compressible SHC-shaped JSON payload, compress it with raw DEFLATE (`new Deflater(9, true)`), Base64URL-encode it as the JWT payload, and set the JWT header to `{\"zip\":\"DEF\"}`.\n\nLocal verification measured the following expansion through `SHCParser.inflate()`:\n\n```text\nplain=1000066 compressed=1052 inflated=1000066 ratio=950\nplain=16000066 compressed=15626 inflated=16000066 ratio=1023\n```\n\nA small compressed payload can therefore allocate many megabytes of heap. Larger payloads can trigger `OutOfMemoryError` or process instability.\n\n### Impact\nThis is a denial-of-service vulnerability. Any validator service or application that accepts attacker-supplied SHC content can be forced to allocate excessive heap memory. Impact ranges from request failure and severe GC pressure to process termination.\n\n### Credits\n- Thai Son Dinh from VinSOC Labs (R&D)",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Maven",
      "name" : "ca.uhn.hapi.fhir:org.hl7.fhir.r5"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "6.9.12"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 6.9.11"
    }
  }, {
    "package" : {
      "ecosystem" : "Maven",
      "name" : "ca.uhn.hapi.fhir:org.hl7.fhir.validation"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "6.9.12"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 6.9.11"
    }
  }, {
    "package" : {
      "ecosystem" : "Maven",
      "name" : "ca.uhn.hapi.fhir:org.hl7.fhir.validation.cli"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "last_affected" : "5.0.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/hapifhir/org.hl7.fhir.core/security/advisories/GHSA-3w98-rrpr-fprr"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-81875"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/hapifhir/org.hl7.fhir.core/pull/2493"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/hapifhir/org.hl7.fhir.core/commit/fbb94216e0ad21ded75be77e5e20242ba194e83f"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/hapifhir/org.hl7.fhir.core"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/hapifhir/org.hl7.fhir.core/releases/tag/6.9.11"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/hapifhir/org.hl7.fhir.core/releases/tag/6.9.12"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-20", "CWE-400", "CWE-409" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-17T20:32:16Z",
    "nvd_published_at" : "2026-09-16T19:17:44Z"
  }
}