HAPI FHIR is a complete implementation of the HL7 FHIR standard for healthcare interoperability in Java. Prior to version 6.9.12, SHCParser in org.hl7.fhir.r5/src/main/java/org/hl7/fhir/r5/elementmodel/SHCParser.java can consume attacker-controlled Smart Health Card JWT content whose header contains zip: "DEF" and whose small raw-DEFLATE payload expands to a very large value. SHCParser.decodeJWT() passes the decoded payload to SHCParser.inflate(), which accumulates all decompressed bytes in a ByteArrayOutputStream without an output-size limit before JSON parsing, and SHCParser.decompress() contains the same unbounded pattern. An application or validator service that accepts attacker-supplied SHC content can therefore suffer excessive heap allocation, severe garbage-collection pressure, request failure, process instability, or process termination. This issue is fixed in version 6.9.12.

Affected: Maven: ca.uhn.hapi.fhir:org.hl7.fhir.r5, Maven: ca.uhn.hapi.fhir:org.hl7.fhir.validation, Maven: ca.uhn.hapi.fhir:org.hl7.fhir.validation.cli

### Summary `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.

### Details The vulnerable code is in `org.hl7.fhir.r5/src/main/java/org/hl7/fhir/r5/elementmodel/SHCParser.java`.

`decodeJWT()` checks `MAX_ALLOWED_SHC_LENGTH`, but this only logs an error and parsing continues:

```java // SHCParser.java:282-284 if (jwt.length() > MAX_ALLOWED_SHC_LENGTH) { logError(...); } ```

If the header contains `"zip":"DEF"`, the payload is inflated before JSON parsing:

```java // SHCParser.java:300-304 if ("DEF".equals(res.header.asString("zip"))) { payloadJson = inflate(payloadJson); } res.payload = JsonParser.parseObject(FileUtilities.bytesToString(payloadJson), true); ```

`inflate()` accumulates all decompressed output in a `ByteArrayOutputStream` and has no maximum output size:

```java // SHCParser.java:455-468 while (!inflater.finished()) { final int count = inflater.inflate(buffer); outputStream.write(buffer, 0, count); } return outputStream.toByteArray(); ```

The same unbounded decompression pattern exists in `decompress()` at `SHCParser.java:410-423`.

### PoC Create a highly compressible SHC-shaped JSON payload, compress it with raw DEFLATE (`new Deflater(9, true)`), Base64URL-encode it as the JWT payload, and…