CVE-2026-81875: HAPI FHIR: SHCParser unbounded DEFLATE decompression causes denial of service

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 MAXALLOWEDSHCLENGTH, but this only logs an error and parsing continues:

java // SHCParser.java:282-284 if (jwt.length() > MAXALLOWEDSHCLENGTH) { 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 set the JWT header to {"zip":"DEF"}.

Local verification measured the following expansion through SHCParser.inflate():

text plain=1000066 compressed=1052 inflated=1000066 ratio=950 plain=16000066 compressed=15626 inflated=16000066 ratio=1023

A small compressed payload can therefore allocate many megabytes of heap. Larger payloads can trigger OutOfMemoryError or process instability.

Impact This 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.

Credits - Thai Son Dinh from VinSOC Labs (R&D)