CVE-2026-81876: HAPI FHIR: SHCParser DEFLATE infinite loop causes denial of service

Summary A malformed Smart Health Card (SHC) JWT with zip: "DEF" and an empty or truncated DEFLATE payload causes SHCParser.inflate() to loop forever. This allows an attacker who can submit SHC content for validation to pin a JVM worker thread indefinitely, causing denial of service.

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

decodeJWT() inflates the JWT payload when the header contains "zip":"DEF":

java // SHCParser.java:300-302 if ("DEF".equals(res.header.asString("zip"))) { payloadJson = inflate(payloadJson); }

inflate() loops only on !inflater.finished() and does not check inflater.needsInput(), inflater.needsDictionary(), or zero-progress output:

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

For empty or truncated raw DEFLATE input, Inflater.inflate() returns 0, finished() remains false, and needsInput() becomes true, producing an infinite tight loop. The same unsafe loop pattern also exists in decompress() at SHCParser.java:410-423.

The validator reaches this path during SHC validation and during file-format detection for SHC-looking input (ResourceChecker.java:115-118).

PoC Compile the project, then run a minimal local harness that calls:

java SHCParser.inflate(new byte[0]);

This never returns. Local verification:

bash timeout 3s java -cp ... VerifyDoSFindings shcHang echo $? 124 = timeout killed the hung process

A JWT PoC uses:

- header: Base64URL({"zip":"DEF"}) - payload: empty or truncated raw DEFLATE bytes - signature: arbitrary

Submit the resulting token as SHC content, for example via a .shc file or content beginning with shc:/ that reaches the validator's SHC detection path.

Impact This is a denial-of-service vulnerability. A single malformed SHC validation request can consume a worker thread indefinitely. In services that validate uploaded SHC content, a small number of concurrent malformed requests can exhaust all validation workers.

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