Summary
HAPI FHIR: SHCParser DEFLATE infinite loop causes denial of service
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":
// 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:
// 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:
SHCParser.inflate(new byte[0]);
This never returns. Local verification:
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.
Credits
- Thai Son Dinh from VinSOC Labs (R&D)
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.
The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths. Typical impact: varies by context: data corruption, logic bypass, or denial of service.
CVE-2026-81876 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (6.9.12); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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See it in your environmentNew to Kodem? Get a demo →Remediation advice
ca.uhn.hapi.fhir:org.hl7.fhir.r5 to 6.9.12 or later; ca.uhn.hapi.fhir:org.hl7.fhir.validation to 6.9.12 or later; ca.uhn.hapi.fhir:org.hl7.fhir.validation.cli to 6.9.12 or later
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
Frequently Asked Questions
- What is CVE-2026-81876? CVE-2026-81876 is a high-severity improper input validation vulnerability in ca.uhn.hapi.fhir:org.hl7.fhir.r5 (maven), affecting versions <= 6.9.11. It is fixed in 6.9.12. The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths.
- How severe is CVE-2026-81876? CVE-2026-81876 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which packages are affected by CVE-2026-81876?
ca.uhn.hapi.fhir:org.hl7.fhir.r5(maven) (versions <= 6.9.11)ca.uhn.hapi.fhir:org.hl7.fhir.validation(maven) (versions <= 6.9.11)ca.uhn.hapi.fhir:org.hl7.fhir.validation.cli(maven) (versions <= 6.9.11)
- Is there a fix for CVE-2026-81876? Yes. CVE-2026-81876 is fixed in 6.9.12. Upgrade to this version or later.
- Is CVE-2026-81876 exploitable, and should I be worried? Whether CVE-2026-81876 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-81876 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-81876?
- Upgrade
ca.uhn.hapi.fhir:org.hl7.fhir.r5to 6.9.12 or later - Upgrade
ca.uhn.hapi.fhir:org.hl7.fhir.validationto 6.9.12 or later - Upgrade
ca.uhn.hapi.fhir:org.hl7.fhir.validation.clito 6.9.12 or later
- Upgrade