CVE-2026-55470 is a high-severity inefficient regular expression (ReDoS) vulnerability in ca.uhn.hapi.fhir:org.hl7.fhir.dstu2 (maven), affecting versions <= 6.9.9. It is fixed in 6.9.10.
Summary The fix for CVE-2026-45367 added RegexTimeout protection to the matches() function in DSTU2016MAY, DSTU3, R4, R4B, and R5, but the DSTU2 module was incompletely patched. In org.hl7.fhir.dstu2, replaceMatches() was updated while matches() at line 2462 still calls the raw String.matches(sw) without any timeout, allowing an unauthenticated attacker to trigger catastrophic regex backtracking and exhaust server CPU. Details Incomplete patch Within the same file (org.hl7.fhir.dstu2/utils/FHIRPathEngine.java), the two functions were patched inconsistently: Line 2226, replaceMatches(), PATCHED: Line 2462, matches(), NOT PATCHED: DSTU3 line 2447, matches(), PATCHED (for comparison): Module-by-module status | Module | matches() | replaceMatches() | |---|---|---| | DSTU2 | ❌ raw str.matches(sw) | ✅ RegexTimeout.replaceAll() | | DSTU2016MAY | ✅ RegexTimeout.matches() | ✅ | | DSTU3 | ✅ RegexTimeout.matches() | ✅ | | R4 | ✅ RegexTimeout.matches() | ✅ | | R4B | ✅ RegexTimeout.matches() | ✅ | | R5 | ✅ RegexTimeout.matches() | ✅ | PoC Requirements: Java 17+, Maven 3.8+ pom.xml dependencies: Test code (reproduces the exact behaviour of DSTU2 line 2462): Verified output (JDK 25.0.3, Linux): The patched approach cuts off the evaluation at 508 ms. The unpatched DSTU2 code runs for 1410 ms on this input with no mechanism to stop it. Longer inputs or more complex patterns produce proportionally worse results. Impact Vulnerability type: Regular Expression Denial of Service (ReDoS) causing CPU exhaustion and service disruption. Who is impacted: Any application using the ca.uhn.hapi.fhir:org.hl7.fhir.dstu2 module that evaluates user-supplied FHIRPath expressions, including the FHIR Validator HTTP endpoint, FHIR servers applying FHIRPath invariants from user-provided resources or profiles, and any system embedding FHIRPathEngine from the DSTU2 module. No authentication is required; an attacker needs only to submit a FHIR resource or FHIRPath expression whose matches() argument contains a catastrophically backtracking regular expression.
A regular expression with worst-case exponential or polynomial matching time is applied to untrusted input, causing excessive CPU use. Typical impact: denial of service when input is crafted to trigger backtracking.
CVE-2026-55470 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 (6.9.10). Upgrading removes the vulnerable code path.
maven
ca.uhn.hapi.fhir:org.hl7.fhir.dstu2 (<= 6.9.9)ca.uhn.hapi.fhir:org.hl7.fhir.convertors (<= 6.9.9)ca.uhn.hapi.fhir:org.hl7.fhir.validation (<= 6.9.9)ca.uhn.hapi.fhir:org.hl7.fhir.validation.cli (<= 6.9.9)ca.uhn.hapi.fhir:org.hl7.fhir.dstu2 → 6.9.10 (maven)ca.uhn.hapi.fhir:org.hl7.fhir.convertors → 6.9.10 (maven)ca.uhn.hapi.fhir:org.hl7.fhir.validation → 6.9.10 (maven)ca.uhn.hapi.fhir:org.hl7.fhir.validation.cli → 6.9.10 (maven)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.
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Upgrade the following packages to resolve this vulnerability:
ca.uhn.hapi.fhir:org.hl7.fhir.dstu2 to 6.9.10 or laterca.uhn.hapi.fhir:org.hl7.fhir.convertors to 6.9.10 or laterca.uhn.hapi.fhir:org.hl7.fhir.validation to 6.9.10 or laterca.uhn.hapi.fhir:org.hl7.fhir.validation.cli to 6.9.10 or laterKodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
CVE-2026-55470 is a high-severity inefficient regular expression (ReDoS) vulnerability in ca.uhn.hapi.fhir:org.hl7.fhir.dstu2 (maven), affecting versions <= 6.9.9. It is fixed in 6.9.10. A regular expression with worst-case exponential or polynomial matching time is applied to untrusted input, causing excessive CPU use.
CVE-2026-55470 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.
ca.uhn.hapi.fhir:org.hl7.fhir.dstu2 (maven) (versions <= 6.9.9)ca.uhn.hapi.fhir:org.hl7.fhir.convertors (maven) (versions <= 6.9.9)ca.uhn.hapi.fhir:org.hl7.fhir.validation (maven) (versions <= 6.9.9)ca.uhn.hapi.fhir:org.hl7.fhir.validation.cli (maven) (versions <= 6.9.9)Yes. CVE-2026-55470 is fixed in 6.9.10. Upgrade to this version or later.
Whether CVE-2026-55470 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
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.
ca.uhn.hapi.fhir:org.hl7.fhir.dstu2 to 6.9.10 or laterca.uhn.hapi.fhir:org.hl7.fhir.convertors to 6.9.10 or laterca.uhn.hapi.fhir:org.hl7.fhir.validation to 6.9.10 or laterca.uhn.hapi.fhir:org.hl7.fhir.validation.cli to 6.9.10 or later