CVE-2026-55470

HAPI FHIR: Incomplete fix for CVE-2026-45367: DSTU2 FHIRPathEngine.matches() missing RegexTimeout protection allows ReDoS

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:

result.add(new StringType(
    RegexTimeout.replaceAll(
        convertToString(focus.get(0)), regex, repl, regexTimeoutMillis)));

Line 2462 — matches() — NOT PATCHED:

result.add(new BooleanType(
    convertToString(focus.get(0)).matches(sw)));
// ↑ raw String.matches() — no RegexTimeout, no complexity check

DSTU3 line 2447 — matches() — PATCHED (for comparison):

result.add(new BooleanType(
    RegexTimeout.matches(st, sw, regexTimeoutMillis)));

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:

ca.uhn.hapi.fhir
    org.hl7.fhir.utilities
    6.9.7

Test code (reproduces the exact behaviour of DSTU2 line 2462):

import org.hl7.fhir.utilities.regex.RegexTimeout;
import java.util.concurrent.*;
String regex = "((a|b){0,5}){20}";
String input = "a".repeat(25) + "c";    // no match → full backtracking
// ① Patched approach — RegexTimeout terminates at 500 ms
long t1 = System.currentTimeMillis();
try {
    RegexTimeout.matches(input, regex, 500);
} catch (TimeoutException e) {
    System.out.println("RegexTimeout blocked in " +
        (System.currentTimeMillis() - t1) + " ms");
}
// ② DSTU2 line 2462 — raw String.matches(), no timeout
long t2 = System.currentTimeMillis();
input.matches(regex);                   // equivalent to what FHIRPathEngine does
System.out.println("str.matches() ran for " +
    (System.currentTimeMillis() - t2) + " ms with no timeout");

Verified output (JDK 25.0.3, Linux):

RegexTimeout blocked in 508 ms      ← patched modules: attack stopped
str.matches() ran for 1410 ms       ← DSTU2: no timeout, CPU exhausted

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.