netty-incubator-codec-ohttp: Binary HTTP parser unchecked varint length overflow causes decoder crash

Summary

io.netty.incubator:netty-incubator-codec-bhttp uses attacker-controlled Binary HTTP variable-length integers as long values but accumulates them into int offsets. Large valid varint lengths wrap the internal offset negative, leading to unchecked ArrayIndexOutOfBoundsException / IndexOutOfBoundsException from a tiny malformed BHTTP payload. A remote peer can trigger connection-level denial of service in applications that expose BinaryHttpParser / BinaryHttpDecoder to untrusted input.

Details

In codec-bhttp/src/main/java/io/netty/incubator/codec/bhttp/BinaryHttpParser.java, several parser paths store cumulative byte offsets in int sumBytes and then add attacker-controlled long lengths using compound assignment. In Java, int += long narrows the result back to int, so a length such as 2^31 wraps sumBytes negative.

Primary request-control-data path:

• readRequestHead(...) declares int sumBytes = 0 at BinaryHttpParser.java:386.
• It reads methodLength as a long at BinaryHttpParser.java:394.
• It performs sumBytes += methodLength at BinaryHttpParser.java:395, narrowing the result to int.
• If methodLength is 2^31, sumBytes wraps negative and bypasses if (sumBytes >= in.readableBytes()) return null at BinaryHttpParser.java:396-398.
• The parser then computes schemeLengthIdx = in.readerIndex() + sumBytes and calls in.getByte(schemeLengthIdx) at BinaryHttpParser.java:401-402, producing a negative index exception.

The same pattern is present in header parsing:

• readFieldLine(...) uses int sumBytes and adds long nameLength / long valueLength at BinaryHttpParser.java:659-680.
• valueLengthIdx = nameIdx + (int) nameLength at BinaryHttpParser.java:674 can also overflow.

getIndeterminateLength(...) similarly uses int sumBytes and long possibleTerminator at BinaryHttpParser.java:544-553.

Proof of concept

Safe local verification performed in this repository. After compiling codec-bhttp, the following minimal verifier uses a 15-byte payload:

import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.incubator.codec.bhttp.BinaryHttpParser;

public final class VerifyBhttpOverflow {
  public static void main(String[] args) {
    byte[] payload = new byte[] {
      0x00, (byte)0xc0, 0x00, 0x00, 0x00, (byte)0x80, 0x00, 0x00, 0x00,
      0x47, 0x45, 0x54, 0x58, 0x58, 0x58
    };
    ByteBuf input = Unpooled.wrappedBuffer(payload);
    try {
      new BinaryHttpParser(8192).parse(input, false);
      System.out.println("returned");
    } catch (Throwable t) {
      System.out.println(t.getClass().getName());
      System.out.println(t.getMessage());
    }
  }
}

Payload interpretation:

• 00: known-length request frame indicator.
• c000000080000000: valid 8-byte varint encoding of 0x80000000 (2^31) as the method length.
• 474554585858: a few dummy bytes so the parser proceeds far enough to compute the next index.

Observed result:

java.lang.ArrayIndexOutOfBoundsException
Index -2147483639 out of bounds for length 15

The parser should reject the malformed/incomplete message with a controlled decoder exception or return null awaiting more bytes; it should not allow integer wraparound to reach unchecked buffer indexing.

Impact

A remote peer can trigger an unchecked exception in the Binary HTTP decoder using a tiny payload. In typical Netty pipelines this closes or fails the affected channel. Depending on application-level exception handling, repeated payloads can cause sustained denial of service for exposed BHTTP endpoints. No memory corruption or information disclosure was observed because the failure occurs in Java/Netty bounds checks.

Suggested remediation

• Use long for all cumulative byte counts derived from protocol lengths.
• Before converting any protocol length to int, verify it is non-negative, no larger than Integer.MAX_VALUE, and no larger than available readable bytes and configured limits.
• Replace sumBytes >= in.readableBytes() checks with precise checked arithmetic that permits exact-boundary complete fields but rejects impossible lengths.
• Throw a controlled CorruptedFrameException / TooLongFrameException for invalid or unsupported lengths.
• Add regression tests for 8-byte varint lengths at and above Integer.MAX_VALUE in request control data, response control data, known and indeterminate field sections, and field lines.

References

• codec-bhttp/src/main/java/io/netty/incubator/codec/bhttp/BinaryHttpParser.java:386-402
• codec-bhttp/src/main/java/io/netty/incubator/codec/bhttp/BinaryHttpParser.java:659-680
• codec-bhttp/src/main/java/io/netty/incubator/codec/bhttp/BinaryHttpParser.java:544-553
• RFC 9292: Binary Representation of HTTP Messages
• RFC 9000 variable-length integer encoding