#37246 [BC-Low] lodestar snappy checksum issue

Submitted on Nov 29th 2024 at 22:23:56 UTC by @gln for Attackathon | Ethereum Protocol

• Report ID: #37246

• Report Type: Blockchain/DLT

• Report severity: Low

• Target: https://github.com/chainsafe/lodestar

• Impacts:

  • Unintended permanent chain split affecting greater than or equal to 25% of the network, requiring hard fork (network partition requiring hard fork)

Description

Brief/Intro

Lodestar does not verify checksum in snappy framing uncompressed chunks.

Vulnerability Details

In Req/Resp protocol the messages are encoded by using ssz_snappy encoding, which is a snappy framing compression over ssz encoded message.

In snappy framing format there are uncompressed chunks, each such chunk is prefixed with a checksum.

Let's see how golang implementation parses such chunks - https://github.com/golang/snappy/blob/master/decode.go#L176

	case chunkTypeUncompressedData:
			// Section 4.3. Uncompressed data (chunk type 0x01).
			if chunkLen < checksumSize {
				r.err = ErrCorrupt
				return r.err
			}
			buf := r.buf[:checksumSize]
			if !r.readFull(buf, false) {
				return r.err
			}
			checksum := uint32(buf[0]) | uint32(buf[1])<<8 | uint32(buf[2])<<16 | uint32(buf[3])<<24
			// Read directly into r.decoded instead of via r.buf.
			n := chunkLen - checksumSize
			if n > len(r.decoded) {
				r.err = ErrCorrupt
				return r.err
			}
			if !r.readFull(r.decoded[:n], false) {
				return r.err
			}
			if crc(r.decoded[:n]) != checksum {
				r.err = ErrCorrupt
				return r.err
			}
			r.i, r.j = 0, n
			continue

As you can see, if checksum is incorrect, decoder fails and returns error.

Now let's look at lodestar decoder https://github.com/ChainSafe/lodestar/blob/unstable/packages/reqresp/src/encodingStrategies/sszSnappy/snappyFrames/uncompress.ts#L17

uncompress(chunk: Uint8ArrayList): Uint8ArrayList | null {
    this.buffer.append(chunk);
    const result = new Uint8ArrayList();
    while (this.buffer.length > 0) {
      if (this.buffer.length < 4) break;

      const type = getChunkType(this.buffer.get(0));
      const frameSize = getFrameSize(this.buffer, 1);

      if (this.buffer.length - 4 < frameSize) {
        break;
      }

      const data = this.buffer.subarray(4, 4 + frameSize);
      this.buffer.consume(4 + frameSize);

      if (!this.state.foundIdentifier && type !== ChunkType.IDENTIFIER) {
        throw "malformed input: must begin with an identifier";
      }

      if (type === ChunkType.IDENTIFIER) {
        if (!Buffer.prototype.equals.call(data, IDENTIFIER)) {
          throw "malformed input: bad identifier";
        }
        this.state.foundIdentifier = true;
        continue;
      }

      if (type === ChunkType.COMPRESSED) {
        result.append(uncompress(data.subarray(4)));
      }
      if (type === ChunkType.UNCOMPRESSED) {
1)        result.append(data.subarray(4));
      }
    }
    if (result.length === 0) {
      return null;
    }
    return result;
  }

1. As you can see, checksum is not verified, bytes are appended to 'result'

Impact Details

Faulty nodes may trigger chain stall by sending p2p messages with incorrect checksum. Such messages will be parsed by lodestar clients by will be rejected by another implementations.

Link to Proof of Concept

https://gist.github.com/gln7/aab55674431b1c8d42a59ccf9d7cbf60

Proof of Concept

Proof of Concept

How to reproduce:

1. get poc via gist link and run it:

$ node dec1.mjs 
checking chunk type=255
checking chunk type=1
got uncompressed chunk..
Decompressed ok 124 bytes