GHSA-m9c9-mc2h-9wjw: Lodestar snappy checksum issue

Impact

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

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;
  }

As you can see, checksum is not verified, bytes are appended to ‘result’

Proof of Concept

How to reproduce:

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

References

• GHSA-m9c9-mc2h-9wjw
• ChainSafe/lodestar@18a0d68