Impact

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

Description

Lodestar client may fail to decode snappy framing compressed messages.

Vulnerability Details

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

It's mentioned here - https://github.com/ethereum/consensus-specs/blob/dev/specs/phase0/p2p-interface.md

The token of the negotiated protocol ID specifies the type of encoding to be used for the req/resp interaction. Only one value is possible at this time:

ssz_snappy: The contents are first SSZ-encoded and then compressed with Snappy frames compression. For objects containing a single field, only the field is SSZ-encoded not a container with a single field. For example, the BeaconBlocksByRoot request is an SSZ-encoded list of Root's. This encoding type MUST be supported by all clients.

In snappy framing format there a few types of chunks.
We are interested in so called reserved skippable chunks. These are chunks with chunk type in range [0x80, 0xfd]
Let's see how rust snappy handles them https://github.com/BurntSushi/rust-snappy/blob/master/src/read.rs#L137

impl<R: io::Read> io::Read for FrameDecoder<R> {
    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
 		   ... 
           ...
  		    let len = len64 as usize;
            match ty {
                Err(b) if 0x02 <= b && b <= 0x7F => {
                    // Spec says that chunk types 0x02-0x7F are reserved and
                    // conformant decoders must return an error.
                    fail!(Error::UnsupportedChunkType { byte: b });
                }
                Err(b) if 0x80 <= b && b <= 0xFD => {
                    // Spec says that chunk types 0x80-0xFD are reserved but
                    // skippable.
                    self.r.read_exact(&mut self.src[0..len])?;
                }

Similar code can be found in golang implementation - https://github.com/golang/snappy/blob/master/decode.go#L221

func (r *Reader) fill() error {
	...
	if chunkType <= 0x7f {
			// Section 4.5. Reserved unskippable chunks (chunk types 0x02-0x7f).
			r.err = ErrUnsupported
			return r.err
		}
		// Section 4.4 Padding (chunk type 0xfe).
		// Section 4.6. Reserved skippable chunks (chunk types 0x80-0xfd).
		if !r.readFull(r.buf[:chunkLen], false) {
			return r.err
		}

Now let's see how lodestar handles such chunks 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) {
        result.append(data.subarray(4));
      }
    }
    if (result.length === 0) {
      return null;
    }
    return result;
  }

 function getChunkType(value: number): ChunkType {
  switch (value) {
    case ChunkType.IDENTIFIER:
      return ChunkType.IDENTIFIER;
    case ChunkType.COMPRESSED:
      return ChunkType.COMPRESSED;
    case ChunkType.UNCOMPRESSED:
      return ChunkType.UNCOMPRESSED;
    case ChunkType.PADDING:
      return ChunkType.PADDING;
    default:
      throw new Error("Unsupported snappy chunk type");
  }

As you can see, lodestar does not recognize such chunks.

If it sees such chunk, function getChunkType() throws an exception and decoding fails.

Impact Details

Faulty nodes may trigger chain stall by sending messages which lodestar fails to parse, while other clients will be able to handle.

Proof of Concept

How to reproduce:

1. get archive (via provided gist link), decode and unpack it:

$ base64 -d poc.txt > poc.tgz
$ tar zxf poc.tgz

2. run dec1.go to verify that our snappy file decompressed successfully

$ go run dec1.go

reading 1.snappy...
read 124 bytes, err <nil>

3. run dec1.mjs to verify that lodestar fails to decode such file

checking chunk type=255
checking chunk type=1
got uncompressed chunk..
checking chunk type=129
file:///../poc/dec1.mjs:74
            throw new Error("Unsupported snappy chunk type");

References

• GHSA-53rv-hcvm-rpp9
• ChainSafe/lodestar@18a0d68