以太坊UDP流量放大反射DDOS漏洞-安全客

漏洞影响

该漏洞表面上是一个放大5倍udp反射DDOS漏洞，但其对ETH的P2P网络的影响是非常大的，但是这个漏洞有很大的两个副作用，一个是ETH的发现节点池会不断的被堆满，导致正常节点无法加入，二是可屏蔽被攻击节点无法探索到任意子网的节点。

漏洞分析

先让我们来看看ETH P2P发现协议的文档，在https://github.com/ethereum/devp2p/blob/master/discv4.md这篇ETH P2P发现协议文档里是有对udp反射DDOS做防御的。

Pong Packet (0x02)
packet-data = [to, ping-hash, expiration]
Pong is the reply to ping.ping-hash should be equal to hash of the corresponding ping packet. Implementations should ignore unsolicited pong packets that do not contain the hash of the most recent ping packet.

其方法就是通过签名PIng包并且让对方主机回复的Pong包要带上之前Ping包的hash才可以通过校验。这个从设计上来说是没什么问题的，然而go-ethereum在实现该协议的时候出了问题。

让我们来看看go-ethereum是怎么实现该协议的，在https://github.com/ethereum/go-ethereum/blob/master/p2p/discover/udp.go#L618

func (req *ping) handle(t *udp, from *net.UDPAddr, fromKey encPubkey, mac []byte) error {
    if expired(req.Expiration) {
        return errExpired
    }
    key, err := decodePubkey(fromKey)
    if err != nil {
        return fmt.Errorf("invalid public key: %v", err)
    }
    //收到ping包后马上回复pong包
    t.send(from, pongPacket, &pong{
        To:         makeEndpoint(from, req.From.TCP),
        ReplyTok:   mac,
        Expiration: uint64(time.Now().Add(expiration).Unix()),
    })
    n := wrapNode(enode.NewV4(key, from.IP, int(req.From.TCP), from.Port))
    t.handleReply(n.ID(), pingPacket, req)
    //如果没通过pong校验则发送一个ping包进行pong校验，如果通过pong校验则加入发现节点池
    if time.Since(t.db.LastPongReceived(n.ID())) > bondExpiration {
        t.sendPing(n.ID(), from, func() { t.tab.addThroughPing(n) })
    } else {
        t.tab.addThroughPing(n)
    }
    t.localNode.UDPEndpointStatement(from, &net.UDPAddr{IP: req.To.IP, Port: int(req.To.UDP)})
    t.db.UpdateLastPingReceived(n.ID(), time.Now())
    return nil
}

由于我们是第一次连接，所以要进行pong校验，我们来看看go-ethereum是怎么实现协议中的pong校验的，在https://github.com/ethereum/go-ethereum/blob/master/p2p/discover/udp.go#L645

func (req *pong) handle(t *udp, from *net.UDPAddr, fromKey encPubkey, mac []byte) error {
    if expired(req.Expiration) {
        return errExpired
    }
    fromID := fromKey.id()
    //开始处理pong，如果没有请求过pong，就返回错误
    if !t.handleReply(fromID, pongPacket, req) {
        return errUnsolicitedReply
    }
    t.localNode.UDPEndpointStatement(from, &net.UDPAddr{IP: req.To.IP, Port: int(req.To.UDP)})
    //刷新pong时间，通过pong校验
    t.db.UpdateLastPongReceived(fromID, time.Now())
    return nil
}

可见只要通过t.handleReply的校验我们就可以刷新pong时间通过校验了，让我们来看看t.handleReply 是怎么处理的https://github.com/ethereum/go-ethereum/blob/master/p2p/discover/udp.go#L369

func (t *udp) handleReply(from enode.ID, ptype byte, req packet) bool {
    matched := make(chan bool, 1)
    select {
    //放入gotreply等待返回matched
    case t.gotreply <- reply{from, ptype, req, matched}:
        // loop will handle it
        return <-matched
    case <-t.closing:
        return false
    }
}

我们继续往下追

case r := <-t.gotreply:
            var matched bool
            for el := plist.Front(); el != nil; el = el.Next() {
                p := el.Value.(*pending)
                if p.from == r.from && p.ptype == r.ptype {
                    //是否有拉取过请求，有则matched为true
                    matched = true
                    // Remove the matcher if its callback indicates
                    // that all replies have been received. This is
                    // required for packet types that expect multiple
                    // reply packets.
                    //对应的callback校验，然而就算p.callback返回为false，matched也为true
                    if p.callback(r.data) {
                        p.errc <- nil
                        plist.Remove(el)
                    }
                    // Reset the continuous timeout counter (time drift detection)
                    contTimeouts = 0
                }
            }
            r.matched <- matched

这里基本能看出问题了，p.callback返回了false，只要不抛出错误，matched还是true，校验就算通过了，我们再来看看pong校验的callback是怎么处理的https://github.com/ethereum/go-ethereum/blob/master/p2p/discover/udp.go#L294

func (t *udp) sendPing(toid enode.ID, toaddr *net.UDPAddr, callback func()) <-chan error {
    req := &ping{
        Version:    4,
        From:       t.ourEndpoint(),
        To:         makeEndpoint(toaddr, 0), // TODO: maybe use known TCP port from DB
        Expiration: uint64(time.Now().Add(expiration).Unix()),
    }
    packet, hash, err := encodePacket(t.priv, pingPacket, req)
    if err != nil {
        errc := make(chan error, 1)
        errc <- err
        return errc
    }
    //这里就是pong校验的callback，可见就算没通过校验也只是返回了false
    errc := t.pending(toid, pongPacket, func(p interface{}) bool {
        ok := bytes.Equal(p.(*pong).ReplyTok, hash)
        if ok && callback != nil {
            callback()
        }
        return ok
    })
    t.localNode.UDPContact(toaddr)
    t.write(toaddr, req.name(), packet)
    return errc
}

所有，实际上go-ethereum并没有很好的实现pong校验，导致协议设计的防御机制彻底失效。