server: disable redirects in peer communication

Disable following redirects from peer HTTP communication on the client's side.
Etcd server may run into SSRF (Server-side request forgery) when adding a new
member. If users provide a malicious peer URL, the existing etcd members may be
redirected to another unexpected internal URL when getting the new member's
version.

Signed-off-by: Ivan Valdes <ivan@vald.es>

server/etcdserver/cluster_util.go

@@ -70,6 +70,9 @@ func getClusterFromRemotePeers(lg *zap.Logger, urls []string, timeout time.Durat
 	cc := &http.Client{
 		Transport: rt,
 		Timeout:   timeout,
+		CheckRedirect: func(req *http.Request, via []*http.Request) error {
+			return http.ErrUseLastResponse
+		},
 	}
 	for _, u := range urls {
 		addr := u + "/members"
@@ -324,7 +327,12 @@ func getVersion(lg *zap.Logger, m *membership.Member, rt http.RoundTripper) (*ve
 }
 
 func promoteMemberHTTP(ctx context.Context, url string, id uint64, peerRt http.RoundTripper) ([]*membership.Member, error) {
-	cc := &http.Client{Transport: peerRt}
+	cc := &http.Client{
+		Transport: peerRt,
+		CheckRedirect: func(req *http.Request, via []*http.Request) error {
+			return http.ErrUseLastResponse
+		},
+	}
 	// TODO: refactor member http handler code
 	// cannot import etcdhttp, so manually construct url
 	requestUrl := url + "/members/promote/" + fmt.Sprintf("%d", id)
@@ -396,6 +404,9 @@ func getDowngradeEnabledFromRemotePeers(lg *zap.Logger, cl *membership.RaftClust
 func getDowngradeEnabled(lg *zap.Logger, m *membership.Member, rt http.RoundTripper) (bool, error) {
 	cc := &http.Client{
 		Transport: rt,
+		CheckRedirect: func(req *http.Request, via []*http.Request) error {
+			return http.ErrUseLastResponse
+		},
 	}
 	var (
 		err  error

server/etcdserver/corrupt.go

@@ -390,7 +390,12 @@ func (s *EtcdServer) getPeerHashKVs(rev int64) []*peerHashKVResp {
 
 	lg := s.Logger()
 
-	cc := &http.Client{Transport: s.peerRt}
+	cc := &http.Client{
+		Transport: s.peerRt,
+		CheckRedirect: func(req *http.Request, via []*http.Request) error {
+			return http.ErrUseLastResponse
+		},
+	}
 	var resps []*peerHashKVResp
 	for _, p := range peers {
 		if len(p.eps) == 0 {

server/lease/leasehttp/http.go

@@ -150,7 +150,12 @@ func RenewHTTP(ctx context.Context, id lease.LeaseID, url string, rt http.RoundT
 		return -1, err
 	}
 
-	cc := &http.Client{Transport: rt}
+	cc := &http.Client{
+		Transport: rt,
+		CheckRedirect: func(req *http.Request, via []*http.Request) error {
+			return http.ErrUseLastResponse
+		},
+	}
 	req, err := http.NewRequest("POST", url, bytes.NewReader(lreq))
 	if err != nil {
 		return -1, err
@@ -210,7 +215,12 @@ func TimeToLiveHTTP(ctx context.Context, id lease.LeaseID, keys bool, url string
 
 	req = req.WithContext(ctx)
 
-	cc := &http.Client{Transport: rt}
+	cc := &http.Client{
+		Transport: rt,
+		CheckRedirect: func(req *http.Request, via []*http.Request) error {
+			return http.ErrUseLastResponse
+		},
+	}
 	var b []byte
 	// buffer errc channel so that errc don't block inside the go routinue
 	resp, err := cc.Do(req)