diff --git a/lib/protocol/protocol.go b/lib/protocol/protocol.go
index 210d889f9..fa4746cbe 100644
--- a/lib/protocol/protocol.go
+++ b/lib/protocol/protocol.go
@@ -49,6 +49,10 @@ const (
 	// MaxBlockSize is the maximum block size allowed
 	MaxBlockSize = 16 << MiB
 
+	// MaxRequestSize is the largest amount of data that can be read in a
+	// single request
+	MaxRequestSize = 2 * MaxBlockSize
+
 	// DesiredPerFileBlocks is the number of blocks we aim for per file
 	DesiredPerFileBlocks = 2000
 
@@ -457,14 +461,6 @@ func (c *rawConnection) dispatcherLoop() (err error) {
 			}
 		}
 
-		switch msg := msg.(type) {
-		case *bep.Request:
-			err = checkFilename(msg.Name)
-		}
-		if err != nil {
-			return newProtocolError(err, msgContext)
-		}
-
 		switch msg := msg.(type) {
 		case *bep.ClusterConfig:
 			err = c.model.ClusterConfig(clusterConfigFromWire(msg))
@@ -484,6 +480,15 @@ func (c *rawConnection) dispatcherLoop() (err error) {
 			err = c.handleIndexUpdate(idxUp)
 
 		case *bep.Request:
+			if err := checkFilename(msg.Name); err != nil {
+				return newProtocolError(err, msgContext)
+			}
+			if msg.Size <= 0 {
+				return newProtocolError(fmt.Errorf("request size %d too small", msg.Size), msgContext)
+			}
+			if msg.Size > MaxRequestSize {
+				return newProtocolError(fmt.Errorf("request size %d exceeds maximum allowed", msg.Size), msgContext)
+			}
 			go c.handleRequest(requestFromWire(msg))
 
 		case *bep.Response:
diff --git a/lib/protocol/protocol_test.go b/lib/protocol/protocol_test.go
index 4873ec987..7f3d23f1b 100644
--- a/lib/protocol/protocol_test.go
+++ b/lib/protocol/protocol_test.go
@@ -11,7 +11,9 @@ import (
 	"context"
 	"encoding/hex"
 	"errors"
+	"fmt"
 	"io"
+	"strings"
 	"sync"
 	"testing"
 	"time"
@@ -541,6 +543,84 @@ func TestDispatcherToCloseDeadlock(t *testing.T) {
 	}
 }
 
+func TestRequestMaxSize(t *testing.T) {
+	invalidSize := []int{-65536, 0, MaxRequestSize + 1}
+	for _, s := range invalidSize {
+		t.Run(fmt.Sprintf("invalid/%d", s), func(t *testing.T) {
+			m := newTestModel()
+			rw := testutil.NewBlockingRW()
+			c := getRawConnection(NewConnection(c0ID, rw, &testutil.NoopRW{}, testutil.NoopCloser{}, m, new(mockedConnectionInfo), CompressionAlways, testKeyGen))
+			c.Start()
+			defer closeAndWait(c, rw)
+
+			c.inbox <- &bep.ClusterConfig{}
+
+			// A request at exactly MaxRequestSize should be accepted.
+			c.inbox <- &bep.Request{
+				Id:   1,
+				Name: "valid",
+				Size: MaxRequestSize,
+			}
+
+			res := <-c.outbox
+			if msg, ok := res.msg.(*bep.Response); !ok || msg.Id != 1 {
+				t.Errorf("bad response %#v", msg)
+			}
+
+			// A request with an invalid size should cause the dispatcher to
+			// return with a protocol error.
+			c.inbox <- &bep.Request{
+				Id:   2,
+				Name: "invalid",
+				Size: int32(s),
+			}
+
+			select {
+			case <-c.dispatcherLoopStopped:
+			case <-time.After(time.Second):
+				t.Fatal("timed out before dispatcher loop terminated")
+			}
+
+			err := m.closedError()
+			if err == nil {
+				t.Fatal("expected connection to be closed with an error")
+			}
+			if !strings.Contains(err.Error(), "protocol error") {
+				t.Errorf("expected a protocol error, got %v", err)
+			}
+		})
+	}
+}
+
+func TestRequestInvalidFilename(t *testing.T) {
+	m := newTestModel()
+	rw := testutil.NewBlockingRW()
+	c := getRawConnection(NewConnection(c0ID, rw, &testutil.NoopRW{}, testutil.NoopCloser{}, m, new(mockedConnectionInfo), CompressionAlways, testKeyGen))
+	c.Start()
+	defer closeAndWait(c, rw)
+
+	c.inbox <- &bep.ClusterConfig{}
+	c.inbox <- &bep.Request{
+		Id:   1,
+		Name: "../escape",
+		Size: 1024,
+	}
+
+	select {
+	case <-c.dispatcherLoopStopped:
+	case <-time.After(time.Second):
+		t.Fatal("timed out before dispatcher loop terminated")
+	}
+
+	err := m.closedError()
+	if err == nil {
+		t.Fatal("expected connection to be closed with an error")
+	}
+	if !strings.Contains(err.Error(), "protocol error") {
+		t.Errorf("expected a protocol error, got %v", err)
+	}
+}
+
 func TestIndexIDString(t *testing.T) {
 	// Index ID is a 64 bit, zero padded hex integer.
 	var i IndexID = 42