diff --git a/antireplay/stable_bloom_filter.go b/antireplay/stable_bloom_filter.go
index 6eef3f5..b52631e 100644
--- a/antireplay/stable_bloom_filter.go
+++ b/antireplay/stable_bloom_filter.go
@@ -1,6 +1,8 @@
 package antireplay
 
 import (
+	"sync"
+
 	"github.com/9seconds/mtg/v2/mtglib"
 	"github.com/OneOfOne/xxhash"
 	boom "github.com/tylertreat/BoomFilters"
@@ -8,9 +10,13 @@ import (
 
 type stableBloomFilter struct {
 	filter boom.StableBloomFilter
+	mutex  sync.Mutex
 }
 
 func (s *stableBloomFilter) SeenBefore(digest []byte) bool {
+	s.mutex.Lock()
+	defer s.mutex.Unlock()
+
 	return s.filter.TestAndAdd(digest)
 }