diff --git a/src/net.h b/src/net.h
--- a/src/net.h
+++ b/src/net.h
@@ -1018,7 +1018,9 @@
     };
 
     // m_avalanche_state == nullptr if we're not using avalanche with this peer
-    std::unique_ptr<AvalancheState> m_avalanche_state;
+    std::unique_ptr<AvalancheState>
+        m_avalanche_state GUARDED_BY(cs_avalanche_state);
+    Mutex cs_avalanche_state;
 
     // Used for headers announcements - unfiltered blocks to relay
     std::vector<BlockHash> vBlockHashesToAnnounce GUARDED_BY(cs_inventory);
diff --git a/src/net_processing.cpp b/src/net_processing.cpp
--- a/src/net_processing.cpp
+++ b/src/net_processing.cpp
@@ -4141,6 +4141,8 @@
     }
 
     if (msg_type == NetMsgType::AVAHELLO) {
+        LOCK(pfrom.cs_avalanche_state);
+
         if (!pfrom.m_avalanche_state) {
             pfrom.m_avalanche_state = std::make_unique<CNode::AvalancheState>();
         }
diff --git a/src/rpc/avalanche.cpp b/src/rpc/avalanche.cpp
--- a/src/rpc/avalanche.cpp
+++ b/src/rpc/avalanche.cpp
@@ -101,9 +101,7 @@
 
     NodeContext &node = EnsureNodeContext(request.context);
     if (!node.connman->ForNode(nodeid, [&](CNode *pnode) {
-            // FIXME This is not thread safe, and might cause issues if the
-            // unlikely event the peer sends an avahello message at the same
-            // time.
+            LOCK(pnode->cs_avalanche_state);
             if (!pnode->m_avalanche_state) {
                 pnode->m_avalanche_state =
                     std::make_unique<CNode::AvalancheState>();