diff --git a/test/extensions/filters/http/ext_proc/unit_test_fuzz/ext_proc_corpus/crash-df781e3c1eede590dddbb6d3fd7e58f87b8d89cd b/test/extensions/filters/http/ext_proc/unit_test_fuzz/ext_proc_corpus/crash-df781e3c1eede590dddbb6d3fd7e58f87b8d89cd
new file mode 100644
index 000000000000..8d60ca08e841
--- /dev/null
+++ b/test/extensions/filters/http/ext_proc/unit_test_fuzz/ext_proc_corpus/crash-df781e3c1eede590dddbb6d3fd7e58f87b8d89cd
@@ -0,0 +1,28 @@
+config {
+  grpc_service {
+    envoy_grpc {
+      cluster_name: ":"
+    }
+  }
+  processing_mode {
+    response_header_mode: SKIP
+    request_trailer_mode: SKIP
+    response_trailer_mode: SKIP
+  }
+  max_message_timeout {
+    nanos: 89
+  }
+}
+request {
+  http_body {
+    data: "\177\177\177\177"
+  }
+}
+response {
+  request_headers {
+  }
+  mode_override {
+    request_header_mode: SKIP
+    request_body_mode: BUFFERED
+  }
+}
diff --git a/test/extensions/filters/http/ext_proc/unit_test_fuzz/ext_proc_unit_test_fuzz.cc b/test/extensions/filters/http/ext_proc/unit_test_fuzz/ext_proc_unit_test_fuzz.cc
index 477a04bb2841..8ca81765cb27 100644
--- a/test/extensions/filters/http/ext_proc/unit_test_fuzz/ext_proc_unit_test_fuzz.cc
+++ b/test/extensions/filters/http/ext_proc/unit_test_fuzz/ext_proc_unit_test_fuzz.cc
@@ -18,13 +18,18 @@ namespace UnitTestFuzz {
 
 class FuzzerMocks {
 public:
-  FuzzerMocks() : addr_(std::make_shared<Network::Address::PipeInstance>("/test/test.sock")) {
+  FuzzerMocks()
+      : addr_(std::make_shared<Network::Address::PipeInstance>("/test/test.sock")), buffer_("foo") {
     ON_CALL(decoder_callbacks_, connection())
         .WillByDefault(Return(OptRef<const Network::Connection>{connection_}));
     connection_.stream_info_.downstream_connection_info_provider_->setRemoteAddress(addr_);
     connection_.stream_info_.downstream_connection_info_provider_->setLocalAddress(addr_);
     ON_CALL(decoder_callbacks_, addDecodedTrailers()).WillByDefault(ReturnRef(request_trailers_));
     ON_CALL(encoder_callbacks_, addEncodedTrailers()).WillByDefault(ReturnRef(response_trailers_));
+    ON_CALL(decoder_callbacks_, decodingBuffer()).WillByDefault(Return(&buffer_));
+    ON_CALL(encoder_callbacks_, encodingBuffer()).WillByDefault(Return(&buffer_));
+    ON_CALL(decoder_callbacks_, decoderBufferLimit()).WillByDefault(Return(1024));
+    ON_CALL(encoder_callbacks_, encoderBufferLimit()).WillByDefault(Return(1024));
   }
 
   NiceMock<Http::MockStreamDecoderFilterCallbacks> decoder_callbacks_;
@@ -33,6 +38,7 @@ class FuzzerMocks {
   NiceMock<Envoy::Network::MockConnection> connection_;
   NiceMock<Http::TestRequestTrailerMapImpl> request_trailers_;
   NiceMock<Http::TestResponseTrailerMapImpl> response_trailers_;
+  NiceMock<Buffer::OwnedImpl> buffer_;
   testing::NiceMock<StreamInfo::MockStreamInfo> async_client_stream_info_;
 };
 
@@ -72,8 +78,8 @@ DEFINE_PROTO_FUZZER(
   filter->setDecoderFilterCallbacks(mocks.decoder_callbacks_);
   filter->setEncoderFilterCallbacks(mocks.encoder_callbacks_);
 
-  ON_CALL(*client, start(_, _, _))
-      .WillByDefault(Invoke(
+  EXPECT_CALL(*client, start(_, _, _))
+      .WillRepeatedly(Invoke(
           [&](ExternalProcessing::ExternalProcessorCallbacks&,
               const envoy::config::core::v3::GrpcService&,
               const StreamInfo::StreamInfo&) -> ExternalProcessing::ExternalProcessorStreamPtr {
@@ -88,7 +94,7 @@ DEFINE_PROTO_FUZZER(
                     }));
             EXPECT_CALL(*stream, streamInfo())
                 .WillRepeatedly(ReturnRef(mocks.async_client_stream_info_));
-            ON_CALL(*stream, close()).WillByDefault(Return(false));
+            EXPECT_CALL(*stream, close()).WillRepeatedly(Return(false));
             return stream;
           }));