diff --git a/proxygen/external/http_parser/http_parser_cpp.cpp b/proxygen/external/http_parser/http_parser_cpp.cpp
index bf2c5f716a..6b21a1509f 100644
--- a/proxygen/external/http_parser/http_parser_cpp.cpp
+++ b/proxygen/external/http_parser/http_parser_cpp.cpp
@@ -530,7 +530,7 @@ parse_url_char(enum state s, const char ch, int strict_flag)
 #if __cplusplus
       [[fallthrough]];
 #else /* __cplusplus */
-    /* FALLTHROUGH */
+    __attribute__((fallthrough));
 #endif /* __cplusplus */
     case s_req_server_start:
     case s_req_server:
@@ -2252,7 +2252,7 @@ http_parse_host_char(enum http_host_state s, const char ch) {
 #if __cplusplus
       [[fallthrough]];
 #else /* __cplusplus */
-    /* FALLTHROUGH */
+    __attribute__((fallthrough));
 #endif /* __cplusplus */
     case s_http_host_v6_end:
       if (ch == ':') {
@@ -2269,7 +2269,7 @@ http_parse_host_char(enum http_host_state s, const char ch) {
 #if __cplusplus
       [[fallthrough]];
 #else /* __cplusplus */
-    /* FALLTHROUGH */
+    __attribute__((fallthrough));
 #endif /* __cplusplus */
     case s_http_host_v6_start:
       if (IS_HEX(ch) || ch == ':' || ch == '.') {
@@ -2412,7 +2412,7 @@ http_parser_parse_url_options(const char *buf, size_t buflen, int is_connect,
 #if __cplusplus
         [[fallthrough]];
 #else /* __cplusplus */
-      /* FALLTHROUGH */
+      __attribute__((fallthrough));
 #endif /* __cplusplus */
       case s_req_server:
         uf = UF_HOST;
diff --git a/proxygen/lib/http/experimental/RFC1867.cpp b/proxygen/lib/http/experimental/RFC1867.cpp
index fd8198d84f..7c4c0ed2c8 100644
--- a/proxygen/lib/http/experimental/RFC1867.cpp
+++ b/proxygen/lib/http/experimental/RFC1867.cpp
@@ -80,7 +80,7 @@ std::unique_ptr<IOBuf> RFC1867Codec::onIngress(std::unique_ptr<IOBuf> data) {
         input_.trimStart(boundary_.length() - 1);
         bytesProcessed_ += boundary_.length() - 1;
         state_ = ParserState::HEADERS_START;
-        // fall through
+        [[fallthrough]];
 
       case ParserState::HEADERS_START: {
         if (input_.chainLength() < 3) {
@@ -117,7 +117,7 @@ std::unique_ptr<IOBuf> RFC1867Codec::onIngress(std::unique_ptr<IOBuf> data) {
         headerParser_.onIngress(*dummyBuf);
         CHECK(!parseError_);
         state_ = ParserState::HEADERS;
-        // fall through
+        [[fallthrough]];
 
       case ParserState::HEADERS:
         while (!parseError_ && input_.front() &&