diff --git a/src/shrpx-unittest.cc b/src/shrpx-unittest.cc
index a02fd2e5..70230e82 100644
--- a/src/shrpx-unittest.cc
+++ b/src/shrpx-unittest.cc
@@ -178,6 +178,7 @@ int main(int argc, char *argv[]) {
       !CU_add_test(pSuite, "util_strifind", shrpx::test_util_strifind) ||
       !CU_add_test(pSuite, "util_random_alpha_digit",
                    shrpx::test_util_random_alpha_digit) ||
+      !CU_add_test(pSuite, "util_format_hex", shrpx::test_util_format_hex) ||
       !CU_add_test(pSuite, "gzip_inflate", test_nghttp2_gzip_inflate) ||
       !CU_add_test(pSuite, "buffer_write", nghttp2::test_buffer_write) ||
       !CU_add_test(pSuite, "pool_recycle", nghttp2::test_pool_recycle) ||
diff --git a/src/shrpx_client_handler.cc b/src/shrpx_client_handler.cc
index 43c28649..900d4db3 100644
--- a/src/shrpx_client_handler.cc
+++ b/src/shrpx_client_handler.cc
@@ -1522,4 +1522,6 @@ void ClientHandler::set_tls_sni(const StringRef &sni) {
 
 StringRef ClientHandler::get_tls_sni() const { return sni_; }
 
+BlockAllocator &ClientHandler::get_block_allocator() { return balloc_; }
+
 } // namespace shrpx
diff --git a/src/shrpx_client_handler.h b/src/shrpx_client_handler.h
index 8a27c24b..c441a5bd 100644
--- a/src/shrpx_client_handler.h
+++ b/src/shrpx_client_handler.h
@@ -162,6 +162,8 @@ public:
   // Returns TLS SNI extension value client sent in this connection.
   StringRef get_tls_sni() const;
 
+  BlockAllocator &get_block_allocator();
+
 private:
   BlockAllocator balloc_;
   Connection conn_;
diff --git a/src/shrpx_ssl.cc b/src/shrpx_ssl.cc
index c59251be..7601a19d 100644
--- a/src/shrpx_ssl.cc
+++ b/src/shrpx_ssl.cc
@@ -243,6 +243,7 @@ int tls_session_new_cb(SSL *ssl, SSL_SESSION *session) {
   auto handler = static_cast<ClientHandler *>(conn->data);
   auto worker = handler->get_worker();
   auto dispatcher = worker->get_session_cache_memcached_dispatcher();
+  auto &balloc = handler->get_block_allocator();
 
   const unsigned char *id;
   unsigned int idlen;
@@ -256,7 +257,8 @@ int tls_session_new_cb(SSL *ssl, SSL_SESSION *session) {
   auto req = make_unique<MemcachedRequest>();
   req->op = MEMCACHED_OP_ADD;
   req->key = MEMCACHED_SESSION_CACHE_KEY_PREFIX.str();
-  req->key += util::format_hex(id, idlen);
+  req->key +=
+      util::format_hex(balloc, StringRef{id, static_cast<size_t>(idlen)});
 
   auto sessionlen = i2d_SSL_SESSION(session, nullptr);
   req->value.resize(sessionlen);
@@ -295,6 +297,7 @@ SSL_SESSION *tls_session_get_cb(SSL *ssl,
   auto handler = static_cast<ClientHandler *>(conn->data);
   auto worker = handler->get_worker();
   auto dispatcher = worker->get_session_cache_memcached_dispatcher();
+  auto &balloc = handler->get_block_allocator();
 
   if (conn->tls.cached_session) {
     if (LOG_ENABLED(INFO)) {
@@ -318,7 +321,8 @@ SSL_SESSION *tls_session_get_cb(SSL *ssl,
   auto req = make_unique<MemcachedRequest>();
   req->op = MEMCACHED_OP_GET;
   req->key = MEMCACHED_SESSION_CACHE_KEY_PREFIX.str();
-  req->key += util::format_hex(id, idlen);
+  req->key +=
+      util::format_hex(balloc, StringRef{id, static_cast<size_t>(idlen)});
   req->cb = [conn](MemcachedRequest *, MemcachedResult res) {
     if (LOG_ENABLED(INFO)) {
       LOG(INFO) << "Memcached: returned status code " << res.status_code;
diff --git a/src/util.cc b/src/util.cc
index 19428acf..056c7cd9 100644
--- a/src/util.cc
+++ b/src/util.cc
@@ -379,6 +379,21 @@ std::string format_hex(const unsigned char *s, size_t len) {
   return res;
 }
 
+StringRef format_hex(BlockAllocator &balloc, const StringRef &s) {
+  auto iov = make_byte_ref(balloc, s.size() * 2 + 1);
+  auto p = iov.base;
+
+  for (auto cc : s) {
+    uint8_t c = cc;
+    *p++ = LOWER_XDIGITS[c >> 4];
+    *p++ = LOWER_XDIGITS[c & 0xf];
+  }
+
+  *p = '\0';
+
+  return StringRef{iov.base, p};
+}
+
 void to_token68(std::string &base64str) {
   std::transform(std::begin(base64str), std::end(base64str),
                  std::begin(base64str), [](char c) {
diff --git a/src/util.h b/src/util.h
index db823f4c..e192864d 100644
--- a/src/util.h
+++ b/src/util.h
@@ -145,6 +145,8 @@ template <size_t N> std::string format_hex(const std::array<uint8_t, N> &s) {
   return format_hex(s.data(), s.size());
 }
 
+StringRef format_hex(BlockAllocator &balloc, const StringRef &s);
+
 std::string http_date(time_t t);
 
 // Returns given time |t| from epoch in Common Log format (e.g.,
diff --git a/src/util_test.cc b/src/util_test.cc
index 66f4085e..e8e24fcf 100644
--- a/src/util_test.cc
+++ b/src/util_test.cc
@@ -552,4 +552,12 @@ void test_util_random_alpha_digit(void) {
   }
 }
 
+void test_util_format_hex(void) {
+  BlockAllocator balloc(4096, 4096);
+
+  CU_ASSERT("0ff0" ==
+            util::format_hex(balloc, StringRef::from_lit("\x0f\xf0")));
+  CU_ASSERT("" == util::format_hex(balloc, StringRef::from_lit("")));
+}
+
 } // namespace shrpx
diff --git a/src/util_test.h b/src/util_test.h
index 6741da9e..c498cbad 100644
--- a/src/util_test.h
+++ b/src/util_test.h
@@ -63,6 +63,7 @@ void test_util_make_http_hostport(void);
 void test_util_make_hostport(void);
 void test_util_strifind(void);
 void test_util_random_alpha_digit(void);
+void test_util_format_hex(void);
 
 } // namespace shrpx