diff --git a/td/telegram/UpdatesManager.cpp b/td/telegram/UpdatesManager.cpp
index 5bac3c7c..476230a3 100644
--- a/td/telegram/UpdatesManager.cpp
+++ b/td/telegram/UpdatesManager.cpp
@@ -187,7 +187,7 @@ void UpdatesManager::fill_gap(void *td, const char *source) {
 
 string UpdatesManager::get_state() const {
   char buff[1024];
-  StringBuilder sb({buff, sizeof(buff)});
+  StringBuilder sb(MutableSlice{buff, sizeof(buff)});
   sb << "UpdatesManager is in state ";
   switch (state_.type) {
     case State::Type::General:
diff --git a/tdnet/td/net/HttpHeaderCreator.h b/tdnet/td/net/HttpHeaderCreator.h
index ffde46f9..96144d71 100644
--- a/tdnet/td/net/HttpHeaderCreator.h
+++ b/tdnet/td/net/HttpHeaderCreator.h
@@ -16,26 +16,26 @@ namespace td {
 class HttpHeaderCreator {
  public:
   static constexpr size_t MAX_HEADER = 4096;
-  HttpHeaderCreator() : sb_({header_, MAX_HEADER}) {
+  HttpHeaderCreator() : sb_(MutableSlice{header_, MAX_HEADER}) {
   }
   void init_ok() {
-    sb_ = StringBuilder({header_, MAX_HEADER});
+    sb_ = StringBuilder(MutableSlice{header_, MAX_HEADER});
     sb_ << "HTTP/1.1 200 OK\r\n";
   }
   void init_get(Slice url) {
-    sb_ = StringBuilder({header_, MAX_HEADER});
+    sb_ = StringBuilder(MutableSlice{header_, MAX_HEADER});
     sb_ << "GET " << url << " HTTP/1.1\r\n";
   }
   void init_post(Slice url) {
-    sb_ = StringBuilder({header_, MAX_HEADER});
+    sb_ = StringBuilder(MutableSlice{header_, MAX_HEADER});
     sb_ << "POST " << url << " HTTP/1.1\r\n";
   }
   void init_error(int code, Slice reason) {
-    sb_ = StringBuilder({header_, MAX_HEADER});
+    sb_ = StringBuilder(MutableSlice{header_, MAX_HEADER});
     sb_ << "HTTP/1.1 " << code << " " << reason << "\r\n";
   }
   void init_status_line(int http_status_code) {
-    sb_ = StringBuilder({header_, MAX_HEADER});
+    sb_ = StringBuilder(MutableSlice{header_, MAX_HEADER});
     sb_ << "HTTP/1.1 " << http_status_code << " " << get_status_line(http_status_code) << "\r\n";
   }
   void add_header(Slice key, Slice value) {
diff --git a/tdutils/td/utils/OptionsParser.h b/tdutils/td/utils/OptionsParser.h
index d9d6ccfc..0820e31b 100644
--- a/tdutils/td/utils/OptionsParser.h
+++ b/tdutils/td/utils/OptionsParser.h
@@ -58,7 +58,7 @@ class OptionsParser {
     return -1;
 #else
     char buff[1024];
-    StringBuilder sb({buff, sizeof(buff)});
+    StringBuilder sb(MutableSlice{buff, sizeof(buff)});
     for (auto &opt : options_) {
       CHECK(opt.type != Option::OptionalArg);
       sb << opt.short_key;
diff --git a/tdutils/td/utils/StringBuilder.cpp b/tdutils/td/utils/StringBuilder.cpp
index 0a6ecbce..86dfdf87 100644
--- a/tdutils/td/utils/StringBuilder.cpp
+++ b/tdutils/td/utils/StringBuilder.cpp
@@ -65,10 +65,16 @@ bool StringBuilder::reserve_inner(size_t size) {
   if (!use_buffer_) {
     return false;
   }
-  //TODO: check size_t oveflow
+
   size_t old_data_size = current_ptr_ - begin_ptr_;
+  if (size >= std::numeric_limits<size_t>::max() - reserved_size - old_data_size - 1) {
+    return false;
+  }
   size_t need_data_size = old_data_size + size;
   size_t old_buffer_size = end_ptr_ - begin_ptr_;
+  if (old_buffer_size >= (std::numeric_limits<size_t>::max() - reserved_size) / 2 - 2) {
+    return false;
+  }
   size_t new_buffer_size = (old_buffer_size + 1) * 2;
   if (new_buffer_size < need_data_size) {
     new_buffer_size = need_data_size;
@@ -77,13 +83,14 @@ bool StringBuilder::reserve_inner(size_t size) {
     new_buffer_size = 100;
   }
   new_buffer_size += reserved_size;
-  auto new_buffer_ = std::make_unique<char[]>(new_buffer_size);
-  std::memcpy(new_buffer_.get(), begin_ptr_, old_data_size);
-  buffer_ = std::move(new_buffer_);
+  auto new_buffer = std::make_unique<char[]>(new_buffer_size);
+  std::memcpy(new_buffer.get(), begin_ptr_, old_data_size);
+  buffer_ = std::move(new_buffer);
   begin_ptr_ = buffer_.get();
   current_ptr_ = begin_ptr_ + old_data_size;
   end_ptr_ = begin_ptr_ + new_buffer_size - reserved_size;
-  CHECK(end_ptr_ > begin_ptr_);
+  CHECK(end_ptr_ > current_ptr_);
+  CHECK(static_cast<size_t>(end_ptr_ - current_ptr_) >= size);
   return true;
 }
 
@@ -136,7 +143,7 @@ StringBuilder &StringBuilder::operator<<(long long unsigned int x) {
 }
 
 StringBuilder &StringBuilder::operator<<(FixedDouble x) {
-  if (unlikely(!reserve())) {
+  if (unlikely(!reserve(std::numeric_limits<double>::max_exponent10 + x.precision + 4))) {
     return on_error();
   }
 
diff --git a/tdutils/td/utils/StringBuilder.h b/tdutils/td/utils/StringBuilder.h
index fb655dd8..1ee97156 100644
--- a/tdutils/td/utils/StringBuilder.h
+++ b/tdutils/td/utils/StringBuilder.h
@@ -19,16 +19,15 @@ namespace td {
 class StringBuilder {
  public:
   explicit StringBuilder(MutableSlice slice, bool use_buffer = false)
-      : begin_ptr_(slice.begin())
-      , current_ptr_(begin_ptr_)
-      , end_ptr_(slice.end() - reserved_size)
-      , use_buffer_(use_buffer) {
+      : begin_ptr_(slice.begin()), current_ptr_(begin_ptr_), use_buffer_(use_buffer) {
     if (slice.size() <= reserved_size) {
       auto buffer_size = reserved_size + 100;
       buffer_ = std::make_unique<char[]>(buffer_size);
       begin_ptr_ = buffer_.get();
       current_ptr_ = begin_ptr_;
       end_ptr_ = begin_ptr_ + buffer_size - reserved_size;
+    } else {
+      end_ptr_ = slice.end() - reserved_size;
     }
   }
 
diff --git a/tdutils/test/json.cpp b/tdutils/test/json.cpp
index deca81a7..5559e2ce 100644
--- a/tdutils/test/json.cpp
+++ b/tdutils/test/json.cpp
@@ -34,7 +34,7 @@ static void decode_encode(string str, string result = "") {
 
 TEST(JSON, array) {
   char tmp[1000];
-  StringBuilder sb({tmp, sizeof(tmp)});
+  StringBuilder sb(MutableSlice{tmp, sizeof(tmp)});
   JsonBuilder jb(std::move(sb));
   jb.enter_value().enter_array() << "Hello" << -123;
   ASSERT_EQ(jb.string_builder().is_error(), false);
@@ -44,7 +44,7 @@ TEST(JSON, array) {
 }
 TEST(JSON, object) {
   char tmp[1000];
-  StringBuilder sb({tmp, sizeof(tmp)});
+  StringBuilder sb(MutableSlice{tmp, sizeof(tmp)});
   JsonBuilder jb(std::move(sb));
   auto c = jb.enter_object();
   c << std::tie("key", "value");
@@ -58,7 +58,7 @@ TEST(JSON, object) {
 
 TEST(JSON, nested) {
   char tmp[1000];
-  StringBuilder sb({tmp, sizeof(tmp)});
+  StringBuilder sb(MutableSlice{tmp, sizeof(tmp)});
   JsonBuilder jb(std::move(sb));
   {
     auto a = jb.enter_array();
diff --git a/tdutils/test/misc.cpp b/tdutils/test/misc.cpp
index 5cf5b362..aacebf5a 100644
--- a/tdutils/test/misc.cpp
+++ b/tdutils/test/misc.cpp
@@ -494,12 +494,12 @@ TEST(Misc, full_split) {
 }
 
 TEST(Misc, StringBuilder) {
-  auto small = std::string{"abcdefghij"};
-  auto big = std::string(1000, 'a');
+  auto small_str = std::string{"abcdefghij"};
+  auto big_str = std::string(1000, 'a');
   using V = std::vector<std::string>;
   for (auto use_buf : {false, true}) {
     for (size_t initial_buffer_size : {0, 1, 5, 10, 100, 1000, 2000}) {
-      for (auto test : {V{small}, {small, big, big, small}, {big, small, big}}) {
+      for (auto test : {V{small_str}, V{small_str, big_str, big_str, small_str}, V{big_str, small_str, big_str}}) {
         std::string buf(initial_buffer_size, '\0');
         td::StringBuilder sb(buf, use_buf);
         std::string res;