diff --git a/esphome/components/anova/anova_base.cpp b/esphome/components/anova/anova_base.cpp
index d55404089e..dcef75e483 100644
--- a/esphome/components/anova/anova_base.cpp
+++ b/esphome/components/anova/anova_base.cpp
@@ -73,51 +73,52 @@ AnovaPacket *AnovaCodec::get_stop_request() {
 }
 
 void AnovaCodec::decode(const uint8_t *data, uint16_t length) {
-  memset(this->buf_, 0, 32);
-  strncpy(this->buf_, (char *) data, length);
+  char buf[32];
+  memset(buf, 0, sizeof(buf));
+  strncpy(buf, (char *) data, std::min<uint16_t>(length, sizeof(buf) - 1));
   this->has_target_temp_ = this->has_current_temp_ = this->has_unit_ = this->has_running_ = false;
   switch (this->current_query_) {
     case READ_DEVICE_STATUS: {
-      if (!strncmp(this->buf_, "stopped", 7)) {
+      if (!strncmp(buf, "stopped", 7)) {
         this->has_running_ = true;
         this->running_ = false;
       }
-      if (!strncmp(this->buf_, "running", 7)) {
+      if (!strncmp(buf, "running", 7)) {
         this->has_running_ = true;
         this->running_ = true;
       }
       break;
     }
     case START: {
-      if (!strncmp(this->buf_, "start", 5)) {
+      if (!strncmp(buf, "start", 5)) {
         this->has_running_ = true;
         this->running_ = true;
       }
       break;
     }
     case STOP: {
-      if (!strncmp(this->buf_, "stop", 4)) {
+      if (!strncmp(buf, "stop", 4)) {
         this->has_running_ = true;
         this->running_ = false;
       }
       break;
     }
     case READ_TARGET_TEMPERATURE: {
-      this->target_temp_ = parse_number<float>(this->buf_, sizeof(this->buf_)).value_or(0.0f);
+      this->target_temp_ = parse_number<float>(buf, sizeof(buf)).value_or(0.0f);
       if (this->fahrenheit_)
         this->target_temp_ = ftoc(this->target_temp_);
       this->has_target_temp_ = true;
       break;
     }
     case SET_TARGET_TEMPERATURE: {
-      this->target_temp_ = parse_number<float>(this->buf_, sizeof(this->buf_)).value_or(0.0f);
+      this->target_temp_ = parse_number<float>(buf, sizeof(buf)).value_or(0.0f);
       if (this->fahrenheit_)
         this->target_temp_ = ftoc(this->target_temp_);
       this->has_target_temp_ = true;
       break;
     }
     case READ_CURRENT_TEMPERATURE: {
-      this->current_temp_ = parse_number<float>(this->buf_, sizeof(this->buf_)).value_or(0.0f);
+      this->current_temp_ = parse_number<float>(buf, sizeof(buf)).value_or(0.0f);
       if (this->fahrenheit_)
         this->current_temp_ = ftoc(this->current_temp_);
       this->has_current_temp_ = true;
@@ -125,8 +126,8 @@ void AnovaCodec::decode(const uint8_t *data, uint16_t length) {
     }
     case SET_UNIT:
     case READ_UNIT: {
-      this->unit_ = this->buf_[0];
-      this->fahrenheit_ = this->buf_[0] == 'f';
+      this->unit_ = buf[0];
+      this->fahrenheit_ = buf[0] == 'f';
       this->has_unit_ = true;
       break;
     }
diff --git a/esphome/components/anova/anova_base.h b/esphome/components/anova/anova_base.h
index 7c1383512d..b831157849 100644
--- a/esphome/components/anova/anova_base.h
+++ b/esphome/components/anova/anova_base.h
@@ -70,7 +70,6 @@ class AnovaCodec {
   bool has_current_temp_;
   bool has_unit_;
   bool has_running_;
-  char buf_[32];
   bool fahrenheit_;
 
   CurrentQuery current_query_;
diff --git a/esphome/components/ezo/ezo.cpp b/esphome/components/ezo/ezo.cpp
index 7f7a41fb41..426f2807c1 100644
--- a/esphome/components/ezo/ezo.cpp
+++ b/esphome/components/ezo/ezo.cpp
@@ -32,7 +32,7 @@ void EZOSensor::update() {
 }
 
 void EZOSensor::loop() {
-  uint8_t buf[20];
+  uint8_t buf[21];
   if (!(this->state_ & EZO_STATE_WAIT)) {
     if (this->state_ & EZO_STATE_SEND_TEMP) {
       int len = sprintf((char *) buf, "T,%0.3f", this->tempcomp_);
@@ -74,7 +74,7 @@ void EZOSensor::loop() {
   if (buf[0] != 1)
     return;
 
-  float val = parse_number<float>((char *) &buf[1], sizeof(buf) - 1).value_or(0);
+  float val = parse_number<float>((char *) &buf[1], sizeof(buf) - 2).value_or(0);
   this->publish_state(val);
 }
 
diff --git a/esphome/core/helpers.h b/esphome/core/helpers.h
index c67ad8eea3..9cdbf7ca16 100644
--- a/esphome/core/helpers.h
+++ b/esphome/core/helpers.h
@@ -362,45 +362,47 @@ std::string str_sanitize(const std::string &str);
 /// @name Parsing & formatting
 ///@{
 
-/// Parse a unsigned decimal number.
+/// Parse an unsigned decimal number (requires null-terminated string).
 template<typename T, enable_if_t<(std::is_integral<T>::value && std::is_unsigned<T>::value), int> = 0>
 optional<T> parse_number(const char *str, size_t len) {
   char *end = nullptr;
   unsigned long value = ::strtoul(str, &end, 10);  // NOLINT(google-runtime-int)
-  if (end == nullptr || end != str + len || value > std::numeric_limits<T>::max())
+  if (end == str || *end != '\0' || value > std::numeric_limits<T>::max())
     return {};
   return value;
 }
+/// Parse an unsigned decimal number.
 template<typename T, enable_if_t<(std::is_integral<T>::value && std::is_unsigned<T>::value), int> = 0>
 optional<T> parse_number(const std::string &str) {
-  return parse_number<T>(str.c_str(), str.length());
+  return parse_number<T>(str.c_str(), str.length() + 1);
 }
-/// Parse a signed decimal number.
+/// Parse a signed decimal number (requires null-terminated string).
 template<typename T, enable_if_t<(std::is_integral<T>::value && std::is_signed<T>::value), int> = 0>
 optional<T> parse_number(const char *str, size_t len) {
   char *end = nullptr;
   signed long value = ::strtol(str, &end, 10);  // NOLINT(google-runtime-int)
-  if (end == nullptr || end != str + len || value < std::numeric_limits<T>::min() ||
-      value > std::numeric_limits<T>::max())
+  if (end == str || *end != '\0' || value < std::numeric_limits<T>::min() || value > std::numeric_limits<T>::max())
     return {};
   return value;
 }
+/// Parse a signed decimal number.
 template<typename T, enable_if_t<(std::is_integral<T>::value && std::is_signed<T>::value), int> = 0>
 optional<T> parse_number(const std::string &str) {
-  return parse_number<T>(str.c_str(), str.length());
+  return parse_number<T>(str.c_str(), str.length() + 1);
 }
-/// Parse a decimal floating-point number.
+/// Parse a decimal floating-point number (requires null-terminated string).
 template<typename T, enable_if_t<(std::is_same<T, float>::value), int> = 0>
 optional<T> parse_number(const char *str, size_t len) {
   char *end = nullptr;
   float value = ::strtof(str, &end);
-  if (end == nullptr || end != str + len || value == HUGE_VALF)
+  if (end == str || *end != '\0' || value == HUGE_VALF)
     return {};
   return value;
 }
+/// Parse a decimal floating-point number.
 template<typename T, enable_if_t<(std::is_same<T, float>::value), int> = 0>
 optional<T> parse_number(const std::string &str) {
-  return parse_number<T>(str.c_str(), str.length());
+  return parse_number<T>(str.c_str(), str.length() + 1);
 }
 
 ///@}