diff --git a/os/hal/ports/STM32/LLD/USARTv1/hal_serial_lld.c b/os/hal/ports/STM32/LLD/USARTv1/hal_serial_lld.c
index e373529a18..64edbde8c9 100644
--- a/os/hal/ports/STM32/LLD/USARTv1/hal_serial_lld.c
+++ b/os/hal/ports/STM32/LLD/USARTv1/hal_serial_lld.c
@@ -728,7 +728,7 @@ void sd_lld_stop(SerialDriver *sdp) {
  */
 void sd_lld_serve_interrupt(SerialDriver *sdp) {
   USART_TypeDef *u = sdp->usart;
-  uint16_t cr1 = u->CR1;
+  uint16_t cr1;
   uint16_t sr = u->SR;
 
   /* Special case, LIN break detection.*/
@@ -755,6 +755,9 @@ void sd_lld_serve_interrupt(SerialDriver *sdp) {
   }
   osalSysUnlockFromISR();
 
+  /* Caching CR1.*/
+  cr1 = u->CR1;
+
   /* Transmission buffer empty.*/
   if ((cr1 & USART_CR1_TXEIE) && (sr & USART_SR_TXE)) {
     msg_t b;
@@ -762,7 +765,7 @@ void sd_lld_serve_interrupt(SerialDriver *sdp) {
     b = oqGetI(&sdp->oqueue);
     if (b < MSG_OK) {
       chnAddFlagsI(sdp, CHN_OUTPUT_EMPTY);
-      u->CR1 = cr1 & ~USART_CR1_TXEIE;
+      cr1 &= ~USART_CR1_TXEIE;
     }
     else
       u->DR = b;
@@ -774,10 +777,13 @@ void sd_lld_serve_interrupt(SerialDriver *sdp) {
     osalSysLockFromISR();
     if (oqIsEmptyI(&sdp->oqueue)) {
       chnAddFlagsI(sdp, CHN_TRANSMISSION_END);
-      u->CR1 = cr1 & ~USART_CR1_TCIE;
+      cr1 &= ~USART_CR1_TCIE;
     }
     osalSysUnlockFromISR();
   }
+
+  /* Writing CR1 once.*/
+  u->CR1 = cr1;
 }
 
 #endif /* HAL_USE_SERIAL */
diff --git a/os/hal/ports/STM32/LLD/USARTv2/hal_serial_lld.c b/os/hal/ports/STM32/LLD/USARTv2/hal_serial_lld.c
index a020ed5ad5..c90d63336e 100644
--- a/os/hal/ports/STM32/LLD/USARTv2/hal_serial_lld.c
+++ b/os/hal/ports/STM32/LLD/USARTv2/hal_serial_lld.c
@@ -843,7 +843,7 @@ void sd_lld_stop(SerialDriver *sdp) {
  */
 void sd_lld_serve_interrupt(SerialDriver *sdp) {
   USART_TypeDef *u = sdp->usart;
-  uint32_t cr1 = u->CR1;
+  uint32_t cr1;
   uint32_t isr;
 
   /* Reading and clearing status.*/
@@ -874,6 +874,9 @@ void sd_lld_serve_interrupt(SerialDriver *sdp) {
     isr = u->ISR;
   }
 
+  /* Caching CR1.*/
+  cr1 = u->CR1;
+
   /* Transmission buffer empty, note it is a while in order to handle two
      situations:
      1) The data registers has been emptied immediately after writing it, this
@@ -888,7 +891,7 @@ void sd_lld_serve_interrupt(SerialDriver *sdp) {
       b = oqGetI(&sdp->oqueue);
       if (b < MSG_OK) {
         chnAddFlagsI(sdp, CHN_OUTPUT_EMPTY);
-        u->CR1 = cr1 & ~USART_CR1_TXEIE;
+        cr1 &= ~USART_CR1_TXEIE;
         osalSysUnlockFromISR();
         break;
       }
@@ -904,10 +907,13 @@ void sd_lld_serve_interrupt(SerialDriver *sdp) {
     osalSysLockFromISR();
     if (oqIsEmptyI(&sdp->oqueue)) {
       chnAddFlagsI(sdp, CHN_TRANSMISSION_END);
-      u->CR1 = cr1 & ~USART_CR1_TCIE;
+      cr1 &= ~USART_CR1_TCIE;
     }
     osalSysUnlockFromISR();
   }
+
+  /* Writing CR1 once.*/
+  u->CR1 = cr1;
 }
 
 #endif /* HAL_USE_SERIAL */
diff --git a/os/hal/ports/STM32/LLD/USARTv3/hal_serial_lld.c b/os/hal/ports/STM32/LLD/USARTv3/hal_serial_lld.c
index d8dc340fe3..fca0443cff 100644
--- a/os/hal/ports/STM32/LLD/USARTv3/hal_serial_lld.c
+++ b/os/hal/ports/STM32/LLD/USARTv3/hal_serial_lld.c
@@ -971,7 +971,7 @@ void sd_lld_stop(SerialDriver *sdp) {
  */
 void sd_lld_serve_interrupt(SerialDriver *sdp) {
   USART_TypeDef *u = sdp->usart;
-  uint32_t cr1 = u->CR1;
+  uint32_t cr1;
   uint32_t isr;
 
   /* Reading and clearing status.*/
@@ -1002,6 +1002,9 @@ void sd_lld_serve_interrupt(SerialDriver *sdp) {
     isr = u->ISR;
   }
 
+  /* Caching CR1.*/
+  cr1 = u->CR1;
+
   /* Transmission buffer empty, note it is a while in order to handle two
      situations:
      1) The data registers has been emptied immediately after writing it, this
@@ -1016,7 +1019,7 @@ void sd_lld_serve_interrupt(SerialDriver *sdp) {
       b = oqGetI(&sdp->oqueue);
       if (b < MSG_OK) {
         chnAddFlagsI(sdp, CHN_OUTPUT_EMPTY);
-        u->CR1 = cr1 & ~USART_CR1_TXEIE;
+        cr1 &= ~USART_CR1_TXEIE;
         osalSysUnlockFromISR();
         break;
       }
@@ -1032,10 +1035,13 @@ void sd_lld_serve_interrupt(SerialDriver *sdp) {
     osalSysLockFromISR();
     if (oqIsEmptyI(&sdp->oqueue)) {
       chnAddFlagsI(sdp, CHN_TRANSMISSION_END);
-      u->CR1 = cr1 & ~USART_CR1_TCIE;
+      cr1 &= ~USART_CR1_TCIE;
     }
     osalSysUnlockFromISR();
   }
+
+  /* Writing CR1 once.*/
+  u->CR1 = cr1;
 }
 
 #endif /* HAL_USE_SERIAL */
diff --git a/readme.txt b/readme.txt
index eac6d9c95f..05c8dd2bd2 100644
--- a/readme.txt
+++ b/readme.txt
@@ -86,6 +86,7 @@
 - NEW: Reworked STM32 SDMMCv1 and SDMMCv2 drivers, better timeout and clock
        handling, improved speed for aligned buffers.
 - FIX: Fixed wrong STM32 LSI activation check (bug #1279).
+- FIX: Fixed STM32 HAL UART ISR flaw (bug #1278).
 - FIX: Fixed race condition caused by chGuardedPoolAllocI() (bug #1277).
 - FIX: Fixed avoid shadowing with build-in pow10 function in chprintf.c
        (bug #1274).