diff --git a/NEWS b/NEWS
index 26b93c5bcb6..85f1b21a986 100644
--- a/NEWS
+++ b/NEWS
@@ -5,11 +5,13 @@ PHP                                                                        NEWS
 - Core:
   . Fixed zend_call_stack build with Linux/uclibc-ng without thread support.
     (Fabrice Fontaine)
-
-- Core:
   . Fixed bug GH-13772 (Invalid execute_data->opline pointers in observer fcall
     handlers when JIT is enabled). (Bob)
 
+- Fibers:
+  . Fixed bug GH-13903 (ASAN false positive underflow when executing copy()).
+    (nielsdos)
+
 - FPM:
   . Fixed bug GH-13563 (Setting bool values via env in FPM config fails).
     (Jakub Zelenka)
diff --git a/Zend/zend_fibers.c b/Zend/zend_fibers.c
index e669ab6b533..d6a4e5f3693 100644
--- a/Zend/zend_fibers.c
+++ b/Zend/zend_fibers.c
@@ -62,6 +62,7 @@
 #endif
 
 #ifdef __SANITIZE_ADDRESS__
+# include <sanitizer/asan_interface.h>
 # include <sanitizer/common_interface_defs.h>
 #endif
 
@@ -300,6 +301,12 @@ static void zend_fiber_stack_free(zend_fiber_stack *stack)
 
 	void *pointer = (void *) ((uintptr_t) stack->pointer - ZEND_FIBER_GUARD_PAGES * page_size);
 
+#ifdef __SANITIZE_ADDRESS__
+	/* If another mmap happens after unmapping, it may trigger the stale stack red zones
+	 * so we have to unpoison it before unmapping. */
+	ASAN_UNPOISON_MEMORY_REGION(pointer, stack->size + ZEND_FIBER_GUARD_PAGES * page_size);
+#endif
+
 #ifdef ZEND_WIN32
 	VirtualFree(pointer, 0, MEM_RELEASE);
 #else