diff --git a/NEWS b/NEWS
index f535cf7187c..06be3911241 100644
--- a/NEWS
+++ b/NEWS
@@ -19,6 +19,7 @@ PHP                                                                        NEWS
   . Fixed bug GH-16630 (UAF in lexer with encoding translation and heredocs).
     (nielsdos)
   . Fix is_zend_ptr() huge block comparison. (nielsdos)
+  . Fixed potential OOB read in zend_dirname() on Windows. (cmb)
 
 - Curl:
   . Fixed bug GH-16802 (open_basedir bypass using curl extension). (nielsdos)
diff --git a/Zend/zend_compile.c b/Zend/zend_compile.c
index 6de663df1fc..a78523d493f 100644
--- a/Zend/zend_compile.c
+++ b/Zend/zend_compile.c
@@ -2122,7 +2122,7 @@ ZEND_API size_t zend_dirname(char *path, size_t len)
 	}
 
 	/* Strip trailing slashes */
-	while (end >= path && IS_SLASH_P(end)) {
+	while (end >= path && IS_SLASH_P_EX(end, end == path)) {
 		end--;
 	}
 	if (end < path) {
@@ -2133,7 +2133,7 @@ ZEND_API size_t zend_dirname(char *path, size_t len)
 	}
 
 	/* Strip filename */
-	while (end >= path && !IS_SLASH_P(end)) {
+	while (end >= path && !IS_SLASH_P_EX(end, end == path)) {
 		end--;
 	}
 	if (end < path) {
@@ -2144,7 +2144,7 @@ ZEND_API size_t zend_dirname(char *path, size_t len)
 	}
 
 	/* Strip slashes which came before the file name */
-	while (end >= path && IS_SLASH_P(end)) {
+	while (end >= path && IS_SLASH_P_EX(end, end == path)) {
 		end--;
 	}
 	if (end < path) {
diff --git a/Zend/zend_virtual_cwd.h b/Zend/zend_virtual_cwd.h
index 15c3b984a9d..92bdede0528 100644
--- a/Zend/zend_virtual_cwd.h
+++ b/Zend/zend_virtual_cwd.h
@@ -73,8 +73,11 @@ typedef unsigned short mode_t;
 #define DEFAULT_SLASH '\\'
 #define DEFAULT_DIR_SEPARATOR	';'
 #define IS_SLASH(c)	((c) == '/' || (c) == '\\')
+// IS_SLASH_P() may read the previous char on Windows, which may be OOB; use IS_SLASH_P_EX() instead
 #define IS_SLASH_P(c)	(*(c) == '/' || \
         (*(c) == '\\' && !IsDBCSLeadByte(*(c-1))))
+#define IS_SLASH_P_EX(c, first_byte)	(*(c) == '/' || \
+        (*(c) == '\\' && ((first_byte) || !IsDBCSLeadByte(*(c-1)))))
 
 /* COPY_WHEN_ABSOLUTE is 2 under Win32 because by chance both regular absolute paths
    in the file system and UNC paths need copying of two characters */
@@ -108,7 +111,9 @@ typedef unsigned short mode_t;
 #endif
 
 #define IS_SLASH(c)	((c) == '/')
+// IS_SLASH_P() may read the previous char on Windows, which may be OOB; use IS_SLASH_P_EX() instead
 #define IS_SLASH_P(c)	(*(c) == '/')
+#define IS_SLASH_P_EX(c, first_byte) IS_SLASH_P(c)
 
 #endif