diff --git a/ext/pcre/php_pcre.c b/ext/pcre/php_pcre.c
index 5671957daac52..5150ce97202a7 100644
--- a/ext/pcre/php_pcre.c
+++ b/ext/pcre/php_pcre.c
@@ -734,6 +734,8 @@ PHPAPI pcre_cache_entry* pcre_get_compiled_regex_cache_ex(zend_string *regex, bo
 #ifdef PCRE2_UCP
 						coptions |= PCRE2_UCP;
 #endif
+						/* The \C escape sequence is unsafe in PCRE2_UTF mode */
+						coptions |= PCRE2_NEVER_BACKSLASH_C;
 				break;
 			case 'J':	coptions |= PCRE2_DUPNAMES;		break;
 
@@ -787,7 +789,11 @@ PHPAPI pcre_cache_entry* pcre_get_compiled_regex_cache_ex(zend_string *regex, bo
 		if (key != regex) {
 			zend_string_release_ex(key, 0);
 		}
-		pcre2_get_error_message(errnumber, error, sizeof(error));
+		if (errnumber == PCRE2_ERROR_BACKSLASH_C_CALLER_DISABLED) {
+			strlcpy((char*)error, "using \\C is incompatible with the 'u' modifier", sizeof(error));
+		} else {
+			pcre2_get_error_message(errnumber, error, sizeof(error));
+		}
 		php_error_docref(NULL,E_WARNING, "Compilation failed: %s at offset %zu", error, erroffset);
 		pcre_handle_exec_error(PCRE2_ERROR_INTERNAL);
 		efree(pattern);
diff --git a/ext/pcre/tests/gh21134.phpt b/ext/pcre/tests/gh21134.phpt
new file mode 100644
index 0000000000000..ddcf19a6e7a27
--- /dev/null
+++ b/ext/pcre/tests/gh21134.phpt
@@ -0,0 +1,15 @@
+--TEST--
+GH-21134: ASan negative-size-param in preg_match_all() with \C + UTF-8 multibyte input
+--CREDITS--
+vi3tL0u1s
+--FILE--
+<?php
+
+$r = preg_match_all("/(.*)\\C/u", "à", $m);
+var_dump($r, $m);
+
+?>
+--EXPECTF--
+Warning: preg_match_all(): Compilation failed: using \C is incompatible with the 'u' modifier at offset 6 in %s on line %d
+bool(false)
+NULL