Heap buffer overflow in zval_undefined_cv() via invalid CV dimension unset during destructor

Description

The following code:

class test
{
    static x$instances;

    public function __construct($id) {
        $this->_id = $id;
    }

    function __destruct() {
        unset(self::$instances[$x]->x);
    }
}
$test = new test(1);
x ^ new test(1);

Resulted in this output:

=================================================================
==354402==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x602000003838 at pc 0x55c9627dd661 bp 0x7fffd13e6510 sp 0x7fffd13e6508
READ of size 8 at 0x602000003838 thread T0
    #0 0x55c9627dd660 in zval_undefined_cv /path/to/php-src/Zend/zend_execute.c:280:21
    #1 0x55c9627dc190 in _zval_undefined_op1 /path/to/php-src/Zend/zend_execute.c:288:9
    #2 0x55c9628019e4 in zend_fetch_dimension_address /path/to/php-src/Zend/zend_execute.c:3000:5
    #3 0x55c9628025b3 in zend_fetch_dimension_address_UNSET /path/to/php-src/Zend/zend_execute.c:3054:2
    #4 0x55c9626f2d8c in ZEND_FETCH_DIM_UNSET_SPEC_VAR_CV_HANDLER /path/to/php-src/Zend/zend_vm_execute.h:32041:2
    #5 0x55c9625abf42 in execute_ex /path/to/php-src/Zend/zend_vm_execute.h:116212:12
    #6 0x55c962586571 in zend_call_function /path/to/php-src/Zend/zend_execute_API.c:1014:3
    #7 0x55c962588782 in zend_call_known_function /path/to/php-src/Zend/zend_execute_API.c:1108:23
    #8 0x55c96295ba5a in zend_call_known_instance_method /path/to/php-src/Zend/zend_API.h:862:2
    #9 0x55c9629592bb in zend_call_known_instance_method_with_0_params /path/to/php-src/Zend/zend_API.h:868:2
    #10 0x55c962958cb1 in zend_objects_destroy_object /path/to/php-src/Zend/zend_objects.c:172:3
    #11 0x55c962956812 in zend_objects_store_del /path/to/php-src/Zend/zend_objects_API.c:181:4
    #12 0x55c9629c01d6 in rc_dtor_func /path/to/php-src/Zend/zend_variables.c:57:2
    #13 0x55c9629c02c4 in i_zval_ptr_dtor /path/to/php-src/Zend/zend_variables.h:45:4
    #14 0x55c9629c0214 in zval_ptr_dtor /path/to/php-src/Zend/zend_variables.c:84:2
    #15 0x55c96284c759 in _zend_hash_del_el_ex /path/to/php-src/Zend/zend_hash.c:1493:3
    #16 0x55c96284b82a in _zend_hash_del_el /path/to/php-src/Zend/zend_hash.c:1520:2
    #17 0x55c96285478a in zend_hash_reverse_apply /path/to/php-src/Zend/zend_hash.c:2236:5
    #18 0x55c96257b410 in shutdown_destructors /path/to/php-src/Zend/zend_execute_API.c:262:4
    #19 0x55c9629d931e in zend_call_destructors /path/to/php-src/Zend/zend.c:1336:3
    #20 0x55c9621da74a in php_request_shutdown /path/to/php-src/main/main.c:1980:3
    #21 0x55c9629ea00b in do_cli /path/to/php-src/sapi/cli/php_cli.c:1158:3
    #22 0x55c9629e4e2c in main /path/to/php-src/sapi/cli/php_cli.c:1362:18
    #23 0x7f4533006d8f  (/lib/x86_64-linux-gnu/libc.so.6+0x29d8f) (BuildId: 4f7b0c955c3d81d7cac1501a2498b69d1d82bfe7)
    #24 0x7f4533006e3f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e3f) (BuildId: 4f7b0c955c3d81d7cac1501a2498b69d1d82bfe7)
    #25 0x55c960e03374 in _start (/path/to/php-src/sapi/cli/php+0x603374) (BuildId: 7e30eed09f929fbd58cc0b481c8117cf438919d5)

0x602000003838 is located 0 bytes to the right of 8-byte region [0x602000003830,0x602000003838)
allocated by thread T0 here:
    #0 0x55c960e865e6 in __interceptor_realloc (/path/to/php-src/sapi/cli/php+0x6865e6) (BuildId: 7e30eed09f929fbd58cc0b481c8117cf438919d5)
    #1 0x55c96242f29b in __zend_realloc /path/to/php-src/Zend/zend_alloc.c:3562:6
    #2 0x55c96242eccc in _erealloc /path/to/php-src/Zend/zend_alloc.c:2801:10
    #3 0x55c962969a6f in pass_two /path/to/php-src/Zend/zend_opcode.c:1069:36
    #4 0x55c9624f9437 in zend_compile_func_decl_ex /path/to/php-src/Zend/zend_compile.c:8721:2
    #5 0x55c9624ef79c in zend_compile_func_decl /path/to/php-src/Zend/zend_compile.c:8743:9
    #6 0x55c9624f232a in zend_compile_stmt /path/to/php-src/Zend/zend_compile.c:11853:4
    #7 0x55c96250ed91 in zend_compile_stmt_list /path/to/php-src/Zend/zend_compile.c:7143:3
    #8 0x55c9624f2200 in zend_compile_stmt /path/to/php-src/Zend/zend_compile.c:11800:4
    #9 0x55c9624f0a42 in zend_compile_class_decl /path/to/php-src/Zend/zend_compile.c:9448:2
    #10 0x55c9624ef61b in zend_compile_top_stmt /path/to/php-src/Zend/zend_compile.c:11775:3
    #11 0x55c9624ef443 in zend_compile_top_stmt /path/to/php-src/Zend/zend_compile.c:11764:4
    #12 0x55c9628e1dbf in zend_compile /path/to/php-src/Zend/zend_language_scanner.l:621:3
    #13 0x55c9628e17e7 in compile_file /path/to/php-src/Zend/zend_language_scanner.l:661:14
    #14 0x55c961bc7d37 in phar_compile_file /path/to/php-src/ext/phar/phar.c:3236:9
    #15 0x55c9629dff39 in zend_execute_script /path/to/php-src/Zend/zend.c:1968:28
    #16 0x55c9621e07ab in php_execute_script_ex /path/to/php-src/main/main.c:2640:13
    #17 0x55c9621e0ca8 in php_execute_script /path/to/php-src/main/main.c:2680:9
    #18 0x55c9629e7ed2 in do_cli /path/to/php-src/sapi/cli/php_cli.c:951:5
    #19 0x55c9629e4e2c in main /path/to/php-src/sapi/cli/php_cli.c:1362:18
    #20 0x7f4533006d8f  (/lib/x86_64-linux-gnu/libc.so.6+0x29d8f) (BuildId: 4f7b0c955c3d81d7cac1501a2498b69d1d82bfe7)

SUMMARY: AddressSanitizer: heap-buffer-overflow /path/to/php-src/Zend/zend_execute.c:280:21 in zval_undefined_cv
Shadow bytes around the buggy address:
  0x0c047fff86b0: fa fa fd fd fa fa fd fd fa fa fd fd fa fa fd fd
  0x0c047fff86c0: fa fa fd fd fa fa fd fd fa fa 00 04 fa fa 00 05
  0x0c047fff86d0: fa fa 00 05 fa fa fd fd fa fa fd fd fa fa fd fd
  0x0c047fff86e0: fa fa fd fd fa fa fd fd fa fa fd fd fa fa fd fa
  0x0c047fff86f0: fa fa 00 00 fa fa 04 fa fa fa 00 fa fa fa fd fa
=>0x0c047fff8700: fa fa 04 fa fa fa 00[fa]fa fa fd fa fa fa fd fa
  0x0c047fff8710: fa fa fd fd fa fa 00 00 fa fa fa fa fa fa fa fa
  0x0c047fff8720: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff8730: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff8740: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff8750: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
==354402==ABORTING

Commit:

d3c5163c25c0bc82b72af2f1b09e9d85d92c3668

Build configuration:

CC="clang" CXX="clang++" CFLAGS="-fsanitize=address -g -O0" CXXFLAGS="-fsanitize=address -g -O0" ./configure --enable-debug --enable-address-sanitizer --disable-shared --with-pic

PHP Version

PHP 8.6.0-dev (cli) (built: Nov  6 2025 19:53:33) (NTS DEBUG)
Copyright (c) The PHP Group
Zend Engine v4.6.0-dev, Copyright (c) Zend Technologies
    with Zend OPcache v8.6.0-dev, Copyright (c), by Zend Technologies

Operating System

Ubuntu 22.04

Smaller reproducer:

class test
{
    static x $instances;
}

$test = new test;
unset($test::$instances[$x]->x);

This branch is entered:

php-src/Zend/zend_execute.c

Lines 2999 to 3000 in 7815ab9

    
           if (type != BP_VAR_W && UNEXPECTED(Z_TYPE_P(container) == IS_UNDEF)) { 
        
           	ZVAL_UNDEFINED_OP1();

But OP1 is not a CV, so we get invalid memory accesses when trying to fetch a CV name.

Interestingly, the following code will trigger "Warning: Undefined variable $foo", but this looks incorrect:

unset($foo[$x]->x);

Possible fix would be to evaluate ZVAL_UNDEFINED_OP1() only when type is BP_VAR_R|BP_VAR_RW:

diff --git a/Zend/zend_execute.c b/Zend/zend_execute.c
index d411dcbc3b9..81373fa6d13 100644
--- a/Zend/zend_execute.c
+++ b/Zend/zend_execute.c
@@ -2996,7 +2996,7 @@ static zend_always_inline void zend_fetch_dimension_address(zval *result, zval *
                }
        } else {
                if (EXPECTED(Z_TYPE_P(container) <= IS_FALSE)) {
-                       if (type != BP_VAR_W && UNEXPECTED(Z_TYPE_P(container) == IS_UNDEF)) {
+                       if ((type == BP_VAR_R || type == BP_VAR_RW) && UNEXPECTED(Z_TYPE_P(container) == IS_UNDEF)) {
                                ZVAL_UNDEFINED_OP1();
                        }
                        if (type != BP_VAR_UNSET) {

Rationale:

The warning should only be triggered in R/RW modes
I believe that in these modes, Z_TYPE_P(container) == IS_UNDEF is only possible when OP1 is a CV. In other modes, an exception would have been thrown, or container would be NULL, not UNDEF.

I think that's just another duplicate of #19160, which I should have merged long ago. I wanted to check the logic again but didn't get around to it.

This still reproduces on PHP-8.4 and PHP-8.5 (heap-buffer-overflow in zval_undefined_cv() with the smaller reproducer above); master and PHP-8.6 are fixed by 8b4ef3a (GH-19160), which was only merged into master. I opened #24788 to backport it to PHP-8.4, with a regression test.

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Heap buffer overflow in zval_undefined_cv() via invalid CV dimension unset during destructor #20403

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	if (type != BP_VAR_W && UNEXPECTED(Z_TYPE_P(container) == IS_UNDEF)) {
	ZVAL_UNDEFINED_OP1();

Heap buffer overflow in zval_undefined_cv() via invalid CV dimension unset during destructor #20403

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arnaud-lb commented on Nov 6, 2025

arnaud-lb commented on Nov 6, 2025

iluuu1994 commented on Nov 6, 2025

Truta446 commented on Oct 27, 2026

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