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Fix GH-23204: use-after-free when __toString() destroys an array argument #23207

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Fix GH-23204: use-after-free when __toString() destroys an array argument #23207

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6c2ad99
Fix GH-23204: use-after-free in implode() when __toString() destroys …
lazerg Aug 10, 2026
127f016
Fix the same use-after-free in strtr() and str_replace()
lazerg Aug 10, 2026
b9443b8
Credit the reporter in the GH-23204 test
lazerg Aug 11, 2026
36daac5
Credit e1abrador and assert the exception class in the GH-23204 test
lazerg Aug 11, 2026
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2 changes: 2 additions & 0 deletions NEWS
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Original file line number Diff line number Diff line change
Expand Up @@ -78,6 +78,8 @@ PHP NEWS
nested arrays). (Lazizbek Ergashev)
. Fixed bug GH-23115 (Stack overflow in compact() with deeply nested
arrays). (Lazizbek Ergashev)
. Fixed bug GH-23204 (Use-after-free in implode(), strtr() and str_replace()
when __toString() destroys an array argument). (Lazizbek Ergashev)

- Streams:
. Fixed bug GH-15836 (Use-after-free when a user stream filter accesses
Expand Down
36 changes: 34 additions & 2 deletions ext/standard/string.c
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Expand Up @@ -983,6 +983,9 @@ PHPAPI void php_implode(const zend_string *glue, HashTable *pieces, zval *return

uint32_t flags = ZSTR_GET_COPYABLE_CONCAT_PROPERTIES(glue);

/* Converting an element may call __toString(), which can destroy pieces. */
GC_TRY_ADDREF(pieces);
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lazerg marked this conversation as resolved.

ZEND_HASH_FOREACH_VAL(pieces, tmp) {
if (EXPECTED(Z_TYPE_P(tmp) == IS_STRING)) {
ptr->str = Z_STR_P(tmp);
Expand Down Expand Up @@ -1042,6 +1045,7 @@ PHPAPI void php_implode(const zend_string *glue, HashTable *pieces, zval *return
}

free_alloca(strings, use_heap);
GC_TRY_DTOR_NO_REF(pieces);
RETURN_NEW_STR(str);
}
/* }}} */
Expand Down Expand Up @@ -3392,7 +3396,12 @@ static void php_strtr_array(zval *return_value, zend_string *str, HashTable *fro
{
if (zend_hash_num_elements(from_ht) < 1) {
RETURN_STR_COPY(str);
} else if (zend_hash_num_elements(from_ht) == 1) {
}

/* Converting a replacement may call __toString(), which can destroy from_ht. */
GC_TRY_ADDREF(from_ht);

if (zend_hash_num_elements(from_ht) == 1) {
zend_long num_key;
zend_string *str_key, *tmp_str, *replace, *tmp_replace;
zval *entry;
Expand Down Expand Up @@ -3421,11 +3430,13 @@ static void php_strtr_array(zval *return_value, zend_string *str, HashTable *fro
}
zend_tmp_string_release(tmp_str);
zend_tmp_string_release(tmp_replace);
return;
break;
} ZEND_HASH_FOREACH_END();
} else {
php_strtr_array_ex(return_value, str, from_ht);
}

GC_TRY_DTOR_NO_REF(from_ht);
}

/* {{{ Translates characters in str using given translation tables */
Expand Down Expand Up @@ -4485,6 +4496,17 @@ static void _php_str_replace_common(
RETURN_THROWS();
}

/* Converting an element may call __toString(), which can destroy the arrays. */
if (search_ht) {
GC_TRY_ADDREF(search_ht);
}
if (replace_ht) {
GC_TRY_ADDREF(replace_ht);
}
if (subject_ht) {
GC_TRY_ADDREF(subject_ht);
}

/* if subject is an array */
if (subject_ht) {
array_init(return_value);
Expand All @@ -4511,6 +4533,16 @@ static void _php_str_replace_common(
if (zcount) {
ZEND_TRY_ASSIGN_REF_LONG(zcount, count);
}

if (search_ht) {
GC_TRY_DTOR_NO_REF(search_ht);
}
if (replace_ht) {
GC_TRY_DTOR_NO_REF(replace_ht);
}
if (subject_ht) {
GC_TRY_DTOR_NO_REF(subject_ht);
}
}

/* {{{ php_str_replace_common */
Expand Down
109 changes: 109 additions & 0 deletions ext/standard/tests/strings/gh23204.phpt
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@@ -0,0 +1,109 @@
--TEST--
GH-23204 (Use-after-free when __toString() destroys the array being read)
--CREDITS--
e1abrador
--FILE--

@iluuu1994 iluuu1994 Aug 11, 2026

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Please credit the reporter within --CREDITS--.

@lazerg lazerg Aug 11, 2026

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Added, credited as iluuu1994.

@iluuu1994 iluuu1994 Aug 11, 2026

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Not me, @e1abrador.

@NickSdot NickSdot Aug 11, 2026

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The docs say the CREDITS section should no longer be used. Should the wording be extended with a "unless reporters have to be credited"?

@lazerg lazerg Aug 11, 2026

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My mistake, sorry. Changed to e1abrador.

@ndossche ndossche Aug 11, 2026

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The docs say the CREDITS section should no longer be used. Should the wording be extended with a "unless reporters have to be credited"?

The reasoning behind this is that git commits can track authors via Co-authored-by, which is complete bs. CREDITS section is a much more precise indication of authorship than a commit-wide author tag. So I purposefully ignore that docs and I believe that line should be removed.

@arnaud-lb arnaud-lb Aug 11, 2026 •
edited
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It looks like that this paragraph of the docs is only considering the case of the commit authors crediting themselves. It makes sense to me that no CREDITS section should be used in this case.

We could expand this paragraph with the case of committing a reproducer that was contributed by someone else.

@NickSdot NickSdot Aug 11, 2026

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I proposed a change in #23215

<?php
class Unset_ implements Stringable {
public function __toString(): string {
global $a;
$a = null;
return "X";
}
}

$a = [new Unset_, 2, 3, 4];
echo "destroyed: ", implode(",", $a), "\n";
var_dump($a);

class Append implements Stringable {
public function __toString(): string {
global $b;
$b[] = str_repeat("y", 32);
return "X";
}
}

$b = [new Append, 2, 3, 4];
echo "appended: ", implode(",", $b), "\n";
echo "count: ", count($b), "\n";

class Boom implements Stringable {
public function __toString(): string {
global $c;
$c = null;
throw new Exception("boom");
}
}

$c = [new Boom, 2, 3, 4];
try {
implode(",", $c);
} catch (Exception $e) {
echo $e::class, ': ', $e->getMessage(), "\n";
}

class UnsetPats implements Stringable {
public function __toString(): string {
global $d;
$d = null;
return "X";
}
}

$d = ["aa" => new UnsetPats, "bb" => "2", "cc" => "3", "dd" => "4"];
echo "strtr: ", strtr("aabbccdd", $d), "\n";

$e = ["aa" => new UnsetPats];
$d = &$e;
echo "strtr single: ", strtr("aabb", $e), "\n";

class UnsetSearch implements Stringable {
public function __toString(): string {
global $f;
$f = null;
return "a";
}
}

$f = [new UnsetSearch, "b", "c", "d"];
echo "str_replace search: ", str_replace($f, "z", "abcd"), "\n";

class UnsetReplace implements Stringable {
public function __toString(): string {
global $g;
$g = null;
return "z";
}
}

$g = [new UnsetReplace, "y", "y", "y"];
echo "str_replace replace: ", str_replace(["a", "b", "c", "d"], $g, "abcd"), "\n";

class UnsetSubject implements Stringable {
public function __toString(): string {
global $h;
$h = null;
return "abcd";
}
}

$h = [new UnsetSubject, "abcd"];
var_dump(str_replace("a", "z", $h));
?>
--EXPECT--
destroyed: X,2,3,4
NULL
appended: X,2,3,4
count: 5
Exception: boom
strtr: X234
strtr single: Xbb
str_replace search: zzzz
str_replace replace: zyyy
array(2) {
[0]=>
string(4) "zbcd"
[1]=>
string(4) "zbcd"
}
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