php2shell/uaf-rce/FINDINGS.md

# CVE-2026-109618 / CVE-2026-109619: Vulnerability Analysis

## Overview

PHP's multipart parser (`rfc1867.c`) and session upload progress handler
(`ext/session/session.c`) contain a use-after-free exploitable over HTTP
against Apache mod_php with **default PHP configuration**. No LFI, no
framework dependency, no non-default settings.

| Property | Value |
|---|---|
| CVE | CVE-2026-109618, CVE-2026-109619 |
| CVSS | 9.2 (both, per Rapid7) |
| Affected | PHP 5.4.0 through 8.3.35, 8.4.0-8.4.26, 8.5.0-8.5.11 |
| Patched | PHP 8.3.36, 8.4.27, 8.5.12 |
| Prerequisites | Apache mod_php (prefork), session.upload_progress.enabled=1 (default) |
| Exploit status | UAF crash confirmed, full RCE not achieved |

## Root cause

### CVE-2026-109618: OOM triggers longjmp past cleanup

When `memory_limit` is exceeded during `php_session_decode()` inside
the RFC1867 progress callback, PHP calls `zend_mm_safe_error()` which
invokes `zend_bailout()`. This is a `longjmp` that unwinds the call
stack without running cleanup code, specifically
`php_var_unserialize_destroy()`.

The state that survives the bailout (in `BG` module globals):

```c
// ext/standard/basic_functions.c
struct {
    uint32_t level;                    // should be 0, stays at 1
    php_unserialize_data_t data;       // freed 8216-byte struct pointer
} unserialize;
```

`php_var_unserialize_init()` allocates a `php_unserialize_data` struct
(8216 bytes = 3 ZendMM pages) and sets `level = 1`.
`php_var_unserialize_destroy()` frees it and sets `level = 0`. When OOM
fires between init and destroy, level stays at 1 and data points to
freed memory.

### CVE-2026-109619: Stale state reused on next request

The request startup flow in `main/main.c` (lines 1784-1838) wraps the
entire block from `zend_activate()` through `zend_activate_modules()` in
one `zend_try/zend_catch`. The execution order:

1. `zend_activate()` (line 1796)
2. `sapi_activate()` (line 1797): multipart parsing and RFC1867 callback
3. `php_hash_environment()` (line 1833)
4. `zend_activate_modules()` (line 1834): module RINITs run here, including `PHP_RINIT_FUNCTION(basic)` which zeros BG state
5. `PG(modules_activated) = 1` (line 1835)

The RFC1867 callback at step 2 runs BEFORE RINIT at step 4. So stale
BG state from the previous request IS used during RFC1867 decode.

When `php_var_unserialize_init()` is called during RFC1867 decode:
- It checks `BG(unserialize).level`
- If level > 0 AND `BG(serialize_lock) == 0`, it returns
  `BG(unserialize).data` (the stale, freed pointer) and increments level

### Why Apache prefork survives (and FPM does not)

**Apache prefork**: The bailout is caught by Apache's request handler.
The child process continues its event loop. BG globals persist because
they are module-level, not request-scoped.

**PHP-FPM**: The bailout propagates to `fpm_main.c`'s outer
`zend_try/zend_catch`. The catch block sets `exit_status = 70` and the
worker exits. A new clean worker is spawned.

## OOM trigger mechanism

A session file with 33000 entries (~682KB) forces
`nTableSize = 65536` during decode. The `arData` allocation:
65536 * 36 = 2,359,296 bytes (~2.25MB). With `memory_limit = 4MB`,
this exceeds the limit during `HashTable` resize in
`zend_hash_real_init_mixed_ex()`.

The OOM triggers during `sapi_activate()` -> RFC1867 callback ->
`php_session_initialize()` -> `php_session_decode()` -> unserialize ->
`HashTable` resize.

After OOM:
- `php_session_cancel_decode()` calls `php_session_destroy()`
- Session file is DELETED (on unpatched PHP)
- `BG(unserialize).level = 1`, `.data` = freed pointer to 8216-byte struct
- Worker survives, serves new connections

## The UAF mechanism

### php_unserialize_data struct layout (8216 bytes, 3 pages)

```
Offset  Field                   Size    Notes
------  -----                   ----    -----
0       last (var_entries*)      8       Points to inline entries
8       first_dtor              8       NULL initially
16      last_dtor               8       NULL initially
24      allowed_classes         8       NULL = allow all
32      ref_props               8       NULL
40      cur_depth               4       0
44      (padding)               4
48      max_depth               8       From ini setting
56      entries.used_slots      8       0 initially
64      entries.next            8       NULL
72      entries.data[1018]      8144    zval* array
8216    (end)
```

### Why var_push crashes

After OOM, the heap is destroyed and rebuilt for the next request. The
stale pointer references freed pages in the ZendMM chunk. Filler form
fields in the crash trigger create 3-page `zend_string` allocations
that consume pages sequentially. When one lands on the stale struct's
pages (~page 288 of 512), the `zend_string` GC header overlaps the
struct's pointer fields:

```
Struct offset 0 (last)      <- gc_refcount(4) + gc_type_info(4)
Struct offset 8 (first_dtor) <- hash(8)
Struct offset 16 (last_dtor) <- len(8)
Struct offset 24+            <- string value (attacker-controlled)
```

`var_push` (var_unserializer.re line 118) reads `(*var_hashx)->last`:
```c
var_entries *var_hash = (*var_hashx)->last;  // = GC header ~= 0x0000030600000001
var_hash->data[var_hash->used_slots++] = rval;  // dereferences invalid address
// SIGSEGV
```

## Confirmed primitives

Tested on PHP 8.3.35 (source build), Apache 2.4.58 (prefork,
MaxRequestWorkers=1), Ubuntu 24.04, x86_64:

| Primitive | Status | Notes |
|---|---|---|
| OOM during RFC1867 session decode | Confirmed | 33000-entry session, 4MB limit |
| Apache prefork worker survival | Confirmed | Same PID after OOM |
| Stale BG(unserialize) state | Confirmed | level=1, data=freed ptr (GDB) |
| Page reclamation via filler spray | Confirmed | 100 x 8200B fields |
| UAF crash (SIGSEGV in var_push) | Confirmed | Reproducible, first attempt |
| Full RCE | Not achieved | GC header overlap blocks pointer control |

## Exploitation constraints

### Why full RCE is not achieved

The struct's three pointer fields at offsets 0-23 overlap with the
`zend_string` header, which is set by the allocator:

- `last` (offset 0) = `gc_refcount` + `gc_type_info`: the allocator sets
  refcount=1 and type_info=IS_STRING with GC flags. This produces a low,
  invalid address like `0x0000030600000001`. The attacker cannot control
  these bytes.

- `first_dtor` (offset 8) = `hash`: lazily computed, initially 0 (NULL).
  This is safe: `var_destroy` checks `first_dtor != NULL` before
  iterating the dtor chain.

- `last_dtor` (offset 16) = `len`: the string length (e.g., 8200 =
  `0x2008`). This is also a low, unmapped address.

The attacker controls bytes from offset 24 onward (`allowed_classes`,
`ref_props`, `max_depth`, `entries.used_slots`, `entries.next`,
`entries.data[]`), but `var_push` crashes reading `last` (offset 0)
before any of these fields are reached.

### What would be needed for full RCE

1. **Info leak**: A separate vulnerability to leak a valid heap address.
   The leaked address would need to be placed at offset 0 of the
   allocation, which is impossible with a `zend_string` overlay (GC
   header is set by the allocator).

2. **Alternative allocation type**: A different 3-page (8216-byte)
   allocation where the attacker controls offset 0. No such allocation
   type has been identified during multipart parsing.

3. **Alternative reclamation path**: A way to overwrite only the struct
   fields WITHOUT overlapping a `zend_string` header. This would require
   a non-contiguous write primitive.

## The patch (PHP 8.3.36, 8.4.27, 8.5.12)

Three independent defense layers, any one sufficient:

### 1. php_basic_early_rinit()

New function called from `php_request_startup()` AFTER `zend_activate()`
but BEFORE `sapi_activate()`:

```c
PHPAPI void php_basic_early_rinit(void) {
    BG(serialize_lock) = 0;
    memset(&BG(serialize), 0, sizeof(BG(serialize)));
    memset(&BG(unserialize), 0, sizeof(BG(unserialize)));
}
```

Eliminates the stale state window entirely. Even if OOM occurs, the
BG state is zeroed before RFC1867 processing on the next request.

### 2. Delayed OOM

`zend_bailout()` on OOM is completely removed. The allocator now either:
- Returns NULL (fallible mode) for checked allocations
- Succeeds beyond the soft limit (non-fallible mode) and queues a
  delayed error via `EG(delayed_oom)` + VM interrupt

No longjmp means all cleanup code runs normally, including
`php_var_unserialize_destroy()`.

A hard limit (`memory_hard_limit_percent`, default 200%) calls
`_exit(137)` as a safety net.

### 3. Session handler hardening

- `PS(in_rfc1867_update)` flag forces `incomplete_classes_only` mode
  during RFC1867 session decode (no `__wakeup`/`__unserialize`)
- `php_rinit_session_globals()` called at `MULTIPART_EVENT_START`
- `$_SESSION` cleanup at `MULTIPART_EVENT_END`
- `PG(modules_activated)` check prevents RFC1867 callback during
  `request_parse_body()`
- ~30 `ZEND_HAS_OOM()` checks throughout `rfc1867.c` for graceful
  handling of allocation failures

## Why this does not apply to latest PHP

The latest stable PHP versions (8.3.36+, 8.4.27+, 8.5.12+) include all
three defense layers. The patch is comprehensive:

- Layer 1 alone blocks the attack (no stale state)
- Layer 2 alone blocks the attack (no longjmp, cleanup runs)
- Layer 3 alone limits exploitability (no object instantiation)

No bypass path has been identified for any of the three layers.