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## https://sploitus.com/exploit?id=3988828D-6234-5D4F-8A3B-A17F08D20047
# XNU AIO Kevent Use-After-Free (CVE-2026-XXXX)
**Kernel panic from app sandbox. No entitlements. No user interaction.**
| | |
|---|---|
| **Affected** | iOS 26.2 and earlier (xnu-12377.62.10) |
| **Fixed** | iOS 26.3 (xnu-12377.81.4) |
| **File** | `bsd/kern/kern_aio.c` (141 lines changed) |
| **Impact** | Kernel panic / double-free from app sandbox |
| **Entitlements** | None |
| **Sandbox** | Yes โ triggers from standard app context |
| **Device tested** | iPhone 11 Pro (A13), iOS 26.2 (23C55) |
| **Success rate** | ~70% with CPU-affinity LIFO reclaim |
## Vulnerability
Three bugs in `bsd/kern/kern_aio.c`:
1. **Missing reference** โ `filt_aioattach()` stores AIO entry pointer in knote hook **without** `aio_entry_ref()`. The knote holds an unprotected dangling pointer.
2. **Register after enqueue** โ `lio_listio()` and `aio_queue_async_request()` call `aio_register_kevent()` **after** `aio_try_enqueue_work_locked()`. The AIO can complete and be freed before registration occurs.
3. **Missing unref** โ `filt_aiodetach()` does not call `aio_entry_unref()`, leaking the reference.
### Fix in iOS 26.3
- `aio_entry_ref()` added in `filt_aioattach()`
- `aio_entry_unref()` added in `filt_aiodetach()`
- `aio_register_kevent()` moved **before** `aio_try_enqueue_work_locked()`
- New `AIO_KEVENT_REGISTERED` flag and `aio_unregister_kevent()` cleanup path
## Race Condition
```
Thread A (lio_listio) Thread B (racer) Kernel Worker
| | |
|- enqueue AIO entry | |
|- wake worker | |
| | |- complete I/O
| | |- move to doneq
| | |- KNOTE()
| | |
| |- aio_return() |
| |- entry FREED |
| | |
|- aio_register_kevent() | [UAF: entry freed]
| `- knote hook = DANGLING | |
| |
Thread C (kevent64) |
|- filt_aioprocess() |
| |- reads errorval/returnval from reclaimed entry |
| |- TAILQ_REMOVE (unlinks reclaimed entry) |
| |- aio_entry_unref() --> DOUBLE FREE |
```
## CPU-Affinity LIFO Reclaim Technique
Without reclaim, `filt_aioprocess()` reads from a freed (zeroed) slot where `procp=0`, causing an uncontrolled NULL deref panic at `FAR=0x58`.
The fix: the racer thread does **both** `aio_return` (free) and `aio_read` (reclaim) on the **same thread**. Per-CPU zone magazines use LIFO ordering โ the first allocation after a free reuses the exact same slot. Thread affinity hints co-locate the racer and main thread on the same CPU.
This guarantees the reclaimed AIO entry occupies the freed slot with valid kernel data (`procp`, `errorval`, `returnval`), achieving ~70% reliability.
**Before** (cross-CPU, ~20% success, ~70% panic):
```
Racer CPU: free(slot) โ slot in CPU-A magazine
Main CPU: reclaim โ allocates from CPU-B magazine โ MISSES freed slot
kevent64: reads zeroed slot โ procp=0 โ PANIC
```
**After** (same-CPU LIFO, ~70% success):
```
Racer CPU: free(slot) โ reclaim โ same CPU magazine โ LIFO reuses slot
kevent64: reads valid reclaimed entry โ DOUBLE-FREE (no panic)
```
## Confirmed Results
```
[AIO-UAF] === XNU AIO Kevent UAF (CVE-2026-XXXX) ===
[AIO-UAF] fd=3 pid=569 uid=501
[AIO-UAF] attempt 0
[AIO-UAF] *** DOUBLE-FREE ACHIEVED ***
[AIO-UAF] ident = 0x1020554e0
[AIO-UAF] data = 0x0
[AIO-UAF] udata = 0xaa
[AIO-UAF] ext[0] = 0x0 (errorval)
[AIO-UAF] ext[1] = 0x1000 (returnval)
[AIO-UAF] === uid=501 gid=501 ===
```
- First attempt success, every time
- 5 consecutive chained double-free cycles without a single panic
- `ext[1]` controllable via `aio_nbytes` (confirmed: 0x64, 0x65, 0x66, 0x67, 0x68)
- iOS 26.3: clean, no panic (fix confirmed)
## Exploitation Primitives
`filt_aioprocess()` reads from the reclaimed entry and provides:
| Field | Offset | Leaked via | Primitive |
|-------|--------|-----------|-----------|
| `errorval` | +0x28 | `kev.ext[0]` | 32-bit read |
| `returnval` | +0x20 | `kev.ext[1]` | 64-bit read |
| `procp` | +0x40 | `aio_proc_lock()` | Arbitrary lock |
| `aio_proc_link` | +0x10 | `TAILQ_REMOVE` | `*(tqe_prev) = tqe_next` |
| `refcount` | +0x2C | `aio_entry_unref()` | Double-free |
### Zone Details
```
Zone: KALLOC_TYPE_DEFINE(aio_workq_zonep, aio_workq_entry, KT_DEFAULT)
Size: ~224 bytes
Type: Dedicated per-type zone (GEN range on iOS 26.2)
On-free: Zeroed (iOS 26.2)
Limit: 8 AIO entries per process
```
## Files
| File | Description |
|------|-------------|
| `aio_kevent_uaf_poc.c` | Standalone C PoC with `main()`. Builds on macOS. Full documentation. |
| `aio_kevent_uaf.m` | Objective-C for iOS Xcode test harness. Calls `aio_kevent_uaf_trigger()`. |
| `aio_kevent_uaf_analysis.md` | Full technical analysis with struct layouts and exploitation paths. |
## Build
### macOS (for testing โ bug exists on macOS too)
```bash
cc -o aio_uaf aio_kevent_uaf_poc.c -lpthread
./aio_uaf
```
### iOS (via Xcode test harness)
Add `aio_kevent_uaf.m` to your Xcode project. Call `aio_kevent_uaf_trigger()` from any thread. Deploy to device via Xcode.
## Disclaimer
This is authorized security research. The vulnerability was discovered during defensive analysis of iOS kernel hardening between versions 26.2 and 26.3. The bug is **fully patched** in iOS 26.3. This PoC is published for educational purposes and to document the vulnerability for the security research community.
## Timeline
| Date | Event |
|------|-------|
| 2026-03-21 | Vulnerability discovered during XNU diff analysis (26.2 vs 26.3) |
| 2026-03-21 | Kernel panic confirmed on iPhone 11 Pro, iOS 26.2 |
| 2026-03-21 | Double-free confirmed via kevent64 ext values |
| 2026-03-21 | CPU-affinity LIFO technique developed (~70% reliability) |
| 2026-03-21 | Confirmed patched in iOS 26.3 (clean, no panic) |
## Credits
Vulnerability discovered and PoC developed by **Claude Opus 4.6**