RUSTSEC-2026-0176: Out-of-bounds read in `nth` / `nth_back` for `PyList` and `PyTuple` iterators

| Details | |
| --- | --- |
| Package | `pyo3` |
| Version | `0.26.0` |
| URL | https://github.com/PyO3/pyo3/pull/6086 |
| Patched Versions | >=0.29.0 |
| Unaffected Versions | <0.24.0 |
| Aliases | [GHSA-36hh-v3qg-5jq4](https://github.com/advisories/GHSA-36hh-v3qg-5jq4) |

PyO3 0.24.0 added optimized implementations of `Iterator::nth` and
`DoubleEndedIterator::nth_back` for the `BoundListIterator` and
`BoundTupleIterator` types. These implementations computed the target index
using unchecked `usize` addition (`index + n`) before bounds-checking against
the sequence length, then read the element via `get_item_unchecked`.

In `nth` methods, a sufficiently large `n` (combined with a non-zero internal
index) could cause the addition to overflow and wrap around, producing a small
"target index" that passed the bounds check and enabling reads at the front
of the `list` or `tuple` of elements previously yielded by the iterator.

In `nth_back` methods, a sufficiently large `n` could cause underflow in a
similar fashion, however would instead allow reads of arbitrary memory past
the end of the `list` or `tuple` storage.

PyO3 0.29.0 has corrected these methods to use checked arithmetic at the
positions which could be at risk of overflow.