RUSTSEC-2026-0176: Out-of-bounds read in `nth` / `nth_back` for `PyList` and `PyTuple` iterators
6 affectedPyO3 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.