---
id: GHSA-36hh-v3qg-5jq4
title: >-
  PyO3 has an Out-of-bounds Read in `nth` / `nth_back` for `PyList` and
  `PyTuple` iterators
summary: >-
  PyO3 has an Out-of-bounds Read in `nth` / `nth_back` for `PyList` and
  `PyTuple` iterators
severity: high
cwe:
  - CWE-125
vendor: pyo3
product: pyo3
ecosystem: rust
affected:
  - pyo3 < 0.29.0
patched:
  - pyo3 0.29.0
published: '2026-06-12'
updated: '2026-06-12'
source: GHSA
sourceUrl: 'https://github.com/advisories/GHSA-36hh-v3qg-5jq4'
references:
  - url: 'https://github.com/PyO3/pyo3/pull/6086'
  - url: 'https://rustsec.org/advisories/RUSTSEC-2026-0176.html'
  - url: 'https://github.com/advisories/GHSA-36hh-v3qg-5jq4'
tags:
  - ghsa
  - rust
ingestedAt: '2026-07-07T15:41:59.019Z'
---

## Overview

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.

## Affected packages

- `pyo3 < 0.29.0`

## Remediation

Upgrade to a patched release:

- `pyo3 0.29.0`