---
id: RUSTSEC-2026-0409
aliases:
  - GHSA-cc53-p78w-23w9
title: Use-after-free from a comparator that answers inconsistently
summary: Use-after-free from a comparator that answers inconsistently
severity: none
vendor: skiplist
product: skiplist
ecosystem: rust
affected:
  - 'skiplist >= 0.0.0-0, < 1.3.0'
patched:
  - skiplist 1.3.0
published: '2026-10-24'
updated: '2026-10-26'
sourceUpdated: '2026-10-26T14:00:04.241377560Z'
source: OSV
sourceUrl: 'https://osv.dev/vulnerability/RUSTSEC-2026-0409'
references:
  - url: 'https://crates.io/crates/skiplist'
  - url: 'https://rustsec.org/advisories/RUSTSEC-2026-0409.html'
  - url: 'https://github.com/JP-Ellis/rust-skiplist/issues/401'
  - url: 'https://github.com/JP-Ellis/rust-skiplist/pull/407'
tags:
  - osv
  - rust
ingestedAt: '2026-10-27T16:20:12.120Z'
---

## Overview

`OrderedSkipList`, `SkipSet` and `SkipMap` can free a node that is still
linked when a `Comparator` or `Ord` implementation gives different answers for
the same pair of values during one mutating call. A later call reads the freed
memory (CWE-416). A safe `impl Ord` with the default comparator is enough to
reach it.

The mutating traversals record a precursor at each level: the last node whose
link points at a node comparing not-`Less` than the target. The splice code
assumes every recorded precursor's link points at or past the final position.
If a later comparison reports one of those nodes as `Less`, the walk steps
past it and the assumption breaks. Removals such as `take_first`, `take` and
`remove` then rewire the wrong link and free the target while its real
predecessor still points at it. `insert` builds crossed links and wrong skip
distances from the same precursors.

A comparator that is a pure function of its two arguments cannot trigger
this, even if it is not transitive. The trigger is a stateful or
non-deterministic implementation, for example one that reads a counter, a
cache, a clock or a random source.

The flaw was corrected in 1.3.0. The mutating traversals track the link target
of the lowest recorded precursor and panic with "comparator does not implement
a total order" when a `Less` answer contradicts an earlier one. Every splice
site also checks its precursors before the first write. Each detected
contradiction panics before any link, value or length changes, so the
collection stays valid and safe to drop.

## Affected packages

- `skiplist >= 0.0.0-0, < 1.3.0`

## Remediation

Upgrade to a patched release:

- `skiplist 1.3.0`