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MEDIUM severity

CVE-2020-35905 — futures-util

MEDIUM

CVE-2020-35905 is a medium-severity (CVSS 4.7) CWE-362 vulnerability in futures-util. 1 public exploit or proof-of-concept reference is indexed, so weaponization risk is real. A fix is available for futures-util — see the affected versions and patch details below.

MutexGuard::map can cause a data race in safe code

Also known asGHSA-rh4w-94hh-9943RUSTSEC-2020-0059
Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
1 known
Exploitation data as of Nov 6, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗
0.3%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs18th percentile — riskier than 18% of all scored CVEsHighest risk
0.00%0.25%0.51%0.76%0.1%0.3%May 26Sep 26Nov 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

1 pkg affected
🦀futures-util

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects crates.io packages — download data is not available via public APIs for these ecosystems.

Description

Affected versions of the crate had a Send/Sync implementation for MappedMutexGuard that only considered variance on T, while MappedMutexGuard dereferenced to U.

This could of led to data races in safe Rust code when a closure used in MutexGuard::map() returns U that is unrelated to T.

The issue was fixed by fixing Send and Sync implementations, and by adding a PhantomData<&'a mut U> marker to the MappedMutexGuard type to tell the compiler that the guard is over U too.

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
🦀crates.iofutures-util≥ 0.3.2&&< 0.3.70.3.7cargo update -p futures-util --precise 0.3.7

Affected Products

1 product · 1 configurations
Application
future-utilsrust-lang
< 0.3.7
range
Exploits & PoCs
1

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.

An issue was discovered in the futures-util crate before 0.3.7 for Rust.…

rustsec.orgNVDJan 2021

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for futures-util, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update futures-util to 0.3.7 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2020-35905 is resolved across your whole dependency graph.

  3. Workarounds

    If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Frequently Asked Questions

Affected versions of the crate had a Send/Sync implementation for MappedMutexGuard that only considered variance on T, while MappedMutexGuard dereferenced to U. This could of led to data races in safe Rust code when a closure used in MutexGuard::map() returns U that is unrelated to T. The issue was fixed by fixing Send and Sync implementations, and by adding a PhantomData<&'a mut U> marker to the MappedMutexGuard type to tell the compiler that the guard is over U too.
O3 Security · Impact-Aware SCA

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CVE-2020-35905: futures-util — Fixed in 0.3.7