CVE-2026-34987: Wasmtime is a runtime for WebAssembly

This month

CRITICAL 9.9UPGRADE

CVE-2026-34987 is a critical-severity flaw (CVSS 9.9) affecting wasmtime. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Wasmtime is a runtime for WebAssembly. From 25.0.0 to before 36.0.7, 42.0.2, and 43.0.1, Wasmtime with its Winch (baseline) non-default compiler backend may allow properly constructed guest Wasm to access host memory outside of its linear-memory sandbox. This vulnerability requires use of the Winch compiler (-Ccompiler=winch). By default, Wasmtime uses its Cranelift backend, not Winch. With Winch, the same incorrect assumption is present in theor

Why this priority

Critical severity

A fixed release or distro patch exists.

Risk facts

CVSS score	9.9 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	Low
User interaction	None
EPSS exploit chance	0.3% (top 80%)
CISA exploited list	No
Weakness	CWE-125, CWE-787
Published	2026-04-09
Last modified	2026-10-26

Publisher (CNA): GitHub, Inc. · Vendor: bytecodealliance

Last updated 2026-10-26 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-04-09 19:16 UTCPublished CVE record published
2026-10-26 22:10 UTCUpdated Record last modified

bytecodealliance wasmtime UPGRADE

Affected: >= 25.0.0 and < 36.0.7 · >= 37.0.0 and < 42.0.2 · 43.0.0

Fixed in: 36.0.7, 42.0.2

Safe to downgrade to: a release before 25.0.0

1. Check installed version

apt-cache policy wasmtime || rpm -q wasmtime || wasmtime --version

2. UPGRADE to 36.0.7 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade wasmtime

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade wasmtime

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 25.0.0 (Debian / Ubuntu)

apt-cache madison wasmtime sudo apt install wasmtime=<VERSION_BEFORE_25.0.0> sudo apt-mark hold wasmtime

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list wasmtime sudo dnf downgrade wasmtime-<VERSION_BEFORE_25.0.0> sudo dnf versionlock add wasmtime

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-34987?

wasmtime: upgrade to 36.0.7, 42.0.2. Exact commands for each OS are on this page.

Is CVE-2026-34987 being exploited?

CVE-2026-34987 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-34987 affect?

CVE-2026-34987 affects wasmtime. Severity is critical (CVSS 9.9). Wasmtime is a runtime for WebAssembly. From 25.0.0 to before 36.0.7, 42.0.2, and 43.0.1, Wasmtime with its Winch (baseline) non-default compiler backend may allow properly constructed guest Wasm to access host memory outside of its linear-m

How urgent is CVE-2026-34987?

Priority 40/100. Critical severity

Related CVEs

CVE-2026-34946 HIGH 7.5 UPGRADE Wasmtime is a runtime for WebAssembly
CVE-2026-35186 HIGH 7.5 UPGRADE Wasmtime is a runtime for WebAssembly
CVE-2026-34971 HIGH 7.8 UPGRADE Wasmtime is a runtime for WebAssembly
CVE-2026-34945 MEDIUM 6.5 UPGRADE Wasmtime is a runtime for WebAssembly
CVE-2026-34988 MEDIUM 6.3 UPGRADE Wasmtime is a runtime for WebAssembly

References

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