⚠ Reference material — not professional advice. Test in staging, back up first, verify against your specific version. Use your own judgment for your environment.
● Medium · CVSS 6.9

How to Fix CVE-2026-34941: Out-of-bounds read in wasmtime

By Sai Kiran Pandrala. Last verified: 2026-05-25.

⚡ At a glance
Severity6.9 (Medium)
Actively exploited?No public listing in CISA KEV
Affectedbytecodealliance wasmtime < 24.0.7, >= 25.0.0, < 36.0.7, >= 37.0.0, < 42.0.2, >= 43.0.0, < 44.0.1
Fixed inwasmtime 24.0.7; wasmtime 36.0.7; wasmtime 42.0.2; wasmtime 44.0.1
Type (CWE)CWE-125: CWE-125: Out-of-bounds Read

What is CVE-2026-34941?

Wasmtime is a runtime for WebAssembly. Prior to 24.0.7, 36.0.7, 42.0.2, and 43.0.1, Wasmtime contains a vulnerability where when transcoding a UTF-16 string to the latin1+utf16 component-model encoding it would incorrectly validate the byte length of the input string when performing a bounds check. Specifically the number of code units were checked instead of the byte length, which is twice the size of the code units. This vulnerability can cause the host to read beyond the end of a WebAssembly's linear memory in an attempt to transcode nonexistent bytes.

Am I affected?

Run the version check that matches your platform:


# Linux
dpkg -s wasmtime 2>/dev/null | grep -i version
rpm -q wasmtime 2>/dev/null
wasmtime --version 2>/dev/null

Compare what you see against the Affected row above (bytecodealliance wasmtime < 24.0.7, >= 25.0.0, < 36.0.7, >= 37.0.0, < 42.0.2, >= 43.0.0, < 44.0.1). If your build sits inside that range, you are exposed and should patch.

How to fix CVE-2026-34941

The primary fix is to upgrade wasmtime to the patched build. Use the commands for your platform below; the patched versions listed in the vendor advisory are: wasmtime 24.0.7; wasmtime 36.0.7; wasmtime 42.0.2; wasmtime 44.0.1.

Ubuntu / Debian


sudo apt-get update
sudo apt-get install --only-upgrade wasmtime
wasmtime --version 2>/dev/null || dpkg -s wasmtime | grep -i version

RHEL / CentOS / Rocky / AlmaLinux


sudo dnf upgrade --security wasmtime -y
rpm -q wasmtime

SUSE / openSUSE


sudo zypper patch --category security
rpm -q wasmtime

Complete PowerShell remediation script (Windows)


# Fix script for CVE-2026-34941 affecting wasmtime
# Run as administrator. Detect -> backup -> upgrade -> verify -> log.

$ErrorActionPreference = "Stop"
$LogPath  = "C:\Logs\CVE-2026-34941-fix-$(Get-Date -Format yyyyMMdd-HHmmss).log"
New-Item -ItemType Directory -Force (Split-Path $LogPath) | Out-Null
Start-Transcript -Path $LogPath -Append

try {
    Write-Host "[1/4] Detecting installed version of wasmtime"
    $pkg = winget list --id "wasmtime" 2>$null
    Write-Host $pkg

    Write-Host "[2/4] Backing up configuration"
    $backup = "C:\Backup\wasmtime-$(Get-Date -Format yyyyMMdd)"
    New-Item -ItemType Directory -Force $backup | Out-Null
    Get-ChildItem "C:\ProgramData\wasmtime" -ErrorAction SilentlyContinue |
        Copy-Item -Destination $backup -Recurse -Force -ErrorAction SilentlyContinue

    Write-Host "[3/4] Applying upgrade to wasmtime 24.0.7; wasmtime 36.0.7; wasmtime 42.0.2; wasmtime 44.0.1"
    winget upgrade --id "wasmtime" --silent --accept-source-agreements --accept-package-agreements
    # Fallback: Windows Update for OS-level fixes
    if ($LASTEXITCODE -ne 0) {
        Install-Module -Name PSWindowsUpdate -Force -SkipPublisherCheck -ErrorAction SilentlyContinue
        Import-Module PSWindowsUpdate
        Install-WindowsUpdate -MicrosoftUpdate -AcceptAll -IgnoreReboot
    }

    Write-Host "[4/4] Verifying patched build"
    winget list --id "wasmtime"
    Write-Host "Fix applied. Reboot if prompted."
    exit 0
} catch {
    Write-Error "Patch failed: $_"
    exit 1
} finally {
    Stop-Transcript
}

Complete Bash remediation script (Linux)


#!/usr/bin/env bash
# Fix script for CVE-2026-34941 affecting wasmtime
# Detect -> backup -> upgrade -> verify -> log.

set -euo pipefail
LOG="/var/log/cve-2026-34941-fix-$(date +%Y%m%d-%H%M%S).log"
exec > >(tee -a "$LOG") 2>&1

echo "[1/4] Detecting installed version"
if command -v dpkg >/dev/null; then
    dpkg -s wasmtime 2>/dev/null | grep -i version || echo "wasmtime not installed via dpkg"
elif command -v rpm >/dev/null; then
    rpm -q wasmtime || echo "wasmtime not installed via rpm"
fi

echo "[2/4] Backing up configuration"
BACKUP="/root/backup-cve-2026-34941-$(date +%Y%m%d)"
mkdir -p "$BACKUP"
for d in /etc/wasmtime /etc/wasmtime.d /etc/wasmtime.conf; do
    [ -e "$d" ] && cp -a "$d" "$BACKUP/" || true
done

echo "[3/4] Applying upgrade (target: wasmtime 24.0.7; wasmtime 36.0.7; wasmtime 42.0.2; wasmtime 44.0.1)"
if command -v apt-get >/dev/null; then
    apt-get update
    apt-get install --only-upgrade -y wasmtime
elif command -v dnf >/dev/null; then
    dnf upgrade --security -y wasmtime
elif command -v yum >/dev/null; then
    yum update -y wasmtime
elif command -v zypper >/dev/null; then
    zypper --non-interactive patch --category security
fi

echo "[4/4] Verifying patched build"
if command -v dpkg >/dev/null; then
    dpkg -s wasmtime 2>/dev/null | grep -i version
elif command -v rpm >/dev/null; then
    rpm -q wasmtime
fi
echo "Done. Restart any running daemons that loaded the old library."

If you can't patch immediately

If you cannot apply the patched version today, restrict exposure with one of the following runnable controls. None replace the patch.

Network restriction (Linux, nftables)


# Block inbound traffic to the affected service from untrusted networks
sudo nft add table inet filter
sudo nft 'add chain inet filter input { type filter hook input priority 0 ; }'
sudo nft 'add rule inet filter input tcp dport {443, 80} ip saddr != 10.0.0.0/8 drop'
sudo nft list ruleset

Service-level fallback


# If the affected feature is optional, stop the service until the patch is applied
sudo systemctl stop wasmtime
sudo systemctl disable wasmtime

How to verify the fix worked


# Linux
wasmtime --version 2>/dev/null || dpkg -s wasmtime | grep -i version
rpm -q wasmtime 2>/dev/null || true


# Windows
winget list | findstr /I "wasmtime"
Get-HotFix | Sort-Object InstalledOn -Descending | Select-Object -First 5

Expected: the reported version is at or above wasmtime 24.0.7; wasmtime 36.0.7; wasmtime 42.0.2; wasmtime 44.0.1. Restart any services that loaded the old library (systemctl restart <service> on Linux, restart the Windows service or reboot when prompted). For network appliances, run show version on the device and confirm the build matches the patched release.

Frequently asked questions

Other vulnerabilities in the same area that are worth patching alongside this one:

Is CVE-2026-34941 actually being exploited?

According to the data sources above, no public confirmation of in-the-wild exploitation at this time. Either way, the fix is the same: apply the vendor patch.

Do I need to reboot after patching?

For OS or kernel updates, yes. For most userland packages a systemctl restart <service> is enough. Any process that loaded the old shared library keeps using it until restarted, so when in doubt, reboot.

What is the CVSS score?

6.9 (medium). Refer to the vendor advisory for the exact vector string.

Where is the official advisory?

See the References section at the bottom of this page; the vendor's URL is the authoritative source for affected builds and patched versions.

References

*Written by Sai Kiran Pandrala. Assembled from the official vendor advisory, NVD record, and CISA KEV listing on 2026-05-25. Always confirm against the vendor's advisory before applying changes in production.*