How to Fix CVE-2026-27204: Denial of service via resource consumption in wasmtime

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

CVE-2026-27204 is a denial of service via resource consumption in bytecodealliance wasmtime. The fix is to apply the vendor patch noted below.

What is CVE-2026-27204?

Wasmtime is a runtime for WebAssembly. Prior to versions 24.0.6, 36.0.6, 4.0.04, 41.0.4, and 42.0.0, Wasmtime's implementation of WASI host interfaces are susceptible to guest-controlled resource exhaustion on the host. Wasmtime did not appropriately place limits on resource allocations requested by the guests. This is a Denial of Service vector. Wasmtime 24.0.6, 36.0.6, 40.0.4, 41.0.4, and 42.0.0 have all been released with the fix for this issue. These versions do not prevent this issue in their default configuration to avoid breaking preexisting behaviors. All versions of Wasmtime have appropriate knobs to prevent this behavior, and Wasmtime 42.0.0-and-later will have these knobs tuned by default to prevent this issue from happening. There are no known workarounds for this issue without upgrading. Embedders are recommended to upgrade and configure their embeddings as necessary to prevent possibly-malicious guests from triggering this issue. The CVSS base score is 6.9 (Medium). The official advisory is at https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-852m-cvvp-9p4w.

Am I affected?

Check the version of wasmtime you are running and compare it against the Affected row above (wasmtime < 24.0.6; wasmtime >= 25.0.0, < 36.0.6; wasmtime >= 37.0.0, < 40.0.4; wasmtime >= 41.0.0, < 41.0.4). If your build sits inside the affected range, you must patch.

Run the version check that fits your platform:

# Linux package check
dpkg -s wasmtime 2>/dev/null | grep -i ^Version
rpm -q wasmtime 2>/dev/null
command -v wasmtime >/dev/null && wasmtime --version 2>/dev/null

# Windows (PowerShell)
Get-Package -Name "*wasmtime*" -ErrorAction SilentlyContinue | Select-Object Name, Version
winget list --name "wasmtime" 2>$null

How to fix CVE-2026-27204

Upgrade wasmtime to a patched build: See vendor advisory. The vendor advisory is the source of truth for the exact fixed version.

Ubuntu / Debian

sudo apt-get update
sudo apt-get install --only-upgrade wasmtime
dpkg -s wasmtime | grep -i ^Version

RHEL / CentOS / Rocky / AlmaLinux

sudo dnf upgrade --refresh wasmtime -y
# or for older releases:
sudo yum update wasmtime -y
rpm -q wasmtime

SUSE / openSUSE

sudo zypper refresh
sudo zypper update wasmtime
rpm -q wasmtime

Complete PowerShell remediation script (Windows)

# Fix script for CVE-2026-27204 affecting wasmtime
# Run as administrator. Detect -> backup -> upgrade -> verify -> log.

$ErrorActionPreference = "Stop"
$LogPath  = "C:\Logs\CVE-2026-27204-fix-$(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] Detect installed version"
    $pkg = Get-Package -Name "*wasmtime*" -ErrorAction SilentlyContinue
    if ($pkg) { $pkg | Format-Table Name, Version }
    else { Write-Host "Not detected via Get-Package; try winget list" }

    Write-Host "[2/4] Backup 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] Apply the upgrade to See vendor advisory"
    winget upgrade --name "wasmtime" --silent --accept-source-agreements --accept-package-agreements
    if ($LASTEXITCODE -ne 0) {
        # Fallback: pull latest via OS update channel
        Install-Module -Name PSWindowsUpdate -Force -SkipPublisherCheck -ErrorAction SilentlyContinue
        Import-Module PSWindowsUpdate
        Install-WindowsUpdate -MicrosoftUpdate -AcceptAll -IgnoreReboot
    }

    Write-Host "[4/4] Verify the patched build"
    winget list --name "wasmtime"
    Write-Host "Patch 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-2026-27204 affecting wasmtime
# Detect -> backup -> upgrade -> verify -> log.

set -euo pipefail
LOG="/var/log/cve-2026-27204-fix-$(date +%Y%m%d-%H%M%S).log"
exec > >(tee -a "$LOG") 2>&1

echo "[1/4] Detect 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] Backup configuration"
BACKUP="/root/backup-cve-2026-27204-$(date +%Y%m%d)"
mkdir -p "$BACKUP"
for d in /etc/wasmtime /etc/wasmtime.d /etc/wasmtime.conf; do
    [ -e "$d" ] && cp -a "$d" "$BACKUP/" || true
done

echo "[3/4] Apply the upgrade (target: See vendor advisory)"
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 --refresh -y wasmtime
elif command -v yum >/dev/null; then
    yum update -y wasmtime
elif command -v zypper >/dev/null; then
    zypper --non-interactive update wasmtime
fi

echo "[4/4] Verify the 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 service that loaded the old library."

If you can't patch immediately

Apply at least one of the following inline controls until you can deploy the patched build. None replace the upgrade.

Restrict exposure with nftables (Linux)

# Allow only trusted CIDR to reach the affected service ports
sudo nft add table inet filter 2>/dev/null || true
sudo nft 'add chain inet filter input { type filter hook input priority 0 ; }' 2>/dev/null || true
sudo nft 'add rule inet filter input tcp dport {80, 443} ip saddr != 10.0.0.0/8 drop'
sudo nft list ruleset

Block at the host firewall (Windows)

New-NetFirewallRule -DisplayName "Block-CVE-2026-27204" -Direction Inbound -Action Block -Protocol TCP -LocalPort 80,443 -RemoteAddress Any -Enabled True
Get-NetFirewallRule -DisplayName "Block-CVE-2026-27204"

Disable the affected service (Linux)

sudo systemctl stop wasmtime 2>/dev/null || true
sudo systemctl disable wasmtime 2>/dev/null || true

If the vendor advisory lists an official workaround, prefer that wording verbatim. If no workaround is published, the only safe remediation is the patch.

How to verify the fix worked

After upgrading, confirm the installed version matches the patched build and that no old library is still loaded by a long-running process.

# Linux
dpkg -s wasmtime 2>/dev/null | grep -i ^Version
rpm -q wasmtime 2>/dev/null || true
# Pid map check for old library handles
sudo lsof +c0 2>/dev/null | grep -i "DEL.*lib" || true

# Windows
Get-Package -Name "*wasmtime*" | Select-Object Name, Version
Get-HotFix | Sort-Object InstalledOn -Descending | Select-Object -First 5

Expected: the reported version is at or above See vendor advisory. Restart the affected service (systemctl restart <service> on Linux, or restart the Windows service) so it loads the patched binary.

Frequently asked questions

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Is CVE-2026-27204 actively exploited?

There is no public confirmation of exploitation in the wild listed in CISA KEV at the time of this writing. Patch anyway. Public exploits commonly follow disclosure within weeks.

Do I need to reboot after patching CVE-2026-27204?

For kernel and OS-level updates, yes. For most userland packages a systemctl restart <service> is enough on Linux, and restarting the Windows service or app on Windows. Any process that loaded the old library keeps using it until restarted.

What is the CVSS score for CVE-2026-27204?

6.9 (Medium). Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:P/VC:N/VI:N/VA:H/SC:N/SI:N/SA:H.

Where is the official advisory for CVE-2026-27204?

The vendor advisory is at https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-852m-cvvp-9p4w. The NVD record is at https://nvd.nist.gov/vuln/detail/CVE-2026-27204.

References

• Official vendor advisory: https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-852m-cvvp-9p4w
• NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-27204
• CISA KEV catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog
• Additional reference: https://github.com/bytecodealliance/wasmtime/issues/11552
• Additional reference: https://github.com/bytecodealliance/wasmtime/pull/12599
• Additional reference: https://docs.rs/wasmtime-wasi/latest/wasmtime_wasi/struct.WasiCtxBuilder.html#method.max_random_size

*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.*