⚠ Reference material — not professional advice. Test in staging, back up first, verify against your specific version. Use your own judgment for your environment.
● High · CVSS 8.6

How to Fix CVE-2026-27730: Ssrf in esm.sh

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

CVE-2026-27730 is a SSRF in esm-dev esm.sh. The fix is to apply the vendor patch noted below.

⚡ At a glance
Severity8.6 (High)
Actively exploited?No public listing in CISA KEV
Affectedesm.sh <= 137
Fixed inSee vendor advisory
Type (CWE)CWE-918: Server-Side Request Forgery (SSRF)

What is CVE-2026-27730?

esm.sh is a no-build content delivery network (CDN) for web development. Versions up to and including 137 have an SSRF vulnerability (CWE-918) in esm.sh’s /http(s) fetch route. The service tries to block localhost/internal targets, but the validation is based on hostname string checks and can be bypassed using DNS alias domains. This allows an external requester to make the esm.sh server fetch internal localhost services. As of time of publication, no known patched versions exist. The CVSS base score is 8.6 (High), which puts this in the upper risk band and warrants a fast patch cycle. The official advisory is at https://github.com/esm-dev/esm.sh/security/advisories/GHSA-p2v6-84h2-5x4r.

Am I affected?

Check the version of esm.sh you are running and compare it against the Affected row above (esm.sh <= 137). If your build sits inside the affected range, you must patch.

Run the version check that fits your platform:


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


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

How to fix CVE-2026-27730

Upgrade esm.sh 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 esm-sh
dpkg -s esm-sh | grep -i ^Version

RHEL / CentOS / Rocky / AlmaLinux


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

SUSE / openSUSE


sudo zypper refresh
sudo zypper update esm-sh
rpm -q esm-sh

Node.js / npm


# Update the affected package in your project
npm install esm.sh@latest
npm ls esm.sh
npm audit fix

Complete PowerShell remediation script (Windows)


# Fix script for CVE-2026-27730 affecting esm.sh
# Run as administrator. Detect -> backup -> upgrade -> verify -> log.

$ErrorActionPreference = "Stop"
$LogPath  = "C:\Logs\CVE-2026-27730-fix-$(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"
    $pkg = Get-Package -Name "*esm.sh*" -ErrorAction SilentlyContinue
    if ($pkg) { $pkg | Format-Table Name, Version }
    else { Write-Host "Not detected via Get-Package; try winget list" }

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

    Write-Host "[3/4] Apply the upgrade to See vendor advisory"
    winget upgrade --name "esm.sh" --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 "esm.sh"
    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-2026-27730 affecting esm.sh
# Detect -> backup -> upgrade -> verify -> log.

set -euo pipefail
LOG="/var/log/cve-2026-27730-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 esm-sh 2>/dev/null | grep -i ^Version || echo "esm-sh not installed via dpkg"
elif command -v rpm >/dev/null; then
    rpm -q esm-sh || echo "esm-sh not installed via rpm"
fi

echo "[2/4] Backup configuration"
BACKUP="/root/backup-cve-2026-27730-$(date +%Y%m%d)"
mkdir -p "$BACKUP"
for d in /etc/esm-sh /etc/esm-sh.d /etc/esm-sh.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 esm-sh
elif command -v dnf >/dev/null; then
    dnf upgrade --refresh -y esm-sh
elif command -v yum >/dev/null; then
    yum update -y esm-sh
elif command -v zypper >/dev/null; then
    zypper --non-interactive update esm-sh
fi

echo "[4/4] Verify the patched build"
if command -v dpkg >/dev/null; then
    dpkg -s esm-sh 2>/dev/null | grep -i ^Version
elif command -v rpm >/dev/null; then
    rpm -q esm-sh
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-2026-27730" -Direction Inbound -Action Block -Protocol TCP -LocalPort 80,443 -RemoteAddress Any -Enabled True
Get-NetFirewallRule -DisplayName "Block-CVE-2026-27730"

Disable the affected service (Linux)


sudo systemctl stop esm-sh 2>/dev/null || true
sudo systemctl disable esm-sh 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 esm-sh 2>/dev/null | grep -i ^Version
rpm -q esm-sh 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 "*esm.sh*" | 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

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

Is CVE-2026-27730 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-2026-27730?

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-2026-27730?

8.6 (High). Vector: CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:N/A:N.

Where is the official advisory for CVE-2026-27730?

The vendor advisory is at https://github.com/esm-dev/esm.sh/security/advisories/GHSA-p2v6-84h2-5x4r. The NVD record is at https://nvd.nist.gov/vuln/detail/CVE-2026-27730.

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.*