● Medium · CVSS 5.3

How to Fix CVE-2026-27902: Cross-site scripting in svelte

Q: What is the CVSS score for CVE-2026-27902?

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

Q: Where is the official advisory for CVE-2026-27902?

The vendor advisory is at [https://github.com/sveltejs/svelte/security/advisories/GHSA-qgvg-pr8v-6rr3](https://github.com/sveltejs/svelte/security/advisories/GHSA-qgvg-pr8v-6rr3). The NVD record is at https://nvd.nist.gov/vuln/detail/CVE-2026-27902.

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

CVE-2026-27902 is a cross-site scripting in sveltejs svelte. The fix is to apply the vendor patch noted below.

⚡ At a glance


Severity	5.3 (Medium)
Actively exploited?	No public listing in CISA KEV
Affected	svelte >= 5.53.0, < 5.53.5
Fixed in	See vendor advisory
Type (CWE)	CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

What is CVE-2026-27902?

Svelte performance oriented web framework. Prior to version 5.53.5, errors from transformError were not correctly escaped prior to being embedded in the HTML output, causing potential HTML injection and XSS if attacker-controlled content is returned from transformError. Version 5.53.5 fixes the issue. The CVSS base score is 5.3 (Medium). The official advisory is at https://github.com/sveltejs/svelte/security/advisories/GHSA-qgvg-pr8v-6rr3.

Am I affected?

Check the version of svelte you are running and compare it against the Affected row above (svelte >= 5.53.0, < 5.53.5). If your build sits inside the affected range, you must patch.

Run the version check that fits your platform:


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


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

How to fix CVE-2026-27902

Upgrade svelte 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 svelte
dpkg -s svelte | grep -i ^Version

RHEL / CentOS / Rocky / AlmaLinux


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

SUSE / openSUSE


sudo zypper refresh
sudo zypper update svelte
rpm -q svelte

Complete PowerShell remediation script (Windows)


# Fix script for CVE-2026-27902 affecting svelte
# Run as administrator. Detect -> backup -> upgrade -> verify -> log.

$ErrorActionPreference = "Stop"
$LogPath  = "C:\Logs\CVE-2026-27902-fix-$(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"
    $pkg = Get-Package -Name "*svelte*" -ErrorAction SilentlyContinue
    if ($pkg) { $pkg | Format-Table Name, Version }
    else { Write-Host "Not detected via Get-Package; try winget list" }

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

    Write-Host "[3/4] Apply the upgrade to See vendor advisory"
    winget upgrade --name "svelte" --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 "svelte"
    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-2026-27902 affecting svelte
# Detect -> backup -> upgrade -> verify -> log.

set -euo pipefail
LOG="/var/log/cve-2026-27902-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 svelte 2>/dev/null | grep -i ^Version || echo "svelte not installed via dpkg"
elif command -v rpm >/dev/null; then
    rpm -q svelte || echo "svelte not installed via rpm"
fi

echo "[2/4] Backup configuration"
BACKUP="/root/backup-cve-2026-27902-$(date +%Y%m%d)"
mkdir -p "$BACKUP"
for d in /etc/svelte /etc/svelte.d /etc/svelte.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 svelte
elif command -v dnf >/dev/null; then
    dnf upgrade --refresh -y svelte
elif command -v yum >/dev/null; then
    yum update -y svelte
elif command -v zypper >/dev/null; then
    zypper --non-interactive update svelte
fi

echo "[4/4] Verify the patched build"
if command -v dpkg >/dev/null; then
    dpkg -s svelte 2>/dev/null | grep -i ^Version
elif command -v rpm >/dev/null; then
    rpm -q svelte
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-2026-27902" -Direction Inbound -Action Block -Protocol TCP -LocalPort 80,443 -RemoteAddress Any -Enabled True
Get-NetFirewallRule -DisplayName "Block-CVE-2026-27902"

Disable the affected service (Linux)


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

How to Fix CVE-2026-5781: Minerva (Bundle Sibling) — Minerva (Bundle Sibling)
How to Fix CVE-2026-1860: Critical Vulnerability in Kali Forms, Contact Form & Drag-and-Drop Builder — Critical Vulnerability in Kali Forms, Contact Form & Drag-and-Drop Builder
How to Fix CVE-2026-33077: CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal') , CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')
How to Fix CVE-2026-5790: Cross-Site Scripting in Stel Order , Cross-Site Scripting in Stel Order
How to Fix CVE-2026-3997: Critical Vulnerability in Text Toggle , Critical Vulnerability in Text Toggle

Is CVE-2026-27902 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-2026-27902?

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-2026-27902?

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

Where is the official advisory for CVE-2026-27902?

The vendor advisory is at https://github.com/sveltejs/svelte/security/advisories/GHSA-qgvg-pr8v-6rr3. The NVD record is at https://nvd.nist.gov/vuln/detail/CVE-2026-27902.

References

Official vendor advisory: https://github.com/sveltejs/svelte/security/advisories/GHSA-qgvg-pr8v-6rr3
NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-27902
CISA KEV catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog
Additional reference: https://github.com/sveltejs/svelte/commit/0298e979371bb583855c9810db79a70a551d22b9
Additional reference: https://github.com/sveltejs/svelte/releases/tag/svelte%405.53.5

*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.*