⚠ Reference material — not professional advice. Test in staging, back up first, verify against your specific version. Use your own judgment for your environment.
● Critical · CVSS 9

How to Fix CVE-2026-33805: Cwe-644: improper neutralization of http headers flaw in @fastify/http-proxy

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

⚡ At a glance
Severity9 (Critical)
Actively exploited?No public listing in CISA KEV
Affected@fastify/reply-from 0 to <12.6.2; @fastify/reply-from @fastify/http-proxy 0 to <11.4.4
Fixed in@fastify/reply-from 12.6.2; @fastify/http-proxy 11.4.4
Type (CWE)CWE-644: CWE-644: Improper Neutralization of HTTP Headers for Scripting Syntax

What is CVE-2026-33805?

@fastify/reply-from v12.6.1 and earlier and @fastify/http-proxy v11.4.3 and earlier process the client's Connection header after the proxy has added its own headers via rewriteRequestHeaders. This allows attackers to retroactively strip proxy-added headers from upstream requests by listing them in the Connection header value. Any header added by the proxy for routing, access control, or security purposes can be selectively removed by a client. @fastify/http-proxy is also affected as it delegates to @fastify/reply-from.

Am I affected?

Run the version check that matches your platform:


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

Compare what you see against the Affected row above (@fastify/reply-from 0 to <12.6.2; @fastify/reply-from @fastify/http-proxy 0 to <11.4.4). If your build sits inside that range, you are exposed and should patch.

How to fix CVE-2026-33805

The primary fix is to upgrade @fastify/http-proxy to the patched build. Use the commands for your platform below; the patched versions listed in the vendor advisory are: @fastify/reply-from 12.6.2; @fastify/http-proxy 11.4.4.

Ubuntu / Debian


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

RHEL / CentOS / Rocky / AlmaLinux


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

SUSE / openSUSE


sudo zypper patch --category security
rpm -q fastify

Complete PowerShell remediation script (Windows)


# Fix script for CVE-2026-33805 affecting @fastify/http-proxy
# Run as administrator. Detect -> backup -> upgrade -> verify -> log.

$ErrorActionPreference = "Stop"
$LogPath  = "C:\Logs\CVE-2026-33805-fix-$(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 @fastify/http-proxy"
    $pkg = winget list --id "_fastify_http_proxy" 2>$null
    Write-Host $pkg

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

    Write-Host "[3/4] Applying upgrade to @fastify/reply-from 12.6.2; @fastify/http-proxy 11.4.4"
    winget upgrade --id "_fastify_http_proxy" --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 "_fastify_http_proxy"
    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-2026-33805 affecting @fastify/http-proxy
# Detect -> backup -> upgrade -> verify -> log.

set -euo pipefail
LOG="/var/log/cve-2026-33805-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 fastify 2>/dev/null | grep -i version || echo "fastify not installed via dpkg"
elif command -v rpm >/dev/null; then
    rpm -q fastify || echo "fastify not installed via rpm"
fi

echo "[2/4] Backing up configuration"
BACKUP="/root/backup-cve-2026-33805-$(date +%Y%m%d)"
mkdir -p "$BACKUP"
for d in /etc/fastify /etc/fastify.d /etc/fastify.conf; do
    [ -e "$d" ] && cp -a "$d" "$BACKUP/" || true
done

echo "[3/4] Applying upgrade (target: @fastify/reply-from 12.6.2; @fastify/http-proxy 11.4.4)"
if command -v apt-get >/dev/null; then
    apt-get update
    apt-get install --only-upgrade -y fastify
elif command -v dnf >/dev/null; then
    dnf upgrade --security -y fastify
elif command -v yum >/dev/null; then
    yum update -y fastify
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 fastify 2>/dev/null | grep -i version
elif command -v rpm >/dev/null; then
    rpm -q fastify
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 fastify
sudo systemctl disable fastify

How to verify the fix worked


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


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

Expected: the reported version is at or above @fastify/reply-from 12.6.2; @fastify/http-proxy 11.4.4. 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-2026-33805 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?

9 (critical). 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.*