⚠ 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 4.6

How to Fix CVE-2026-27477: Ssrf in mastodon

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

CVE-2026-27477 is a SSRF in mastodon mastodon. The fix is to apply the vendor patch noted below.

⚡ At a glance
Severity4.6 (Medium)
Actively exploited?No public listing in CISA KEV
Affectedmastodon >= 4.4.0, < 4.4.14; mastodon >= 4.5.0, < 4.5.7
Fixed inSee vendor advisory
Type (CWE)CWE-918: Server-Side Request Forgery (SSRF)

What is CVE-2026-27477?

Mastodon is a free, open-source social network server based on ActivityPub. FASP registration requires manual approval by an administrator. In versions 4.4.0 through 4.4.13 and 4.5.0 through 4.5.6, an unauthenticated attacker can register a FASP with an attacker-chosen base_url that includes or resolves to a local / internal address, leading to the Mastodon server making requests to that address. This only affects Mastodon servers that have opted in to testing the experimental FASP feature by setting the environment variable EXPERIMENTAL_FEATURES to a value including fasp. An attacker can force the Mastodon server to make http(s) requests to internal systems. While they cannot control the full URL that is being requested (only the prefix) and cannot see the result of those requests, vulnerabilities or other undesired behavior could be triggered in those systems. The fix is included in the 4.4.14 and 4.5.7 releases. Admins that are actively testing the experimental "fasp" feature should update their systems. Servers not using the experimental feature flag fasp are not affected. The CVSS base score is 4.6 (Medium). The official advisory is at https://github.com/mastodon/mastodon/security/advisories/GHSA-46w6-g98f-wxqm.

Am I affected?

Check the version of mastodon you are running and compare it against the Affected row above (mastodon >= 4.4.0, < 4.4.14; mastodon >= 4.5.0, < 4.5.7). If your build sits inside the affected range, you must patch.

Run the version check that fits your platform:


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


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

How to fix CVE-2026-27477

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

RHEL / CentOS / Rocky / AlmaLinux


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

SUSE / openSUSE


sudo zypper refresh
sudo zypper update mastodon
rpm -q mastodon

Complete PowerShell remediation script (Windows)


# Fix script for CVE-2026-27477 affecting mastodon
# Run as administrator. Detect -> backup -> upgrade -> verify -> log.

$ErrorActionPreference = "Stop"
$LogPath  = "C:\Logs\CVE-2026-27477-fix-$(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"
    $pkg = Get-Package -Name "*mastodon*" -ErrorAction SilentlyContinue
    if ($pkg) { $pkg | Format-Table Name, Version }
    else { Write-Host "Not detected via Get-Package; try winget list" }

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

    Write-Host "[3/4] Apply the upgrade to See vendor advisory"
    winget upgrade --name "mastodon" --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 "mastodon"
    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-2026-27477 affecting mastodon
# Detect -> backup -> upgrade -> verify -> log.

set -euo pipefail
LOG="/var/log/cve-2026-27477-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 mastodon 2>/dev/null | grep -i ^Version || echo "mastodon not installed via dpkg"
elif command -v rpm >/dev/null; then
    rpm -q mastodon || echo "mastodon not installed via rpm"
fi

echo "[2/4] Backup configuration"
BACKUP="/root/backup-cve-2026-27477-$(date +%Y%m%d)"
mkdir -p "$BACKUP"
for d in /etc/mastodon /etc/mastodon.d /etc/mastodon.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 mastodon
elif command -v dnf >/dev/null; then
    dnf upgrade --refresh -y mastodon
elif command -v yum >/dev/null; then
    yum update -y mastodon
elif command -v zypper >/dev/null; then
    zypper --non-interactive update mastodon
fi

echo "[4/4] Verify the patched build"
if command -v dpkg >/dev/null; then
    dpkg -s mastodon 2>/dev/null | grep -i ^Version
elif command -v rpm >/dev/null; then
    rpm -q mastodon
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-2026-27477" -Direction Inbound -Action Block -Protocol TCP -LocalPort 80,443 -RemoteAddress Any -Enabled True
Get-NetFirewallRule -DisplayName "Block-CVE-2026-27477"

Disable the affected service (Linux)


sudo systemctl stop mastodon 2>/dev/null || true
sudo systemctl disable mastodon 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 mastodon 2>/dev/null | grep -i ^Version
rpm -q mastodon 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 "*mastodon*" | 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-2026-27477 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-2026-27477?

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-2026-27477?

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

Where is the official advisory for CVE-2026-27477?

The vendor advisory is at https://github.com/mastodon/mastodon/security/advisories/GHSA-46w6-g98f-wxqm. The NVD record is at https://nvd.nist.gov/vuln/detail/CVE-2026-27477.

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.*