How to Fix CVE-2026-29178: Ssrf in lemmy

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

CVE-2026-29178 is a SSRF in LemmyNet lemmy. The fix is to apply the vendor patch noted below.

What is CVE-2026-29178?

Lemmy, a link aggregator and forum for the fediverse, is vulnerable to server-side request forgery via a dependency on activitypub_federation, a framework for ActivityPub federation in Rust. Prior to version 0.19.16, the GET /api/v4/image/{filename} endpoint is vulnerable to unauthenticated SSRF through parameter injection in the file_type query parameter. An attacker can inject arbitrary query parameters into the internal request to pict-rs, including the proxy parameter which causes pict-rs to fetch arbitrary URLs. This issue has been patched in version 0.19.16. The CVSS base score is 7.7 (High), which puts this in the upper risk band and warrants a fast patch cycle. The official advisory is at https://github.com/LemmyNet/lemmy/security/advisories/GHSA-jvxv-2jjp-jxc3.

Am I affected?

Check the version of lemmy you are running and compare it against the Affected row above (lemmy < 0.19.16). If your build sits inside the affected range, you must patch.

Run the version check that fits your platform:

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

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

How to fix CVE-2026-29178

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

RHEL / CentOS / Rocky / AlmaLinux

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

SUSE / openSUSE

sudo zypper refresh
sudo zypper update lemmy
rpm -q lemmy

Node.js / npm

# Update the affected package in your project
npm install lemmy@latest
npm ls lemmy
npm audit fix

Complete PowerShell remediation script (Windows)

# Fix script for CVE-2026-29178 affecting lemmy
# Run as administrator. Detect -> backup -> upgrade -> verify -> log.

$ErrorActionPreference = "Stop"
$LogPath  = "C:\Logs\CVE-2026-29178-fix-$(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"
    $pkg = Get-Package -Name "*lemmy*" -ErrorAction SilentlyContinue
    if ($pkg) { $pkg | Format-Table Name, Version }
    else { Write-Host "Not detected via Get-Package; try winget list" }

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

    Write-Host "[3/4] Apply the upgrade to See vendor advisory"
    winget upgrade --name "lemmy" --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 "lemmy"
    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-2026-29178 affecting lemmy
# Detect -> backup -> upgrade -> verify -> log.

set -euo pipefail
LOG="/var/log/cve-2026-29178-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 lemmy 2>/dev/null | grep -i ^Version || echo "lemmy not installed via dpkg"
elif command -v rpm >/dev/null; then
    rpm -q lemmy || echo "lemmy not installed via rpm"
fi

echo "[2/4] Backup configuration"
BACKUP="/root/backup-cve-2026-29178-$(date +%Y%m%d)"
mkdir -p "$BACKUP"
for d in /etc/lemmy /etc/lemmy.d /etc/lemmy.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 lemmy
elif command -v dnf >/dev/null; then
    dnf upgrade --refresh -y lemmy
elif command -v yum >/dev/null; then
    yum update -y lemmy
elif command -v zypper >/dev/null; then
    zypper --non-interactive update lemmy
fi

echo "[4/4] Verify the patched build"
if command -v dpkg >/dev/null; then
    dpkg -s lemmy 2>/dev/null | grep -i ^Version
elif command -v rpm >/dev/null; then
    rpm -q lemmy
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-2026-29178" -Direction Inbound -Action Block -Protocol TCP -LocalPort 80,443 -RemoteAddress Any -Enabled True
Get-NetFirewallRule -DisplayName "Block-CVE-2026-29178"

Disable the affected service (Linux)

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

Related fixes

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

• How to Fix CVE-2026-39847: Emmett has a path traversal in internal assets handler in emmett — Emmett has a path traversal in internal assets handler in emmett
• How to Fix CVE-2026-23479: Use After Free in redis — Use After Free in redis
• How to Fix CVE-2026-2184: Command Injection in Certificate Generation System , Command Injection in Certificate Generation System
• How to Fix CVE-2026-29786: Path traversal in node-tar , Path traversal in node-tar
• How to Fix CVE-2026-7645: Path Traversal in sublinear-time-solver , Path Traversal in sublinear-time-solver

Is CVE-2026-29178 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-2026-29178?

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-2026-29178?

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

Where is the official advisory for CVE-2026-29178?

The vendor advisory is at https://github.com/LemmyNet/lemmy/security/advisories/GHSA-jvxv-2jjp-jxc3. The NVD record is at https://nvd.nist.gov/vuln/detail/CVE-2026-29178.

References

• Official vendor advisory: https://github.com/LemmyNet/lemmy/security/advisories/GHSA-jvxv-2jjp-jxc3
• NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-29178
• CISA KEV catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog
• Additional reference: https://github.com/LemmyNet/lemmy/commit/f47a03f56d1797bceab5f34b6f624c91cecd5871

*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.*