⚠ 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 7

How to Fix CVE-2026-33018: Use-after-free in libsixel

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

⚡ At a glance
Severity7 (High)
Actively exploited?No public listing in CISA KEV
Affectedsaitoha libsixel < 1.8.7-rc1
Fixed inlibsixel 1.8.7-rc1
Type (CWE)CWE-416: CWE-416: Use After Free

What is CVE-2026-33018?

libsixel is a SIXEL encoder/decoder implementation derived from kmiya's sixel. Versions 1.8.7 and prior contain a Use-After-Free vulnerability via the load_gif() function in fromgif.c, where a single sixel_frame_t object is reused across all frames of an animated GIF and gif_init_frame() unconditionally frees and reallocates frame->pixels between frames without consulting the object's reference count. Because the public API explicitly provides sixel_frame_ref() to retain a frame and sixel_frame_get_pixels() to access the raw pixel buffer, a callback following this documented usage pattern will hold a dangling pointer after the second frame is decoded, resulting in a heap use-after-free confirmed by ASAN. Any application using sixel_helper_load_image_file() with a multi-frame callback to process user-supplied animated GIFs is affected, with a reliable crash as the minimum impact and potential for code execution.

Am I affected?

Run the version check that matches your platform:


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

Compare what you see against the Affected row above (saitoha libsixel < 1.8.7-rc1). If your build sits inside that range, you are exposed and should patch.

How to fix CVE-2026-33018

The primary fix is to upgrade libsixel to the patched build. Use the commands for your platform below; the patched version listed in the vendor advisory is: libsixel 1.8.7-rc1.

Ubuntu / Debian


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

RHEL / CentOS / Rocky / AlmaLinux


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

SUSE / openSUSE


sudo zypper patch --category security
rpm -q libsixel

Complete PowerShell remediation script (Windows)


# Fix script for CVE-2026-33018 affecting libsixel
# Run as administrator. Detect -> backup -> upgrade -> verify -> log.

$ErrorActionPreference = "Stop"
$LogPath  = "C:\Logs\CVE-2026-33018-fix-$(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 libsixel"
    $pkg = winget list --id "libsixel" 2>$null
    Write-Host $pkg

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

    Write-Host "[3/4] Applying upgrade to libsixel 1.8.7-rc1"
    winget upgrade --id "libsixel" --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 "libsixel"
    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-2026-33018 affecting libsixel
# Detect -> backup -> upgrade -> verify -> log.

set -euo pipefail
LOG="/var/log/cve-2026-33018-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 libsixel 2>/dev/null | grep -i version || echo "libsixel not installed via dpkg"
elif command -v rpm >/dev/null; then
    rpm -q libsixel || echo "libsixel not installed via rpm"
fi

echo "[2/4] Backing up configuration"
BACKUP="/root/backup-cve-2026-33018-$(date +%Y%m%d)"
mkdir -p "$BACKUP"
for d in /etc/libsixel /etc/libsixel.d /etc/libsixel.conf; do
    [ -e "$d" ] && cp -a "$d" "$BACKUP/" || true
done

echo "[3/4] Applying upgrade (target: libsixel 1.8.7-rc1)"
if command -v apt-get >/dev/null; then
    apt-get update
    apt-get install --only-upgrade -y libsixel
elif command -v dnf >/dev/null; then
    dnf upgrade --security -y libsixel
elif command -v yum >/dev/null; then
    yum update -y libsixel
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 libsixel 2>/dev/null | grep -i version
elif command -v rpm >/dev/null; then
    rpm -q libsixel
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 libsixel
sudo systemctl disable libsixel

How to verify the fix worked


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


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

Expected: the reported version is at or above libsixel 1.8.7-rc1. 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-2026-33018 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?

7 (high). 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.*