⚠ 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 7.1

How to Fix CVE-2026-33020: Heap buffer overflow in libsixel

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

⚡ At a glance
Severity7.1 (High)
Actively exploited?No public listing in CISA KEV
Affectedsaitoha libsixel < 1.8.7-r1
Fixed inlibsixel 1.8.7-r1
Type (CWE)CWE-122: CWE-122: Heap-based Buffer Overflow

What is CVE-2026-33020?

libsixel is a SIXEL encoder/decoder implementation derived from kmiya's sixel. Versions 1.8.7 and prior contain an integer overflow which leads to a heap buffer overflow via sixel_frame_convert_to_rgb888() in frame.c, where allocation size and pointer offset computations for palettised images (PAL1, PAL2, PAL4) are performed using int arithmetic before casting to size_t. For images whose pixel count exceeds INT_MAX / 4, the overflow produces an undersized heap allocation for the conversion buffer and a negative pointer offset for the normalization sub-buffer, after which sixel_helper_normalize_pixelformat() writes the full image data starting from the invalid pointer, causing massive heap corruption confirmed by ASAN. An attacker providing a specially crafted large palettised PNG can corrupt the heap of the victim process, resulting in a reliable crash and potential arbitrary 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-r1). If your build sits inside that range, you are exposed and should patch.

How to fix CVE-2026-33020

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-r1.

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-2026-33020 affecting libsixel
# Run as administrator. Detect -> backup -> upgrade -> verify -> log.

$ErrorActionPreference = "Stop"
$LogPath  = "C:\Logs\CVE-2026-33020-fix-$(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-r1"
    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-2026-33020 affecting libsixel
# Detect -> backup -> upgrade -> verify -> log.

set -euo pipefail
LOG="/var/log/cve-2026-33020-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-2026-33020-$(date +%Y%m%d)"
mkdir -p "$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-r1)"
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-r1. 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-2026-33020 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?

7.1 (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.*