⚠ 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 9.1

How to Fix CVE-2026-6100: Use-after-free in CPython

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

⚡ At a glance
Severity9.1 (Critical)
Actively exploited?No public listing in CISA KEV
AffectedPython Software Foundation CPython 0 to <3.15.0
Fixed inCPython 3.15.0
Type (CWE)CWE-416: Use after free

What is CVE-2026-6100?

Use-after-free (UAF) was possible in the lzma.LZMADecompressor, bz2.BZ2Decompressor, and gzip.GzipFile when a memory allocation fails with a MemoryError and the decompression instance is re-used. This scenario can be triggered if the process is under memory pressure. The fix cleans up the dangling pointer in this specific error condition. The vulnerability is only present if the program re-uses decompressor instances across multiple decompression calls even after a MemoryError is raised during decompression.

Am I affected?

Run the version check that matches your platform:


# Linux
dpkg -s python3 2>/dev/null | grep -i version
rpm -q python3 2>/dev/null
python3 --version 2>/dev/null

Compare what you see against the Affected row above (Python Software Foundation CPython 0 to <3.15.0). If your build sits inside that range, you are exposed and should patch.

How to fix CVE-2026-6100

The primary fix is to upgrade CPython to the patched build. Use the commands for your platform below; the patched version listed in the vendor advisory is: CPython 3.15.0.

Ubuntu / Debian


sudo apt-get update
sudo apt-get install --only-upgrade python3
python3 --version 2>/dev/null || dpkg -s python3 | grep -i version

RHEL / CentOS / Rocky / AlmaLinux


sudo dnf upgrade --security python3 -y
rpm -q python3

SUSE / openSUSE


sudo zypper patch --category security
rpm -q python3

Complete PowerShell remediation script (Windows)


# Fix script for CVE-2026-6100 affecting Python
# Run as administrator. Detect -> backup -> upgrade -> verify -> log.

$ErrorActionPreference = "Stop"
$LogPath  = "C:\Logs\CVE-2026-6100-fix-$(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 Python"
    $pkg = winget list --id "Python" 2>$null
    Write-Host $pkg

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

    Write-Host "[3/4] Applying upgrade to CPython 3.15.0"
    winget upgrade --id "Python" --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 "Python"
    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-2026-6100 affecting Python
# Detect -> backup -> upgrade -> verify -> log.

set -euo pipefail
LOG="/var/log/cve-2026-6100-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 python3 2>/dev/null | grep -i version || echo "python3 not installed via dpkg"
elif command -v rpm >/dev/null; then
    rpm -q python3 || echo "python3 not installed via rpm"
fi

echo "[2/4] Backing up configuration"
BACKUP="/root/backup-cve-2026-6100-$(date +%Y%m%d)"
mkdir -p "$BACKUP"
for d in /etc/python3 /etc/python3.d /etc/python3.conf; do
    [ -e "$d" ] && cp -a "$d" "$BACKUP/" || true
done

echo "[3/4] Applying upgrade (target: CPython 3.15.0)"
if command -v apt-get >/dev/null; then
    apt-get update
    apt-get install --only-upgrade -y python3
elif command -v dnf >/dev/null; then
    dnf upgrade --security -y python3
elif command -v yum >/dev/null; then
    yum update -y python3
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 python3 2>/dev/null | grep -i version
elif command -v rpm >/dev/null; then
    rpm -q python3
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

Reduce attack surface


# python3 is a library or shell, not a service. Restart any daemon that links it
# after applying the patch so the new code is actually loaded in memory.
sudo lsof | grep python3 | awk '{print $1, $2}' | sort -u
# Then restart each listed process group, or schedule a reboot at the next window.

How to verify the fix worked


# Linux
python3 --version 2>/dev/null || dpkg -s python3 | grep -i version
rpm -q python3 2>/dev/null || true


# Windows
winget list | findstr /I "python3"
Get-HotFix | Sort-Object InstalledOn -Descending | Select-Object -First 5

Expected: the reported version is at or above CPython 3.15.0. 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-2026-6100 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?

9.1 (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.*