⚠ 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 8.8

How to Fix CVE-2026-3298: Out-of-bounds write in CPython

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

⚡ At a glance
Severity8.8 (High)
Actively exploited?No public listing in CISA KEV
AffectedPython Software Foundation CPython 3.11.0 to <3.14.5rc1, 3.15.0a1 to <3.15.0b1
Fixed inCPython 3.14.5rc1; CPython 3.15.0b1
Type (CWE)CWE-787: Out-of-bounds write

What is CVE-2026-3298?

The method "sock_recvfrom_into()" of "asyncio.ProacterEventLoop" (Windows only) was missing a boundary check for the data buffer when using nbytes parameter. This allowed for an out-of-bounds buffer write if data was larger than the buffer size. Non-Windows platforms are not affected.

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


# Windows
winget list | findstr /I "python3"
Get-WmiObject Win32_Product | Where-Object { $_.Name -like "*CPython*" } | Select-Object Name, Version

Compare what you see against the Affected row above (Python Software Foundation CPython 3.11.0 to <3.14.5rc1, 3.15.0a1 to <3.15.0b1). If your build sits inside that range, you are exposed and should patch.

How to fix CVE-2026-3298

The primary fix is to upgrade CPython to the patched build. Use the commands for your platform below; the patched versions listed in the vendor advisory are: CPython 3.14.5rc1; CPython 3.15.0b1.

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

Windows (PowerShell, run as administrator)


# Check installed version
Get-HotFix | Sort-Object InstalledOn -Descending | Select-Object -First 5

# Apply latest Microsoft security updates (Windows Update)
Install-Module -Name PSWindowsUpdate -Force -SkipPublisherCheck
Import-Module PSWindowsUpdate
Get-WindowsUpdate -MicrosoftUpdate
Install-WindowsUpdate -MicrosoftUpdate -AcceptAll -AutoReboot

# Or via winget for application-level patches
winget upgrade --all --accept-source-agreements --accept-package-agreements

If the affected component is a third-party application, identify the package and run:


winget upgrade --id <vendor.product>

Replace <vendor.product> with the actual winget identifier (run winget search Python to find it).

Complete PowerShell remediation script (Windows)


# Fix script for CVE-2026-3298 affecting Python
# Run as administrator. Detect -> backup -> upgrade -> verify -> log.

$ErrorActionPreference = "Stop"
$LogPath  = "C:\Logs\CVE-2026-3298-fix-$(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 3.14.5rc1; CPython 3.15.0b1"
    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-2026-3298 affecting Python
# Detect -> backup -> upgrade -> verify -> log.

set -euo pipefail
LOG="/var/log/cve-2026-3298-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-2026-3298-$(date +%Y%m%d)"
mkdir -p "$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 3.14.5rc1; CPython 3.15.0b1)"
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

Windows firewall isolation


# Allow only management subnet to reach the vulnerable service
New-NetFirewallRule -DisplayName "Restrict CPython" `
    -Direction Inbound -Action Block -RemoteAddress Any `
    -Protocol TCP -LocalPort 443
New-NetFirewallRule -DisplayName "Allow Mgmt CPython" `
    -Direction Inbound -Action Allow -RemoteAddress 10.0.0.0/8 `
    -Protocol TCP -LocalPort 443

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 3.14.5rc1; CPython 3.15.0b1. 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-2026-3298 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?

8.8 (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.*