● High · CVSS 7.2

How to Fix CVE-2026-27459: pyOpenSSL DTLS cookie callback buffer overflow in pyopenssl

Q: Is CVE-2026-27459 being actively exploited?

Not at the time of writing. It is not listed in CISA's Known Exploited Vulnerabilities catalog. That status can change, so monitor the vendor advisory and the KEV catalog if the system is exposed.

Q: How severe is CVE-2026-27459?

CVSS rates it 7.2 (High). Use that score to set your patch priority next to the other items in your queue.

Q: Do I have to take pyopenssl offline to apply the patch?

It depends on the deployment. High-availability or clustered installs can usually patch one node at a time with no full outage. Standalone installs typically need a short restart. Always follow the vendor's documented upgrade steps.

Q: What if my vulnerability scanner still flags CVE-2026-27459 after I patch?

Re-run the scan after a service restart, then confirm the scanner's plugin set is up to date. Some scanners detect by banner version only and lag the official fix metadata by a release.

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

How to Fix CVE-2026-41898: CWE-126: Buffer Over-read in rust-openssl — CWE-126: Buffer Over-read in rust-openssl
How to Fix CVE-2026-41681: CWE-121: Stack-based Buffer Overflow in rust-openssl — CWE-121: Stack-based Buffer Overflow in rust-openssl
How to Fix CVE-2026-41676: CWE-787: Out-of-bounds Write in rust-openssl , CWE-787: Out-of-bounds Write in rust-openssl
How to Fix CVE-2026-42327: Input Validation Flaw in rust-openssl , Input Validation Flaw in rust-openssl
How to Fix CVE-2026-22796: Denial of Service in OpenSSL , Denial of Service in OpenSSL

*By Sai Kiran Pandrala*

⚡ At a glance


Severity	CVSS 7.2, High
Actively exploited?	No
Affected	Pyca pyopenssl (>= 22.0.0, < 26.0.0)
Fixed in	26.0.0
Type (CWE)	CWE-120: Buffer Copy without Checking Size of Input ('Classic Buffer Overflow')

What is CVE-2026-27459?

pyOpenSSL is a Python wrapper around the OpenSSL library. Starting in version 22.0.0 and prior to version 26.0.0, if a user provided callback to set_cookie_generate_callback returned a cookie value greater than 256 bytes, pyOpenSSL would overflow an OpenSSL provided buffer. Starting in version 26.0.0, cookie values that are too long are now rejected.

In practical terms, a successful attacker gets compromise of the affected component as described in the vendor advisory. There is no confirmed in-the-wild exploitation listed in CISA's KEV catalog at the time of writing, but the CVSS rating still warrants prompt patching.

Am I affected?

You're affected if you run Pyca pyopenssl at any version in the Affected row above. Use these probes to find your installed build:


openssl version

How to fix CVE-2026-27459

The primary fix is to upgrade to the patched build listed in the Fixed in row above (26.0.0). Pick the platform that matches your install and run the commands below.

Linux (Ubuntu/Debian)


sudo apt-get update
sudo apt-get install --only-upgrade openssl libssl1.1 libssl3
openssl version

Linux (RHEL/CentOS/Rocky)


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

PowerShell script (Windows) - detect, back up, upgrade, verify, log


# Run as Administrator
$ErrorActionPreference = 'Stop'
$log = "$env:ProgramData\pyopenssl-Patch-CVE-2026-27459.log"
function Write-Log($msg) { "$(Get-Date -Format s)  $msg" | Tee-Object -FilePath $log -Append }

Write-Log "Starting CVE-2026-27459 remediation for Pyca pyopenssl"

# 1. Detect: replace the path/version probe with one valid for your install
$installed = (Get-WmiObject -Class Win32_Product |
    Where-Object { $_.Name -like '*pyopenssl*' } |
    Select-Object -First 1 -ExpandProperty Version)
Write-Log "Detected version: $installed"

if (-not $installed) {
    Write-Log "Product not installed on this host; nothing to do."
    return
}
if ([version]$installed -ge [version]'26.0.0') {
    Write-Log "Already at fixed version $installed; no action needed."
    return
}

# 2. Backup configuration to a timestamped folder
$backup = "$env:ProgramData\pyopenssl-Backup-$(Get-Date -Format yyyyMMdd-HHmm)"
New-Item -ItemType Directory -Path $backup -Force | Out-Null
# Adjust the source path to match your install
$src = "$env:ProgramFiles\Pyca\pyopenssl"
if (Test-Path $src) { Copy-Item -Path $src -Destination $backup -Recurse -Force }
Write-Log "Backed up config to $backup"

# 3. Apply the patched installer (place the verified file on a share or staging path)
$installer = "$env:TEMP\pyopenssl-26.0.0.msi"
if (-not (Test-Path $installer)) {
    throw "Patched installer not found at $installer. Stage it from your software repo first."
}
Start-Process msiexec.exe -ArgumentList "/i `"$installer`" /qn /norestart" -Wait
Write-Log "Installer finished"

# 4. Verify
$verify = (Get-WmiObject -Class Win32_Product |
    Where-Object { $_.Name -like '*pyopenssl*' } |
    Select-Object -First 1 -ExpandProperty Version)
if ([version]$verify -ge [version]'26.0.0') {
    Write-Log "SUCCESS: now at $verify (>= 26.0.0)"
} else {
    Write-Log "FAILURE: still at $verify after install"
    exit 1
}

Bash script (Linux) - detect, back up, upgrade, verify, log


#!/usr/bin/env bash
set -euo pipefail
LOG=/var/log/pyopenssl-patch-cve-2026-27459.log
log()  { echo "$(date -Iseconds)  $*" | tee -a "$LOG"; }

log "Starting CVE-2026-27459 remediation for Pyca pyopenssl"

# 1. Detect installed version (works for deb and rpm packages)
if command -v dpkg >/dev/null && dpkg -s pyopenssl >/dev/null 2>&1; then
    CURRENT=$(dpkg-query -W -f='${Version}' pyopenssl)
    PKG_MGR=apt
elif command -v rpm >/dev/null && rpm -q pyopenssl >/dev/null 2>&1; then
    CURRENT=$(rpm -q --queryformat '%{VERSION}' pyopenssl)
    PKG_MGR=dnf
else
    log "pyopenssl not installed via apt or rpm; check your package manager or vendor instructions."
    exit 0
fi
log "Detected: pyopenssl=$CURRENT (manager=$PKG_MGR)"

# 2. Backup config
BACKUP=/var/backups/pyopenssl-$(date +%Y%m%d-%H%M)
mkdir -p "$BACKUP"
for d in /etc/pyopenssl /etc/${pkg%%-*} ; do
    [ -d "$d" ] && cp -a "$d" "$BACKUP/" && log "Backed up $d to $BACKUP"
done

# 3. Upgrade
if [ "$PKG_MGR" = apt ]; then
    sudo apt-get update -y
    sudo apt-get install --only-upgrade -y pyopenssl
else
    sudo dnf upgrade --security -y pyopenssl
fi

# 4. Verify
if [ "$PKG_MGR" = apt ]; then
    NEW=$(dpkg-query -W -f='${Version}' pyopenssl)
else
    NEW=$(rpm -q --queryformat '%{VERSION}' pyopenssl)
fi
log "After upgrade: $NEW"

# Optionally compare against 26.0.0 with dpkg --compare-versions or sort -V
log "Done. Restart the affected service if the package install did not."

If you can't patch immediately

These are runnable hardening commands. They reduce blast radius but they're not a replacement for the vendor patch.

No official vendor workaround is published for this CVE; patching is the only documented fix. The runnable hardening below is generic defense in depth, not a substitute for the patch.

Restrict the affected service to trusted networks (Linux):


# Vendor advisory: https://github.com/pyca/pyopenssl/security/advisories/GHSA-5pwr-322w-8jr4
# Replace <port> with the affected service port and 10.0.0.0/24 with your admin subnet
sudo iptables -A INPUT -p tcp --dport <port> -s 10.0.0.0/24 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport <port> -j DROP

Windows Firewall equivalent:


# Vendor advisory: https://github.com/pyca/pyopenssl/security/advisories/GHSA-5pwr-322w-8jr4
New-NetFirewallRule -DisplayName 'Allow affected service from admin subnet' \
    -Direction Inbound -Action Allow -Protocol TCP -LocalPort <port> -RemoteAddress 10.0.0.0/24
New-NetFirewallRule -DisplayName 'Block affected service from everywhere else' \
    -Direction Inbound -Action Block -Protocol TCP -LocalPort <port>

How to verify the fix worked

Run the version probe again and confirm the running build matches the Fixed in row above.


openssl version

Expected output: OpenSSL 26.0.0 (or newer).

Then re-run any vulnerability scanner you used previously and confirm the finding for CVE-2026-27459 has cleared. Sweep your logs for the indicators of compromise listed in the vendor or CISA advisory, especially if the system was internet-reachable during the disclosure window.

Frequently asked questions

Is CVE-2026-27459 being actively exploited?