⚠ 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 8.1 ⚠ ACTIVELY EXPLOITED — CISA KEV

How to Fix CVE-2020-6820: Use-after-free when handling a ReadableStream in Thunderbird

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

*By Sai Kiran Pandrala*

⚡ At a glance
SeverityCVSS 8.1, High
Actively exploited?Yes, listed in CISA KEV (added 2021-11-03)
AffectedMozilla Thunderbird (< 68.7.0); Mozilla Firefox (< 74.0.1); Mozilla Firefox ESR (< 68.6.1)
Fixed in68.7.0, 74.0.1, 68.6.1
Type (CWE)Use-after-free when handling a ReadableStream
⚠️ Patch immediately. CVE-2020-6820 is in CISA's Known Exploited Vulnerabilities catalog (added 2021-11-03). Federal agencies had until 2022-05-03 to remediate.

What is CVE-2020-6820?

Under certain conditions, when handling a ReadableStream, a race condition can cause a use-after-free. We are aware of targeted attacks in the wild abusing this flaw. This vulnerability affects Thunderbird < 68.7.0, Firefox < 74.0.1, and Firefox ESR < 68.6.1.

In practical terms, a successful attacker gets use-after-free that can lead to heap corruption and code execution. CISA has confirmed exploitation in the wild, so this is not a theoretical risk.

Am I affected?

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


# Confirm the installed version via your package manager
dpkg -l | grep -i thunderbird   # Debian/Ubuntu
rpm -qa | grep -i thunderbird   # RHEL/CentOS/Rocky

How to fix CVE-2020-6820

The primary fix is to upgrade to the patched build listed in the Fixed in row above (68.7.0, 74.0.1, 68.6.1). Pick the platform that matches your install and run the commands below.

Windows (PowerShell, admin)


Stop-Process -Name firefox -Force -ErrorAction SilentlyContinue
$installer = "$env:TEMP\Firefox-Setup-68.7.0.exe"
Invoke-WebRequest -Uri 'https://download.mozilla.org/?product=firefox-latest-ssl&os=win64&lang=en-US' -OutFile $installer
Start-Process -FilePath $installer -ArgumentList '-ms' -Wait
(Get-Item "$env:ProgramFiles\Mozilla Firefox\firefox.exe").VersionInfo.ProductVersion

macOS


brew upgrade --cask firefox

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


# Run as Administrator
$ErrorActionPreference = 'Stop'
$log = "$env:ProgramData\Thunderbird-Patch-CVE-2020-6820.log"
function Write-Log($msg) { "$(Get-Date -Format s)  $msg" | Tee-Object -FilePath $log -Append }

Write-Log "Starting CVE-2020-6820 remediation for Mozilla Thunderbird"

# 1. Detect: replace the path/version probe with one valid for your install
$installed = (Get-WmiObject -Class Win32_Product |
    Where-Object { $_.Name -like '*Thunderbird*' } |
    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]'68.7.0') {
    Write-Log "Already at fixed version $installed; no action needed."
    return
}

# 2. Backup configuration to a timestamped folder
$backup = "$env:ProgramData\Thunderbird-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\Mozilla\Thunderbird"
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\Thunderbird-68.7.0.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 { $_.Name -like '*Thunderbird*' } |
    Select-Object -First 1 -ExpandProperty Version)
if ([version]$verify -ge [version]'68.7.0') {
    Write-Log "SUCCESS: now at $verify (>= 68.7.0)"
} 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/thunderbird-patch-cve-2020-6820.log
log()  { echo "$(date -Iseconds)  $*" | tee -a "$LOG"; }

log "Starting CVE-2020-6820 remediation for Mozilla Thunderbird"

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

# 2. Backup config
BACKUP=/var/backups/thunderbird-$(date +%Y%m%d-%H%M)
mkdir -p "$BACKUP"
for d in /etc/thunderbird /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 thunderbird
else
    sudo dnf upgrade --security -y thunderbird
fi

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

# Optionally compare against 68.7.0 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://www.mozilla.org/security/advisories/mfsa2020-14/
# 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://www.mozilla.org/security/advisories/mfsa2020-14/
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.


# Confirm the running build matches the patched version listed by the vendor
# Example for Linux package installs:
dpkg -l | grep -i "thunderbird"   # Debian/Ubuntu
rpm -qa | grep -i "thunderbird"   # RHEL/CentOS/Rocky

Expected output: the package version should meet or exceed 68.7.0.

Then re-run any vulnerability scanner you used previously and confirm the finding for CVE-2020-6820 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-2020-6820 being actively exploited?

Yes. CISA added it to the Known Exploited Vulnerabilities catalog, which means in-the-wild exploitation has been observed and confirmed.

How severe is CVE-2020-6820?

CVSS rates it 8.1 (High). Use that score to set your patch priority next to the other items in your queue.

Do I have to take Thunderbird 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.

What if my vulnerability scanner still flags CVE-2020-6820 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.

References

*Written by Sai Kiran Pandrala on 2026-05-25. Sourced from the official vendor advisory, the NVD record, and the CISA KEV listing. Always confirm against the vendor advisory before applying changes in production.*