● Low · CVSS 2.3

How to Fix CVE-2026-21620: Relative path traversal in OTP

Q: What is the CVSS score for CVE-2026-21620?

2.3 (Low). Vector: `CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N`.

Q: Where is the official advisory for CVE-2026-21620?

The vendor advisory is at [https://github.com/erlang/otp/security/advisories/GHSA-hmrc-prh3-rpvp](https://github.com/erlang/otp/security/advisories/GHSA-hmrc-prh3-rpvp). The NVD record is at https://nvd.nist.gov/vuln/detail/CVE-2026-21620.

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

CVE-2026-21620 is a relative path traversal in Erlang OTP. The fix is to upgrade to OTP 1.1.1.1; OTP 1.2.2.1; OTP 1.2.4; OTP 26.2.5.17; OTP 27.3.4.8; OTP 28.3.2.

⚡ At a glance


Severity	2.3 (Low)
Actively exploited?	No public listing in CISA KEV
Affected	OTP 17.0 to <; OTP 07b8f441ca711f9812fad9e9115bab3c3aa92f79 to <; OTP 5.10 to <7.0; OTP 1.0 to <*
Fixed in	OTP 1.1.1.1; OTP 1.2.2.1; OTP 1.2.4; OTP 26.2.5.17; OTP 27.3.4.8; OTP 28.3.2
Type (CWE)	CWE-23 Relative Path Traversal

What is CVE-2026-21620?

Relative Path Traversal, Improper Isolation or Compartmentalization vulnerability in erlang otp erlang/otp (tftp_file modules), erlang otp inets (tftp_file modules), erlang otp tftp (tftp_file modules) allows Relative Path Traversal. This vulnerability is associated with program files lib/tftp/src/tftp_file.erl, src/tftp_file.erl.

This issue affects otp: from 17.0, from 07b8f441ca711f9812fad9e9115bab3c3aa92f79; otp: from 5.10 before 7.0; otp: from 1.0. The CVSS base score is 2.3 (Low). The official advisory is at https://github.com/erlang/otp/security/advisories/GHSA-hmrc-prh3-rpvp.

Am I affected?

Check the version of OTP you are running and compare it against the Affected row above (OTP 17.0 to <*; OTP 07b8f441ca711f9812fad9e9115bab3c3aa92f79 to <*; OTP 5.10 to <7.0; OTP 1.0 to <*). If your build sits inside the affected range, you must patch.

Run the version check that fits your platform:


# Linux package check
dpkg -s otp 2>/dev/null | grep -i ^Version
rpm -q otp 2>/dev/null
command -v otp >/dev/null && otp --version 2>/dev/null


# Windows (PowerShell)
Get-Package -Name "*OTP*" -ErrorAction SilentlyContinue | Select-Object Name, Version
winget list --name "OTP" 2>$null

How to fix CVE-2026-21620

Upgrade OTP to a patched build: OTP 1.1.1.1; OTP 1.2.2.1; OTP 1.2.4; OTP 26.2.5.17; OTP 27.3.4.8; OTP 28.3.2. The vendor advisory is the source of truth for the exact fixed version.

Ubuntu / Debian


sudo apt-get update
sudo apt-get install --only-upgrade otp
dpkg -s otp | grep -i ^Version

RHEL / CentOS / Rocky / AlmaLinux


sudo dnf upgrade --refresh otp -y
# or for older releases:
sudo yum update otp -y
rpm -q otp

SUSE / openSUSE


sudo zypper refresh
sudo zypper update otp
rpm -q otp

Node.js / npm


# Update the affected package in your project
npm install OTP@latest
npm ls OTP
npm audit fix

Complete PowerShell remediation script (Windows)


# Fix script for CVE-2026-21620 affecting OTP
# Run as administrator. Detect -> backup -> upgrade -> verify -> log.

$ErrorActionPreference = "Stop"
$LogPath  = "C:\Logs\CVE-2026-21620-fix-$(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] Detect installed version"
    $pkg = Get-Package -Name "*OTP*" -ErrorAction SilentlyContinue
    if ($pkg) { $pkg | Format-Table Name, Version }
    else { Write-Host "Not detected via Get-Package; try winget list" }

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

    Write-Host "[3/4] Apply the upgrade to OTP 1.1.1.1; OTP 1.2.2.1; OTP 1.2.4; OTP 26.2.5.17; OTP 27.3.4.8; OTP 28.3.2"
    winget upgrade --name "OTP" --silent --accept-source-agreements --accept-package-agreements
    if ($LASTEXITCODE -ne 0) {
        # Fallback: pull latest via OS update channel
        Install-Module -Name PSWindowsUpdate -Force -SkipPublisherCheck -ErrorAction SilentlyContinue
        Import-Module PSWindowsUpdate
        Install-WindowsUpdate -MicrosoftUpdate -AcceptAll -IgnoreReboot
    }

    Write-Host "[4/4] Verify the patched build"
    winget list --name "OTP"
    Write-Host "Patch 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-2026-21620 affecting OTP
# Detect -> backup -> upgrade -> verify -> log.

set -euo pipefail
LOG="/var/log/cve-2026-21620-fix-$(date +%Y%m%d-%H%M%S).log"
exec > >(tee -a "$LOG") 2>&1

echo "[1/4] Detect installed version"
if command -v dpkg >/dev/null; then
    dpkg -s otp 2>/dev/null | grep -i ^Version || echo "otp not installed via dpkg"
elif command -v rpm >/dev/null; then
    rpm -q otp || echo "otp not installed via rpm"
fi

echo "[2/4] Backup configuration"
BACKUP="/root/backup-cve-2026-21620-$(date +%Y%m%d)"
mkdir -p "$BACKUP"
for d in /etc/otp /etc/otp.d /etc/otp.conf; do
    [ -e "$d" ] && cp -a "$d" "$BACKUP/" || true
done

echo "[3/4] Apply the upgrade (target: OTP 1.1.1.1; OTP 1.2.2.1; OTP 1.2.4; OTP 26.2.5.17; OTP 27.3.4.8; OTP 28.3.2)"
if command -v apt-get >/dev/null; then
    apt-get update
    apt-get install --only-upgrade -y otp
elif command -v dnf >/dev/null; then
    dnf upgrade --refresh -y otp
elif command -v yum >/dev/null; then
    yum update -y otp
elif command -v zypper >/dev/null; then
    zypper --non-interactive update otp
fi

echo "[4/4] Verify the patched build"
if command -v dpkg >/dev/null; then
    dpkg -s otp 2>/dev/null | grep -i ^Version
elif command -v rpm >/dev/null; then
    rpm -q otp
fi
echo "Done. Restart any service that loaded the old library."

If you can't patch immediately

Apply at least one of the following inline controls until you can deploy the patched build. None replace the upgrade.

Restrict exposure with nftables (Linux)


# Allow only trusted CIDR to reach the affected service ports
sudo nft add table inet filter 2>/dev/null || true
sudo nft 'add chain inet filter input { type filter hook input priority 0 ; }' 2>/dev/null || true
sudo nft 'add rule inet filter input tcp dport {80, 443} ip saddr != 10.0.0.0/8 drop'
sudo nft list ruleset

Block at the host firewall (Windows)


New-NetFirewallRule -DisplayName "Block-CVE-2026-21620" -Direction Inbound -Action Block -Protocol TCP -LocalPort 80,443 -RemoteAddress Any -Enabled True
Get-NetFirewallRule -DisplayName "Block-CVE-2026-21620"

Disable the affected service (Linux)


sudo systemctl stop otp 2>/dev/null || true
sudo systemctl disable otp 2>/dev/null || true

If the vendor advisory lists an official workaround, prefer that wording verbatim. If no workaround is published, the only safe remediation is the patch.

How to verify the fix worked

After upgrading, confirm the installed version matches the patched build and that no old library is still loaded by a long-running process.


# Linux
dpkg -s otp 2>/dev/null | grep -i ^Version
rpm -q otp 2>/dev/null || true
# Pid map check for old library handles
sudo lsof +c0 2>/dev/null | grep -i "DEL.*lib" || true


# Windows
Get-Package -Name "*OTP*" | Select-Object Name, Version
Get-HotFix | Sort-Object InstalledOn -Descending | Select-Object -First 5

Expected: the reported version is at or above OTP 1.1.1.1; OTP 1.2.2.1; OTP 1.2.4; OTP 26.2.5.17; OTP 27.3.4.8; OTP 28.3.2. Restart the affected service (systemctl restart <service> on Linux, or restart the Windows service) so it loads the patched binary.

Frequently asked questions

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Is CVE-2026-21620 actively exploited?

There is no public confirmation of exploitation in the wild listed in CISA KEV at the time of this writing. Patch anyway. Public exploits commonly follow disclosure within weeks.

Do I need to reboot after patching CVE-2026-21620?

For kernel and OS-level updates, yes. For most userland packages a systemctl restart <service> is enough on Linux, and restarting the Windows service or app on Windows. Any process that loaded the old library keeps using it until restarted.

What is the CVSS score for CVE-2026-21620?

2.3 (Low). Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N.

Where is the official advisory for CVE-2026-21620?

The vendor advisory is at https://github.com/erlang/otp/security/advisories/GHSA-hmrc-prh3-rpvp. The NVD record is at https://nvd.nist.gov/vuln/detail/CVE-2026-21620.

References

Official vendor advisory: https://github.com/erlang/otp/security/advisories/GHSA-hmrc-prh3-rpvp
NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-21620
CISA KEV catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog
Additional reference: https://cna.erlef.org/cves/CVE-2026-21620.html
Additional reference: https://osv.dev/vulnerability/EEF-CVE-2026-21620
Additional reference: https://www.erlang.org/doc/system/versions.html#order-of-versions

*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.*