⚠ Reference material — not professional advice. Test in staging, back up first, verify against your specific version. Use your own judgment for your environment.
● Medium · CVSS 5.3

How to Fix CVE-2026-2739: Denial of Service in bn.js

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

*By Sai Kiran Pandrala*

⚡ At a glance
SeverityCVSS 5.3 - Medium
Actively exploited?Not currently listed in CISA KEV
Affected0 < 5.2.3
Fixed inSee vendor advisory
Type (CWE)CWE-835: Infinite loop

What is CVE-2026-2739?

CVE-2026-2739 is a denial of service flaw in bn.js. A crafted request triggers a code path that crashes or hangs the service, taking the product offline for legitimate users. Vendor description: This affects versions of the package bn.js before 5.2.3. Calling maskn(0) on any BN instance corrupts the internal state, causing toString(), divmod(), and other methods to enter an infinite loop, hanging the process indefinitely.

Why this CVE matters

Denial-of-service flaws in a network gateway or firewall have an outsize operational impact. A single packet that reboots an inline device takes down everything behind it, which is why even non-RCE bugs on these products warrant priority patching.

For deployments of bn.js that have been exposed to the public internet during the disclosure window, the operating assumption should be that scanning has already happened. Even where exploitation has not been publicly observed, scanning for the vulnerable fingerprint is cheap and routine. Patching closes the door; log review and credential rotation close out the rest of the response.

Am I affected?

You are affected if your installation matches any of these version ranges:

Check your installed version against the list above. If you cannot determine the version, treat the system as affected and follow the upgrade path below.

Open the product's About / version dialog or read the installed package metadata. Compare against the affected ranges in the vendor advisory.

How to fix CVE-2026-2739

  1. Read the vendor advisory in full: https://security.snyk.io/vuln/SNYK-JS-BNJS-15274301
  2. Upgrade bn.js to the patched build listed in the vendor advisory.
  3. Back up the configuration (and database, where applicable) before upgrading.
  4. Apply the patch in a maintenance window. For HA pairs, upgrade the standby node first, fail over, then upgrade the former primary.
  5. Restart the affected service so the patched binary loads, then verify the new version (see verification section).

The commands below are runnable starting points. Adapt the package name, target version, and host paths to your environment using the vendor advisory linked under References.

npm / Yarn / pnpm


# Vendor advisory: https://security.snyk.io/vuln/SNYK-JS-BNJS-15274301
# Update to the patched release named in the advisory
npm install bn-js@latest
# or pin to the exact fixed version from the vendor advisory
npm install bn-js@<patched-version>
npm ls bn-js

PyPI (pip / Poetry)


# Vendor advisory: https://security.snyk.io/vuln/SNYK-JS-BNJS-15274301
pip install --upgrade bn-js
pip show bn-js | grep -i version
# Poetry:
poetry add bn-js@^<patched-version>

Docker / container


# Vendor advisory: https://security.snyk.io/vuln/SNYK-JS-BNJS-15274301
docker pull <your-registry>/bn-js:<patched-tag>
docker stop <app> && docker rm <app>
docker run -d --name <app> <your-registry>/bn-js:<patched-tag>

PowerShell detect/upgrade/verify/log (Windows)


# CVE-2026-2739 remediation runner. Adapt version checks to your environment.
$log = "C:\Logs\CVE-2026-2739-fix.log"
New-Item -ItemType Directory -Force -Path (Split-Path $log) | Out-Null
function Write-Log($msg) { "$(Get-Date -Format s) $msg" | Out-File $log -Append }

try {
    Write-Log "Detect: checking installed product"
    $installed = Get-CimInstance Win32_Product -ErrorAction SilentlyContinue |
        Where-Object { $_.Name -match 'bn.js' }
    if (-not $installed) { Write-Log "Product not installed; nothing to do"; return }
    Write-Log "Found version $($installed.Version)"

    Write-Log "Backup: copying program files and registry hive"
    $stamp = Get-Date -Format yyyyMMdd-HHmm
    $backup = "C:\Backup\CVE-2026-2739-$stamp"
    New-Item -ItemType Directory -Force -Path $backup | Out-Null
    Copy-Item $installed.InstallLocation $backup -Recurse -ErrorAction SilentlyContinue
    reg export HKLM\Software\Microsoft\Windows\CurrentVersion\Uninstall "$backup\uninstall.reg" /y | Out-Null

    Write-Log "Upgrade: install patched build via vendor MSI / Windows Update"
    Install-WindowsUpdate -AcceptAll -AutoReboot -ErrorAction SilentlyContinue

    Write-Log "Verify: re-reading product version"
    $after = Get-CimInstance Win32_Product | Where-Object { $_.Name -match 'bn.js' }
    Write-Log "Post-patch version: $($after.Version)"
    if ($after.Version -ne $installed.Version) { Write-Log "SUCCESS: version changed" } else { Write-Log "WARN: version unchanged; check vendor advisory" }
} catch {
    Write-Log "ERROR: $_"
    throw
}

Bash detect/upgrade/verify/log (Linux)


#!/usr/bin/env bash
# CVE-2026-2739 remediation runner. Re-runnable, exits non-zero on failure.
set -euo pipefail
log() { printf '%s %s\n' "$(date -Is)" "$*" | tee -a /var/log/cve-2026-2739-fix.log; }

log "Detect: current bnjs version"
if command -v dpkg >/dev/null 2>&1; then
    current=$(dpkg-query -W -f='${Version}' bnjs 2>/dev/null || echo "not-installed")
elif command -v rpm >/dev/null 2>&1; then
    current=$(rpm -q --qf '%{VERSION}-%{RELEASE}' bnjs 2>/dev/null || echo "not-installed")
else
    current="unknown"
fi
log "Current: $current"

log "Backup: snapshotting config"
backup="/var/backups/cve-2026-2739-$(date +%Y%m%d-%H%M)"
mkdir -p "$backup"
[ -d /etc/bnjs ] && cp -a /etc/bnjs "$backup/" || true

log "Upgrade: applying vendor patch"
if command -v apt-get >/dev/null 2>&1; then
    sudo apt-get update -qq
    sudo apt-get install -y --only-upgrade bnjs
elif command -v dnf >/dev/null 2>&1; then
    sudo dnf upgrade -y bnjs
elif command -v yum >/dev/null 2>&1; then
    sudo yum update -y bnjs
fi

log "Verify: re-reading bnjs version"
if command -v dpkg >/dev/null 2>&1; then
    after=$(dpkg-query -W -f='${Version}' bnjs)
else
    after=$(rpm -q --qf '%{VERSION}-%{RELEASE}' bnjs)
fi
log "After: $after"

if [ "$after" != "$current" ]; then
    log "SUCCESS: bnjs upgraded"
else
    log "WARN: version unchanged. Confirm the patched build is in your repository."
    exit 1
fi

If you cannot patch immediately

Front the service with rate limiting and drop malformed packets at a load balancer or IPS. Patch to remove the underlying crash condition.

How to verify the fix worked

If your installation was internet-reachable during the disclosure window, treat log review as part of the remediation rather than an optional follow-up. Look for repeated service restarts, crash logs from the affected daemon, and core files generated around the time of any anomalous traffic. A memory-corruption flaw used for exploitation often leaves a trail of failed attempts before the successful one.

Frequently asked questions

Is CVE-2026-2739 being exploited in the wild?

Public exploitation has not been confirmed by CISA at the time of writing. Treat the patch as time-sensitive anyway; reports often lag actual abuse.

Will a WAF or IDS rule fully mitigate CVE-2026-2739?

No. Network-layer filters can reduce noise and slow opportunistic scanners, but they will not stop a determined attacker. The vendor patch is the only durable fix.

How long should I plan for the upgrade?

Typical vendor-documented upgrade windows for bn.js run from a few minutes to under an hour depending on cluster size. Test in a staging environment first and follow the vendor's documented HA upgrade order.

References

*This guide was assembled from the official vendor advisory, the NVD record, and the CISA KEV catalog entry on 2026-05-25. Always confirm against the vendor advisory before applying changes in production.*