How to Fix CVE-2026-33780: Memory leak in Junos OS

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

What is CVE-2026-33780?

A Missing Release of Memory after Effective Lifetime vulnerability in the Layer 2 Address Learning Daemon (l2ald) of Juniper Networks Junos OS and Junos OS Evolved allows an adjacent, unauthenticated attacker to cause a memory leak ultimately leading to a Denial of Service (DoS). In an EVPN-MPLS scenario, routes learned from remote multi-homed Provider Edge (PE) devices are programmed as ESI routes. Due to a logic issue in the l2ald memory management, memory allocated for these routes is not released when there is churn for these routes. As a result, memory leaks in the l2ald process which will ultimately lead to a crash and restart of l2ald.

Am I affected?

From the device CLI, run the version check:

# Cisco IOS/IOS XE
show version | include Version

# FortiGate (FortiOS)
get system status

# Palo Alto (PAN-OS)
show system info | match sw-version

# Juniper (Junos)
show version

Compare what you see against the Affected row above (Juniper Networks Junos OS 0 to <22.4R3-S5, 23.2 to <23.2R2-S3, 23.4 to <23.4R2-S4, 24.2 to <24.2R2; Juniper Networks Junos OS Evolved all version prior to to <22.4R3-S5-EVO, 23.2 to <23.2R2-S3-EVO, 23.4 to <23.4R2-S4-EVO, 24.2 to <24.2R2-EVO). If your build sits inside that range, the device is exposed and should be upgraded.

How to fix CVE-2026-33780

The primary fix is to upgrade Junos OS to the patched build. Use the commands for your platform below; the patched versions listed in the vendor advisory are: Junos OS 22.4R3-S5; Junos OS 23.2R2-S3; Junos OS 23.4R2-S4; Junos OS 24.2R2 (and 4 more SKUs — see vendor advisory).

Juniper (Junos CLI)

# Vendor advisory: https://kb.juniper.net/JSA107819
show version
request system software add /var/tmp/<patched-image>.tgz no-validate reboot

Complete operator runbook (network appliance)

Run this from your management workstation (Linux/macOS or Windows with a TFTP server reachable). It shells into the device, backs up the running config, transfers the patched image, and reloads.

# Vendor advisory: https://kb.juniper.net/JSA107819
#!/usr/bin/env bash
# CVE-2026-33780 fix runbook for Juniper Networks Junos OS
# Target build: Junos OS 22.4R3-S5; Junos OS 23.2R2-S3; Junos OS 23.4R2-S4; Junos OS 24.2R2 (and 4 more SKUs, see vendor advisory)

set -euo pipefail
DEVICE="${1:?usage: $0 <device-ip> <patched-image-file> <tftp-server-ip>}"
IMAGE="${2:?patched image filename required}"
TFTP="${3:?tftp server ip required}"
LOG="/var/log/cve-2026-33780-$(date +%Y%m%d-%H%M%S).log"

echo "[1/4] Pulling current config from $DEVICE"
ssh admin@"$DEVICE" "show version" | tee -a "$LOG"
ssh admin@"$DEVICE" "copy running-config tftp://$TFTP/cve-2026-33780-pre.cfg"

echo "[2/4] Uploading patched image $IMAGE via TFTP"
ssh admin@"$DEVICE" "copy tftp://$TFTP/$IMAGE flash:$IMAGE" | tee -a "$LOG"

echo "[3/4] Setting boot image and reloading"
ssh admin@"$DEVICE" "configure terminal
 boot system flash:$IMAGE
 end
 write memory
 reload" | tee -a "$LOG"

echo "[4/4] Wait ~5 minutes, then verify"
sleep 300
ssh admin@"$DEVICE" "show version | include Version" | tee -a "$LOG"
echo "Compare reported version against Junos OS 22.4R3-S5; Junos OS 23.2R2-S3; Junos OS 23.4R2-S4; Junos OS 24.2R2 (and 4 more SKUs, see vendor advisory)"

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.

Windows firewall isolation

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

Network appliance ACL

# Cisco IOS example: restrict management plane to a trusted subnet
configure terminal
access-list 99 permit 10.0.0.0 0.255.255.255
line vty 0 15
 access-class 99 in
end
write memory

Service-level fallback

# If the affected feature is optional, stop the service until the patch is applied
sudo systemctl stop junos
sudo systemctl disable junos

How to verify the fix worked

# Linux
junos --version 2>/dev/null || dpkg -s junos | grep -i version
rpm -q junos 2>/dev/null || true

# Windows
winget list | findstr /I "junos"
Get-HotFix | Sort-Object InstalledOn -Descending | Select-Object -First 5

Expected: the reported version is at or above Junos OS 22.4R3-S5; Junos OS 23.2R2-S3; Junos OS 23.4R2-S4; Junos OS 24.2R2 (and 4 more SKUs, see vendor advisory). 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

Related fixes

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

• How to Fix CVE-2026-33774: Check for unusual or exceptional conditions in Junos OS , Check for unusual or exceptional conditions in Junos OS
• How to Fix CVE-2026-33784: Use of default password in JSI LWC , Use of default password in JSI LWC
• How to Fix CVE-2026-21913: Critical Vulnerability in Junos OS , Critical Vulnerability in Junos OS
• How to Fix CVE-2026-33785: Missing authorization in Junos OS , Missing authorization in Junos OS
• How to Fix CVE-2026-21904: Cross-site scripting in Junos Space , Cross-site scripting in Junos Space

Is CVE-2026-33780 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?

6.5 (medium). 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

• Official vendor advisory: https://kb.juniper.net/JSA107819
• NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-33780

*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.*