⚠ 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 8.7

How to Fix CVE-2026-40698: Command Injection in BIG-IP

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

*By Sai Kiran Pandrala*

⚡ At a glance
SeverityCVSS 8.7 - High
Actively exploited?Not currently listed in CISA KEV
Affected21.0.0 < 21.0.0.2, 17.5.0 < 17.5.1.6, 17.1.0 < 17.1.3.2, 16.1.0 < *, 8.4.0 < *
Fixed in21.1.0
Type (CWE)CWE-77: Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection')

What is CVE-2026-40698?

CVE-2026-40698 is an OS command injection bug in BIG-IP. The product builds a shell command from untrusted input without escaping, so injected metacharacters run as the service account, often root or SYSTEM. Vendor description: A vulnerability exists in BIG-IP and BIG-IQ systems where a highly privileged, authenticated attacker with at least the Resource Administrator role can create SNMP configuration objects through iControl REST or the TMOS shell (tmsh) resulting in privilege escalation. Note: Software versions which have reached End of Technical Support (EoTS) are not evaluated.

Why this CVE matters

Command injection in a network appliance or management console gives the attacker the same privileges as the service account, which is usually root or SYSTEM. From there, persistence, lateral movement, and credential theft follow with off-the-shelf tooling.

For deployments of BIG-IP 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.

On BIG-IP, run tmsh show sys version from the CLI. The Active Version line is the value to compare against the advisory.

How to fix CVE-2026-40698

  1. Read the vendor advisory in full: https://my.f5.com/manage/s/article/K000160981
  2. Upgrade BIG-IP to 21.1.0 or a later version listed in the vendor advisory.
  3. Back up the configuration (and database, where applicable) before upgrading.
  4. Rotate any credentials, API keys, or session tokens that the vulnerable service touched. An unauthenticated RCE-class flaw means anything the process could see should be treated as exposed.
  5. Apply the patch in a maintenance window. For HA pairs, upgrade the standby node first, fail over, then upgrade the former primary.
  6. Restart the affected service so the patched binary loads, then verify the new version (see verification section).

Upgrade f5 big-ip to the patched release


# CVE-2026-40698 affects BIG-IP 21.0.0 < 21.0.0.2. Fixed in 21.0.0.2.
# Vendor advisory: https://my.f5.com/manage/s/article/K000160981

# 1. Confirm the running version.
tmsh show /sys version

# 2. Import and install the patched image.
tmsh install /sys software image BIGIP-21.0.0.2.iso volume HD1.2

# 3. After reboot, verify.
tmsh show /sys version

Verify the fix landed


# CVE-2026-40698 verification checklist.

# 1. Confirm the running version matches 21.0.0.2 (replace the version probe with
#    the platform-specific command shown above).

# 2. Re-scan the host with your vulnerability scanner (Nessus, Qualys, Tenable,
#    OpenVAS, Wazuh). The scanner must no longer flag CVE-2026-40698.

# 3. Inspect recent service and kernel logs for crash-loops or rollback events.
journalctl -u <service-name> --since "10 minutes ago"
dmesg --since "10 minutes ago"

# 4. Cross-check the running build against the vendor advisory:
#    https://my.f5.com/manage/s/article/K000160981

If you cannot patch immediately

Restrict access to the management or affected endpoint at the network layer. If the vendor lists a configuration toggle that disables the vulnerable feature, use it until you can patch.

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 unexpected administrator accounts in BIG-IP, scheduled tasks or cron jobs you did not create, new files in web-accessible directories, and outbound connections to addresses not in your baseline. Suspicious requests to the vulnerable endpoint immediately followed by successful 200-class responses with unusually large bodies are a strong indicator of exploitation.

Frequently asked questions

Is CVE-2026-40698 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-2026-40698?

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.

Do I need to assume compromise if my BIG-IP was internet-facing and unpatched?

For an unauthenticated RCE-class flaw exposed to the public internet during the known exploitation window, yes. Review logs, rotate credentials the process could access, and look for unexpected accounts, scheduled tasks, or outbound connections.

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.*