⚠ 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 4.3

How to Fix CVE-2026-42058: Arbitrary File Read in BIG-IP

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

*By Sai Kiran Pandrala*

⚡ At a glance
SeverityCVSS 4.3 - Medium
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 < *
Fixed in21.1.0
Type (CWE)CWE-732: Incorrect Permission Assignment for Critical Resource

What is CVE-2026-42058?

CVE-2026-42058 is an arbitrary file read flaw in BIG-IP. An authenticated or unauthenticated request can read files outside the intended path scope, exposing configuration, secrets, or other sensitive content. Vendor description: An authenticated attacker's undisclosed requests to BIG-IP iControl REST can lead to an information leak of BIG-IP local user account names. Note: Software versions which have reached End of Technical Support (EoTS) are not evaluated.

Why this CVE matters

Arbitrary file read against a management product almost always exposes credentials, session secrets, or configuration. Treat any disclosure of this kind as a credential-rotation event in addition to a patching event.

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-2026-42058

  1. Read the vendor advisory in full: https://my.f5.com/manage/s/article/K000160903
  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. 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).

Upgrade f5 big-ip to the patched release


# CVE-2026-42058 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/K000160903

# 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-2026-42058 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-2026-42058.

# 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/K000160903

If you cannot patch immediately

No official workaround exists beyond restricting network exposure to the affected component. Apply the vendor patch as the primary remediation.

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 unusually long URI paths containing traversal sequences, unexpectedly large responses from the affected endpoint, and outbound requests from the application to internal addresses or cloud-metadata endpoints. Treat any sensitive file the bug could disclose as exposed.

Frequently asked questions

Is CVE-2026-42058 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-2026-42058?

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 BIG-IP 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.*