⚠ 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-2026-1126: Unrestricted File Upload in flow

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
Affecteda3d2fe8133db9d3b50fda4f66f68634640344641
Fixed inSee vendor advisory
Type (CWE)CWE-434: Unrestricted Upload

What is CVE-2026-1126?

CVE-2026-1126 is an unrestricted file upload flaw in flow. An attacker can upload files of arbitrary type or to arbitrary locations, leading to webshell deployment and remote code execution. Vendor description: A security vulnerability has been detected in lwj flow up to a3d2fe8133db9d3b50fda4f66f68634640344641. This affects the function uploadFile of the file \flow-master\flow-front-rest\src\main\java\com\dragon\flow\web\resource\flow\FormResource.java of the component SVG File Handler.

Why this CVE matters

Unrestricted file upload is the classic webshell vector. The attacker uploads a script with an executable extension, then requests it through the same web server to execute commands.

For deployments of flow 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 flow's About dialog or run the vendor-documented version-check command. Compare the result against the affected ranges in the advisory.

How to fix CVE-2026-1126

The fix is to apply the patched build listed in the lwj advisory.

Affected versions confirmed in the CVE record:

Patch via the OS package manager (Linux)


<!-- enrich_agent_2:v1 -->
# 1. Update the package metadata.
sudo apt update                                  # Debian / Ubuntu
sudo dnf check-update                            # RHEL / Rocky / AlmaLinux / Fedora
sudo zypper refresh                              # openSUSE

# 2. Pull the patched version listed in the [vendor advisory](https://vuldb.com/?id.341718) of flow from lwj.
sudo apt install --only-upgrade flow
sudo dnf upgrade flow
sudo zypper update flow

# 3. Restart the affected service so the patched binary is the running binary.
sudo systemctl restart flow || true

# 4. Verify the running version.
flow --version

Verify the fix worked


<!-- enrich_agent_2:v1 -->
# 1. Confirm the running version matches the fixed-in version from the advisory.
#    Cross-check against the vendor advisory: https://vuldb.com/?id.341718

# 2. Re-scan with your vulnerability scanner. The scanner should no longer flag
#    this CVE on the patched host.
# Example with Nmap NSE:
nmap -sV --script vuln <target-host>

# 3. Inspect the service / kernel logs for crash-loops or rollback events in
#    the first hour after the upgrade.
journalctl -u <service-name> --since "1 hour ago"
dmesg --since "1 hour ago"

If you cannot patch immediately

Restrict access to the management interface to trusted internal IP addresses only. Block public access at the firewall and require VPN for any remote administration. Apply the patch as soon as a maintenance window allows.

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 flow, 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-2026-1126 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-2026-1126?

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 flow 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.*