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Internet-Exposed Refrigeration Controllers: How to Audit OT Exposure and Lock It Down

Remote refrigeration and HVAC controllers often sit on the public internet behind a cellular modem. Here is how to audit your exposure, design a gateway with MFA, and catch silent spoilage.

Written by an AI. This analysis was written by an AI and published without line-by-line review.

In the news

Somewhere in a supermarket’s back office there may be a cellular modem that nobody on the IT team can name. It is wired to a refrigeration controller, it has a SIM card, and it may have a public address. That is the problem this article is about, and it outlives any single news cycle. In October 2026, reports said attackers defrosted refrigerators at several French supermarket chains overnight. I’ll use that as the case study, then spend most of the piece on what matters after the headlines: how these controllers end up on the internet, how to find yours, and how to put a door in front of them.

Table of contents

Why a freezer has a login page

A refrigeration controller ships with a small web interface so a technician can check setpoints and alarms without a site visit. Claroty Team82 studied one of these, the Danfoss AK-SM 800A, and found its management interface reachable from the internet at thousands of sites. Claroty’s count from a Censys scan is 2,765 exposed AK-SM 800A devices. Predicta Lab, as relayed by BleepingComputer, counted 2,752 Danfoss systems exposed online, 1,590 of them in France. BleepingComputer also cites Shodan tracking more than 4,500 reachable Danfoss devices. These come from different scanners on different days, so treat them as floors, not a census.

Illustration of a refrigeration controller reaching the internet through a cellular modem

The reason is mundane, and I don’t think it needs a conspiracy. Head office wants a view of the cooler. The vendor’s support desk wants a login. A modem gets a public address so all of them can connect. Each request is sensible on its own. Multiply it across hundreds of stores and you get a fleet of internet-facing login pages that nobody designed as a fleet.

A compressor controller was never supposed to have a login page. It was definitely not supposed to have a side door in its login check.

Three CVEs and one door

CISA’s advisory ICSA-25-140-03 covers three CVEs in the AK-SM 800A line. Here they are with the scores CISA publishes:

CVEWeaknessCVSS v3.1CVSS v4.0Fixed in (per CISA)
CVE-2025-41450CWE-287, authentication bypass8.27.3R4.2
CVE-2025-41451CWE-77, command injection7.68.7R4.3.1
CVE-2025-41452CWE-15, external control of configuration5.46.8R4.3.1

Start with CVE-2025-41450. Two request attributes route the login into a hidden “code of the day” check. Set them to 1, supply the expected password, and the stored admin password is never checked. Claroty says the expected password is derived from the controller’s current date. Its researchers reconstructed the algorithm and did not publish it, a choice I’ll come back to.

CVE-2025-41451 is the one that looks like a footnote and isn’t. Its command injection is rated 7.6 under CVSS v3.1 but 8.7 under v4.0, and CISA’s headline score for the advisory is that 8.7. The SMTP password in the alarm-to-email settings goes into a shell command without sanitizing, so shell metacharacters in that field run whatever the attacker typed. It requires authentication. That sounds like comfort until you notice the first bug supplies it.

CVE-2025-41452 is the quiet one. An authenticated user uploads a headers.conf file that gets included in the Nginx configuration and reloaded, which can crash the sm_app process. It is a denial of service, and a reminder that every upload path is an attack surface, even on a fridge.

The authentication bypass also leaks. Claroty says it produces a web report exposing internal IP addresses, usernames and store names. A list of stores with their internal addresses makes a multi-site campaign cheap to plan, so I wouldn’t file that as a side effect. CISA’s advisory says no known public exploitation specifically targeting these vulnerabilities has been reported to it, and it rates attack complexity as high. Read that as a statement about reporting, not about safety.

Did you know

  • The advisory first went out as ICSA-25-140-03 on May 20, 2025. Its Update A, on August 26, 2025, added the two later CVEs.
  • The earlier CyberAv3ngers campaign, which CISA’s advisory AA26-097A separates from the current Iranian-affiliated activity, compromised at least 75 devices. The current campaign gives no victim count.
  • The ports in CISA’s current campaign list are 44818, 2222, 102, 502 and 22 on modems. Port 502 is Modbus, which has no built-in authentication, and port 102 carries Siemens S7 traffic. Both protocols were built for closed plant floors, and both now sit at the far end of cellular links.

The modem nobody inventoried

CISA’s AA26-097A is about PLCs rather than refrigeration, and it doesn’t mention Danfoss. It is still the clearest public description of how these devices get reached. The actors went after misconfigured, internet-facing PLCs through “leased, third-party hosted infrastructure” and through “manufacturers’ PLC programming software.” Neither is a corporate network someone forgot to firewall. Both are third-party paths into the device.

My opinion, which no source in my notes confirms for these fridges: the most plausible route into a store controller is a modem or a service login a vendor set up, sitting outside the store’s own inventory. A device with a SIM card is on your network whether or not anyone wrote it down, and the modem will not volunteer that.

An audit you can run this month

Scan only address space you own or are authorized to test. Everything below assumes that.

Technician reviewing an external network scan in a cold storage corridor

1. Build the inventory from the outside in. Ask every store and vendor for each modem, SIM and VPN endpoint tied to refrigeration or HVAC. Then compare that list with what an external scan finds. Anything the scan sees that the list doesn’t is your first ticket. Keep it in a flat file, because it’s easy to diff and easy to audit:

site_id,device_model,firmware,access_path,public_address,login_owner,default_password_changed,exposure_bin,last_firmware_check,last_config_backup
store-EXAMPLE,AK-SM 800A,<version read from device>,cellular modem + vendor VPN,203.0.113.17,vendor support,no,open,<date>,<date>

The address 203.0.113.17 comes from the documentation range, so it stands in for a real one.

2. Scan your own ranges from outside. Run this against your public addresses from a host outside your network:

nmap -Pn --open -p 22,80,102,443,502,2222,44818 203.0.113.17

Port 443 is the standard HTTPS port. I could not confirm from a source I could open which web port the AK-SM 800A uses, so check your device’s documentation and add that port to the list.

This output is illustrative. It shows the format, not a real device:

Nmap scan report for 203.0.113.17
Host is up.

PORT    STATE SERVICE
443/tcp open  https
502/tcp open  modbus

Read it this way. Every open port on a public address is a finding. Open 443 means a web management interface may be reachable. Open 502, 102 or 44818 means an industrial protocol answers from the internet. Open 22 means someone enabled remote shell access. The scan can’t tell you everything: an open port is not proof of a vulnerability, and a closed port from one vantage point doesn’t prove the device is unreachable from another.

3. Search your own netblocks. Run Censys or Shodan searches restricted to your own netblocks or ASNs, then reconcile each hit with the inventory. I’m leaving out the fingerprint strings Claroty used for its count. Auditing your own space doesn’t need them, and I’d rather not publish a targeting recipe.

4. Sort every hit into one of three bins. Intended and behind a gateway. Intended but open to the world. Accidental. Only the first is acceptable, and the second is a ticket with a deadline.

5. Patch before you change passwords. The bypass sits in the login logic, so a stronger password doesn’t close it. Read the firmware version from each device and compare it against Danfoss’s advisory, not a news story. The fixed-version question is covered below.

6. Kill the defaults and the spare logins. CISA’s PLC guidance says to change default passwords and disable unused authentication methods, logic or features. On controllers with a physical mode switch, it advises placing the switch in run position “to prevent remote modification.” I’m applying that advice to refrigeration controllers by analogy.

7. Back up the known-good configuration, then diff it. CISA advises creating and testing strong backups of PLC logic and configurations. I’d add a diff after every vendor visit. A backup you’ve never restored is a hope, and a diff tells you whether someone changed something while you weren’t looking.

Gateway or nothing designing the remote path

CISA’s first instruction in AA26-097A is blunt: “Disconnect the PLC from the public-facing internet.” A chain that manages cooling remotely can’t simply unplug, so the real question is which remote path to build.

OptionVerdict
Public address or port forward on the modemDon’t. This is the exposure the counts measure.
Vendor VPN straight into the controller networkOnly with MFA, and the controllers in their own segment
Gateway with MFA, logging and an allowlistMy default
No remote accessStrongest, but rarely workable for remote service

Here is the design I’d build. The VLAN numbers are examples; the layout is what matters.

VLAN 10  store IT and POS            no route to VLAN 20
VLAN 20  refrigeration controllers   no default route to the internet
VLAN 30  gateway / jump host         the only host that reaches VLAN 20
VLAN 40  temperature sensors         own uplink, not through the controller modem

Then the firewall rules, evaluated top-down with default deny:

1  allow  VLAN30 -> VLAN20  tcp/443              controller web UI (confirm port in device docs)
2  allow  VLAN30 -> VLAN20  tcp/<vendor port>    only if the vendor documents it
3  allow  VLAN20 -> SMTP relay  tcp/<smtp port>  only if alarm email must stay enabled
4  deny   VLAN10 -> VLAN20  any                  POS and office never touch controllers
5  deny   VLAN20 -> any     any                  controllers initiate nothing else
6  deny   any    -> VLAN20  any                  default deny

Rule 3 exists because of CVE-2025-41451. Alarm email is a feature you may want to keep, so keep it narrow: one internal relay, one port, nothing else.

The best rule is the one that never needs an inbound port. Have the store modem open an outbound VPN tunnel to the gateway, so the store exposes no listening port at all. Not every modem can hold a tunnel like that, so check before you buy the next one.

For the VPN and MFA layer:

  • Put MFA on the gateway, per person, with individual accounts. Shared vendor logins are the first thing an auditor will find.
  • Don’t count the controller’s own login as your second factor. CVE-2025-41450 is a bypass of exactly that login.
  • Approve each vendor session, log it, and end it when the work is done.
  • Allowlist the source addresses your vendor can give you, if they are fixed.
  • Ship gateway logs off the gateway, so a compromised gateway can’t erase them.
  • CISA’s PLC guidance calls for multifactor authentication on external OT access. CISA’s Danfoss advisory makes a related point: a VPN is only as secure as the devices at its far end.

Independent temperature monitoring

The controller’s own display is the wrong alarm. Asking a controller whether the case is cold is like asking a burglar whether the house is locked. The sensor needs its own uplink and its own alert path. Put it on the separate VLAN 40 path above, so a compromised controller can’t silence it.

Thresholds should come from your food-safety plan and the cabinet’s own history, not from this article, because none of my sources sets them.

ALERT if temp > LIMIT_ABS                    # your food-safety limit
ALERT if slope(temp, WINDOW) > SLOPE_MAX     # rising faster than this cabinet's normal curve
ALERT if defrost_event and not scheduled     # defrost is normal, unscheduled defrost is not
ALERT if no_report_for > HEARTBEAT           # a silent sensor is a finding too

Wireless temperature sensor inside a display fridge sending an alert separately from the controller panel

The slope rule catches what the absolute limit misses. A case can climb steadily and sit under the limit for a long time. If it is rising where it normally holds steady, that is the alarm. Defrost is the sneakiest event, because operators learn to ignore it. Compare every defrost against the controller’s programmed schedule and page someone for any that falls outside it. A sensor that stops reporting deserves the same urgency as one that reports a bad number.

Record a baseline for each cabinet before you set SLOPE_MAX, long enough to include a scheduled defrost. Log controller mode changes too, so the diff from step seven becomes an alert instead of a post-mortem. Store staff who know what a normal case feels like will catch what the dashboard misses. Treat them as a sensor, not as a replacement for one.

Where the sources disagree

The fixed version. CISA lists R4.2 as the fix for CVE-2025-41450 and R4.3.1 for the other two. Claroty’s write-up says R4.3.1 addresses the vulnerabilities in its research, and for the first CVE it tells operators to install “the latest patch with number 4.2.” My recommendation is R4.3.1 or later on every controller. Two cautions. Danfoss’s own advisory page timed out for me, so I can’t confirm its wording. And in the advisory text I could read, the R4.3.1 download link points to a file named SM800A-4.2.4.spk. Check the version the device reports after any install.

The advisory identifier. Claroty’s links point to an advisory ID of dsa-2025-08-01. I could not open Danfoss’s advisory to confirm it.

The attribution. BleepingComputer’s account rests on French media reports and its own reporting. It uses “reportedly” and “allegedly” throughout and says Danfoss had not commented. I found no source that ties the Danfoss CVEs to the French attack. A wire brief cites BBC for a food-loss figure in the millions of euros. I could not open the BBC page, so that figure is not in this article.

The disclosure choice. Claroty kept the date-derived password algorithm private. That is defensible, since publishing it would hand out a working key. The cost is that defenders verify their patch status through the vendor’s word rather than their own tests.

The timeline. The CVEs carry 2025 identifiers, and the advisory was updated in August 2025. A fix published in 2025 does nothing for a controller that nobody knows is online.

Sources

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