UDP out-of-bounds write

1. Summary

The Contec CMS8000 patient monitor (also relabeled and sold as the Epsimed MN-120) contains an out-of-bounds write in the network service that parses incoming UDP requests [1]. A remote attacker with network reachability to the device can send a specially formatted UDP datagram whose contents are written past the bounds of the intended memory region, corrupting memory with attacker-influenced data [1]. Because the write is unauthenticated and requires no user interaction, the flaw can be leveraged toward remote code execution on the monitor, or at minimum to crash or hang it [1][7].

2. Affected products & versions

3. The vulnerability in detail

The defect lives in the CMS8000’s firmware, specifically in the network service that receives and parses incoming UDP requests [1]. The authoritative NVD record classifies the issue as an out-of-bounds write (CWE-787): the device accepts a “specially formatted UDP request,” and in the course of handling that request writes attacker-influenced data to a memory location outside the boundaries of the buffer or structure that was supposed to hold it [1]. NVD states that this “can allow an attacker to write data out of bounds,” and that the condition can lead to remote code execution [1].

The root cause is a missing or inadequate bounds check. In the general shape of a CWE-787 network parser, the handler consumes a length or offset field that is derived from — or influenced by — the incoming datagram and uses it to position or size a write, without first validating that the resulting write stays inside the destination region [1]. When the attacker supplies a request that overstates that length, or steers the write to an offset the code did not anticipate, the write lands in adjacent memory. That adjacent memory may hold other program state, control structures, or, on a memory-unsafe embedded target, data that participates in the device’s control flow. Corrupting it with attacker-influenced bytes is the classic memory-corruption primitive that separates a benign parsing bug from a code-execution vulnerability. The analysis characterizes the primitive more specifically as an out-of-bounds write with a controllable offset — a stronger primitive than a fixed-position overflow, and the property that would make the leap from “corrupt memory” to “redirect execution” more credible; this controllable-offset characterization is inferred and (not stated in public advisories) [1].

Why does an out-of-bounds write matter more than a mere crash? CWE-787 (Out-of-bounds Write) means the software writes past the end (or before the beginning) of the intended buffer. The danger is not the crash itself but that the neighbouring memory the write lands in can hold values the program later trusts — return addresses, function pointers, indices, length fields, or state flags. An attacker who controls what gets written, and (if the offset is controllable) where, can therefore steer the program’s behaviour rather than merely destabilize it. On a low-level embedded RTOS or Linux target without modern exploit mitigations, this is often enough to convert memory corruption into arbitrary code execution [1].

Triggering the flaw has minimal preconditions. The attacker needs (1) network reachability to the monitor over UDP — the same L2/L3 segment, or any routable path to it — and (2) a vulnerable firmware version [1]. No credentials, no prior session, no user interaction, and no physical or RF adjacency are required; this is reflected directly in the CVSS vector’s AV:N/AC:L/PR:N/UI:N metrics [1]. UDP matters here: it is connectionless, so the attacker does not need to complete a handshake or hold a session, and a single datagram is sufficient to reach the vulnerable handler. There is no rate-limiting, sequencing, or authentication step between an inbound packet and the flawed parser that an attacker would have to defeat first [1].

The exploitation chain is short and linear [1][7]:

1. The attacker establishes network reachability to a CMS8000 / Epsimed MN-120 monitor. No credentials are needed [1].
2. The attacker crafts a specially formatted UDP request aimed at the vulnerable request handler [1][7].
3. The attacker sends the datagram(s) to the device over UDP [1].
4. The malformed input is written out of bounds, corrupting memory with attacker-influenced data (CWE-787) — at an attacker-influenced offset per the analysis, which public advisories do not confirm [1].
5. The attacker leverages that corruption to divert execution and achieve remote code execution on the monitor process [1][7]. Alternatively, the same malformed input crashes or hangs the process rather than yielding controlled execution [1].

One important disambiguation: Claroty Team82 did demonstrate a root-shell RCE against the CMS8000, but by a completely different path — impersonating the device’s hard-coded Central Management System IP, uploading a malicious binary over NFS, and installing a busybox reverse shell, which required physically holding button “C” during boot [5]. That work is tied to the hidden-function/NFS-update behavior (associated with CVE-2025-1204 / CVE-2025-0626), and Claroty’s writeup does not reference CVE-2024-12248 at all [5]. The UDP out-of-bounds write here is a distinct, remotely triggered defect with no physical-access precondition [1][5].

One caveat on specificity: public sources confirm only the protocol (UDP) and the class (CWE-787 out-of-bounds write leading to RCE). The specific vulnerable binary, the service/daemon name, the UDP port, and the exact field that seeds the bad length/offset are not identified in any public source [1].

4. Discovery & timeline

The vulnerability originated from an anonymous / unnamed external researcher who tipped CISA about unexpected functionality in the device; CISA then performed its own firmware analysis of three versions [6]. Public coordinated disclosure occurred in late January 2025 through CISA ICS-Medical advisory ICSMA-25-030-01 and a concurrent FDA safety communication, released as one of three coordinated CVEs alongside CVE-2025-0626 (hidden beacon to a hard-coded IP) and CVE-2025-0683 (plaintext patient-data leak) [6][7]. The disclosure date is 2025-01-31 [6]. The FDA safety communication is dated 2025-01-30 with public reporting on 2025-01-31; the exact date split is treated as approximate rather than confirmed (not documented in public advisories) [4]. At the time of the primary advisories no patch was available; the recommended action was to remove or disconnect the device from the network [6][7]. A later FDA update on 2025-07-02 and an Update A of the CISA advisory (and a July 2025 Contec patch) are referenced in some secondary coverage but are not confirmed in public advisories [4][2].

5. Technical reference

• CVE: CVE-2024-12248 [1]
• CWE: CWE-787 — Out-of-bounds Write [1]
• CVSS v3.1: 9.8 CRITICAL — AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H [1]
• CVSS v4.0: 9.3 CRITICAL — AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N [1]
• What the metrics mean here: AV:N (network-reachable over UDP), AC:L (a single crafted datagram, no special conditions), PR:N + UI:N (no auth, no user interaction) — this combination is why the base score is near-maximal [1]. C:H/I:H/A:H (v3.1) reflects the assumption that RCE compromises confidentiality, integrity, and availability of the device. The v4.0 vector scores subsequent-system impact as SC:N/SI:N/SA:N, i.e., the base scoring does not credit downstream consequences beyond the device itself [1].
• Code/commit refs: none available. The specific binary, service/daemon name, and UDP port implementing the flawed handler are not identified in public sources — only the protocol (UDP) is confirmed [1].
• CVSS assigning source: attributed to ICS-CERT; the assigning CNA is not documented in the NVD record, so this is unconfirmed [1].

6. Consequence & CFSE path analysis

Real-world consequence: a network-adjacent attacker who reaches a vulnerable CMS8000/MN-120 can either take control of the monitor’s software — potentially falsifying the vital signs a clinician sees or suppressing alarms that should fire — or knock the monitor offline, degrading patient monitoring. (The specific RCE-enabled outcomes of falsifying vitals or suppressing alarms are drawn from the analysis and are not directly demonstrated in public sources for this CVE.) In a clinical setting where staff act on what the monitor displays, a monitor that lies is worse than a monitor that is visibly broken, and both are unacceptable for a bedside safety device. Scale compounds this: because CMS8000 units in a facility typically share identical firmware, a single crafted datagram can be replayed against every vulnerable monitor on the network, so an attacker can target the whole fleet at once [6].

Why the CFSE Path Score differs from CVSS. CVSS bases this at v3.1 9.8 / v4.0 9.3 CRITICAL, scoring device-local compromise. The v4.0 vector explicitly zeroes the subsequent-system axes (SC/SI/SA:N), so CVSS does not carry the network→device→clinical-safety authority chain into the number [1]. The CFSE path analysis decomposes the single CVE into two co-dominant consequence paths and bands both EMERGENCY, i.e., higher than the CVSS baseline would imply on its own (direction UNDER relative to the published baseline).

• DEVICE_CONTROL_SAFETY (safety path): treats a single unauthenticated UDP datagram triggering an out-of-bounds write with a controllable offset (RE4/EC3) as leading to remote code execution in the monolithic root monitor process, which can falsify displayed vitals or suppress alarms (PH4) and cross a network→device→clinical-safety authority chain (CH4). Its CRITICAL base band is uplifted to EMERGENCY on fleet-wide shared firmware (SR4/SX4) and recall-class recovery (OR4, CISA’s “remove from network” guidance).
• DEVICE_AVAILABILITY (recovery path): treats the same trivially triggered datagram as crashing/hanging the process (EC4), yielding degraded monitoring rather than attacker-directed therapy (PH2, since staff can notice an offline monitor — though a PH4 variant exists if the outage goes unnoticed) with bounded function authority (AT2). It reaches EMERGENCY as its base band with no adjustment.

Both paths are marked LS:ACTIVE with no patch available. The overall path_verdict is EMERGENCY, driven by the safety consequence being the worst of the two paths.

Analysis caveat: several of the uplift inputs (controllable offset RE4/EC3; monolithic root-privileged process; RCE→vitals-falsification/alarm-suppression) are inferred and not demonstrated in public sources for this specific CVE.

7. Remediation & mitigations

At disclosure there was no patch for CVE-2024-12248, so remediation is compensating rather than corrective [6][7]:

• Remove the UDP attack surface (highest-value, bug-specific). The exploit depends entirely on the device receiving an attacker’s UDP datagram, so denying UDP reachability closes the bug. If remote monitoring is not needed, unplug the Ethernet cable and disable wireless so the device operates in local-monitoring mode only — this directly removes the UDP reachability the out-of-bounds write depends on [6].
• Discontinue use / substitute if the networking features cannot be disabled and are required: obtain alternative monitors [6].
• Restrict inbound UDP to the monitors. Where the monitors must stay networked, place them on an isolated segment and apply host/network ACLs that permit UDP only from the specific management hosts they legitimately talk to, dropping all other inbound UDP. This narrows the set of hosts that can reach the vulnerable handler even though it does not fix the parser [6][7].
• Network segmentation / subnet blocking (companion CVEs). Segment the monitors and block the hard-coded subnet 202.114.4.0/24 [7]. Note this specific block primarily targets the companion beacon/data-leak CVEs (CVE-2025-0626 / -0683); it reduces overall exposure but does not by itself close the UDP OOB-write path, which is reachable from any host that can send UDP to the device [7].
• Vendor patch (unverified). FDA metadata references a July 2025 Contec patch that removes networking functionality entirely (local-monitoring only). If real, this would eliminate the network attack surface — but it is not stated as a code-level fix to the bounds-check defect. Treat as (not documented in public advisories) [4].

Because the highest-value mitigation is simply denying UDP reachability, the most useful bug-specific detection is monitoring for unexpected inbound UDP to these monitors on network segments where they should be isolated.

8. Sources

1. NVD — CVE-2024-12248 — NIST NVD — https://nvd.nist.gov/vuln/detail/CVE-2024-12248 — authoritative CVE record: CWE-787 via specially formatted UDP requests leading to RCE, both CVSS vectors, three affected firmware versions, no fixed version. Credibility: primary advisory.
2. ICSMA-25-030-01: Contec Health CMS8000 Patient Monitor (Update A) — CISA — https://www.cisa.gov/news-events/ics-medical-advisories/icsma-25-030-01 — originating ICS-Medical advisory coordinating all three CVEs, scoring source, anonymous-researcher credit, relabeled model, “remove from network” mitigation. Credibility: primary advisory.
3. TLP:CLEAR — Contec CMS8000 Contains a Backdoor (Fact Sheet) — CISA — https://www.cisa.gov/sites/default/files/2025-01/fact-sheet-contec-cms8000-contains-a-backdoor-508c.pdf — CISA firmware analysis; context on the companion backdoor/NFS/port-515 behavior (distinct from this CVE). Credibility: primary advisory.
4. Cybersecurity Vulnerabilities with Certain Patient Monitors from Contec and Epsimed — FDA Safety Communication — US FDA — https://www.fda.gov/medical-devices/safety-communications/cybersecurity-vulnerabilities-certain-patient-monitors-contec-and-epsimed-fda-safety-communication — clinical context, relabeled MN-120, no known incidents, July 2025 networking-removal patch. Credibility: primary advisory.
5. Do the CONTEC CMS8000 Patient Monitors Contain a Chinese Backdoor? The Reality is More Complicated… — Claroty Team82 — https://claroty.com/team82/research/are-contec-cms8000-patient-monitors-infected-with-a-chinese-backdoor-the-reality-is-more-complicated — independent reverse-engineering; disambiguates the physical-access NFS RCE (CVE-2025-1204) from this UDP OOB-write CVE. Credibility: researcher-primary.
6. Patient monitors with backdoor are sending info to China, CISA warns — Help Net Security — https://www.helpnetsecurity.com/2025/01/31/contec-cms8000-patient-monitor-backdoor-china/ — disclosure timeline (2025-01-31), anonymous-researcher tip, FDA mitigation guidance, shared-firmware fleet exposure, no-incidents statement. Credibility: reputable press.
7. CISA and FDA Warn of Critical Backdoor in Contec CMS8000 Patient Monitors — The Hacker News — https://thehackernews.com/2025/01/cisa-and-fda-warn-of-critical-backdoor.html — the three CVEs, the OOB-write UDP→RCE mechanism with no auth, relabeled MN-120, no-patch-at-disclosure, subnet-block mitigation. Credibility: reputable press.