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CVE-2026-18747

Estado: Pendiente de análisisMedia (6.8)—

The MCUmgr SMP-over-console transport decodes a base64 frame, reads a 16-bit packet length from it, verifies a CRC and then unconditionally strips the trailing CRC with rx_ctxt->nb->len -= 2U; in mcumgr_serial_process_frag() (subsys/mgmt/mcumgr/transport/src/serial_util.c). mcumgr_serial_extract_len() accepted any declared length, including 0 and 1, and a packet declaring length 0 passes the checksum test for free because crc16_itu_t() over zero bytes returns the zero seed. Since net_buf::len is a uint16_t, the subtraction underflows and the buffer is handed to SMP claiming roughly 65 KB of payload while its data area is only CONFIG_MCUMGR_TRANSPORT_NETBUF_SIZE bytes (default 384).

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The trigger is a single unauthenticated 7-byte line on the management console — the 0x06 0x09 packet marker followed by the base64 group AAA= and a newline — delivered to any transport built on this helper: CONFIG_MCUMGR_TRANSPORT_UART (smp_uart.c) or CONFIG_MCUMGR_TRANSPORT_SHELL (smp_shell.c), both of which select MCUMGR_TRANSPORT_SERIAL_HAS_SMP_OVER_CONSOLE. No prior session state, fragmentation or credentials are required to trigger the underflow, and the malformed frame is mishandled before any command handler or command-level access control runs. The attacker only needs write access to that console, which on many boards is a USB CDC-ACM port rather than a bare UART header.

With the inflated length, smp_process_request_packet() in subsys/mgmt/mcumgr/smp/src/smp.c loses its bound: cbor_nb_reader_init() gives the CBOR decoder a ~65 KB window into a 384-byte buffer, and each request header's nh_len is checked only against the inflated length. On its own the 7-byte frame re-parses whatever stale bytes the reused pool buffer still holds, typically a replay of the previously received request followed by a parse error, without leaving the buffer. Because the transport is unauthenticated, though, the attacker also controls the frames sent before the trigger, and can stage buffer contents so that a request succeeds with an nh_len larger than the buffer; net_buf_pull(), guarded only by __ASSERT_NO_MSG, then moves the parse cursor out of bounds and the loop reads further headers and CBOR from adjacent memory. The consequence is an out-of-bounds read that can fault the MCUmgr thread (denial of service); memory disclosure is also possible, since the default-enabled os echo handler (CONFIG_MCUMGR_GRP_OS_ECHO) decodes its string inside that window and copies it into its response. There is no integrity gain beyond what the unauthenticated transport already permits.

The fix rejects any declared packet length of two bytes or fewer in mcumgr_serial_extract_len(), so the CRC-strip subtraction can no longer underflow. The identical pattern remains in the test-only loopback transport subsys/mgmt/mcumgr/transport/src/smp_dummy.c (CONFIG_MCUMGR_TRANSPORT_DUMMY), which has no external input path and therefore carries no practical exposure.

CVSS

Probabilidad de explotación (EPSS)

EPSS (Exploit Prediction Scoring System, de FIRST) estima la probabilidad de que una vulnerabilidad sea explotada en 30 días. Complementa a CVSS (impacto) y a CISA KEV (explotación confirmada).

🎯 Técnicas ATT&CK

Cómo se explota esta vulnerabilidad y qué consigue el atacante, en el lenguaje de MITRE ATT&CK.

AV:L sin privilegios permite escalada local mediante underflow de buffer en MCUmgr. La lectura out-of-bounds desde memoria adyacente y el DoS por fault del thread son los impactos confirmados; no hay RCE explícito sin sesión/credenciales.

Inferido por nuestro agente de análisis a partir de la descripción oficial, el vector CVSS y la CWE, y comprobado por un supervisor. Puede contener errores.

🛡️ Mitigaciones ATT&CK que cubren estas técnicas

Threat Intelligence ProgramFilter Network TrafficExecution PreventionApplication Isolation and SandboxingExploit ProtectionUpdate Software

Tecnologías afectadas (1)

⚠ Inferidas por IA a partir de la descripción — NVD aún no ha analizado esta CVE; no son CPE verificados.

Zephyr — McumgrAI

CWE

Referencias

JSON original (NVD)

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{
  "id": "CVE-2026-18747",
  "cveTags": [],
  "metrics": {
    "ssvcV203": [
      {
        "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
        "ssvcData": {
          "id": "CVE-2026-18747",
          "role": "CISA Coordinator",
          "options": [
            {
              "exploitation": "none"
            },
            {
              "automatable": "no"
            },
            {
              "technicalImpact": "partial"
            }
          ],
          "version": "2.0.3",
          "timestamp": "2026-09-30T13:44:27.727115Z"
        }
      }
    ],
    "cvssMetricV31": [
      {
        "type": "Secondary",
        "source": "vulnerabilities@zephyrproject.org",
        "cvssData": {
          "scope": "UNCHANGED",
          "version": "3.1",
          "baseScore": 6.8,
          "attackVector": "LOCAL",
          "baseSeverity": "MEDIUM",
          "vectorString": "CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H",
          "integrityImpact": "NONE",
          "userInteraction": "NONE",
          "attackComplexity": "LOW",
          "availabilityImpact": "HIGH",
          "privilegesRequired": "NONE",
          "confidentialityImpact": "LOW"
        },
        "impactScore": 4.2,
        "exploitabilityScore": 2.5
      }
    ]
  },
  "affected": [
    {
      "source": "vulnerabilities@zephyrproject.org",
      "affectedData": [
        {
          "vendor": "zephyrproject",
          "product": "zephyr",
          "versions": [
            {
              "status": "affected",
              "version": "1.11.0",
              "lessThan": "4.4.2",
              "versionType": "semver"
            }
          ],
          "packageName": "zephyr",
          "programFiles": [
            "subsys/mgmt/mcumgr/transport/src/serial_util.c"
          ],
          "collectionURL": "https://github.com/zephyrproject-rtos/zephyr",
          "defaultStatus": "unaffected",
          "programRoutines": [
            {
              "name": "mcumgr_serial_extract_len"
            },
            {
              "name": "mcumgr_serial_process_frag"
            }
          ]
        }
      ]
    }
  ],
  "published": "2026-09-29T00:17:04.473",
  "references": [
    {
      "url": "https://github.com/zephyrproject-rtos/zephyr/commit/f7fc3e779a59c68c96eaf63a6b03ac7489f5e4a5",
      "source": "vulnerabilities@zephyrproject.org"
    },
    {
      "url": "https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-g6cx-xj75-hvfw",
      "source": "vulnerabilities@zephyrproject.org"
    }
  ],
  "vulnStatus": "Awaiting Analysis",
  "weaknesses": [
    {
      "type": "Secondary",
      "source": "vulnerabilities@zephyrproject.org",
      "description": [
        {
          "lang": "en",
          "value": "CWE-125"
        },
        {
          "lang": "en",
          "value": "CWE-191"
        }
      ]
    }
  ],
  "descriptions": [
    {
      "lang": "en",
      "value": "The MCUmgr SMP-over-console transport decodes a base64 frame, reads a 16-bit packet length from it, verifies a CRC and then unconditionally strips the trailing CRC with rx_ctxt->nb->len -= 2U; in mcumgr_serial_process_frag() (subsys/mgmt/mcumgr/transport/src/serial_util.c). mcumgr_serial_extract_len() accepted any declared length, including 0 and 1, and a packet declaring length 0 passes the checksum test for free because crc16_itu_t() over zero bytes returns the zero seed. Since net_buf::len is a uint16_t, the subtraction underflows and the buffer is handed to SMP claiming roughly 65 KB of payload while its data area is only CONFIG_MCUMGR_TRANSPORT_NETBUF_SIZE bytes (default 384).\n\nThe trigger is a single unauthenticated 7-byte line on the management console — the 0x06 0x09 packet marker followed by the base64 group AAA= and a newline — delivered to any transport built on this helper: CONFIG_MCUMGR_TRANSPORT_UART (smp_uart.c) or CONFIG_MCUMGR_TRANSPORT_SHELL (smp_shell.c), both of which select MCUMGR_TRANSPORT_SERIAL_HAS_SMP_OVER_CONSOLE. No prior session state, fragmentation or credentials are required to trigger the underflow, and the malformed frame is mishandled before any command handler or command-level access control runs. The attacker only needs write access to that console, which on many boards is a USB CDC-ACM port rather than a bare UART header.\n\nWith the inflated length, smp_process_request_packet() in subsys/mgmt/mcumgr/smp/src/smp.c loses its bound: cbor_nb_reader_init() gives the CBOR decoder a ~65 KB window into a 384-byte buffer, and each request header's nh_len is checked only against the inflated length. On its own the 7-byte frame re-parses whatever stale bytes the reused pool buffer still holds, typically a replay of the previously received request followed by a parse error, without leaving the buffer. Because the transport is unauthenticated, though, the attacker also controls the frames sent before the trigger, and can stage buffer contents so that a request succeeds with an nh_len larger than the buffer; net_buf_pull(), guarded only by __ASSERT_NO_MSG, then moves the parse cursor out of bounds and the loop reads further headers and CBOR from adjacent memory. The consequence is an out-of-bounds read that can fault the MCUmgr thread (denial of service); memory disclosure is also possible, since the default-enabled os echo handler (CONFIG_MCUMGR_GRP_OS_ECHO) decodes its string inside that window and copies it into its response. There is no integrity gain beyond what the unauthenticated transport already permits.\n\nThe fix rejects any declared packet length of two bytes or fewer in mcumgr_serial_extract_len(), so the CRC-strip subtraction can no longer underflow. The identical pattern remains in the test-only loopback transport subsys/mgmt/mcumgr/transport/src/smp_dummy.c (CONFIG_MCUMGR_TRANSPORT_DUMMY), which has no external input path and therefore carries no practical exposure."
    }
  ],
  "lastModified": "2026-09-30T14:17:27.877",
  "sourceIdentifier": "vulnerabilities@zephyrproject.org"
}