# CVE-2026-89720: ubifs: fix out-of-bounds read in signature length check

> A crafted UBIFS image triggers an out-of-bounds read in the kernel signature verification, potentially leaking memory contents.

- **Severity:** High 7.7 (`CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 11 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-100085, BELL-CVE-2026-89720, DEBIAN-CVE-2026-89720, GHSA-8388-259p-82vg, UBUNTU-CVE-2026-89720

## What is CVE-2026-89720?

CVE-2026-89720 is a high-severity vulnerability in Linux, published on 11 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

In the Linux kernel, the following vulnerability has been resolved:

ubifs: fix out-of-bounds read in signature length check

ubifs_sb_verify_signature() bounds the on-disk ubifs_sig_node->len field
before handing the signature payload to verify_pkcs7_signature(), but the
check has the wrong sign:

	if (le32_to_cpu(signode->len) > snod->len + sizeof(struct ubifs_sig_node))

The signature bytes start sizeof(struct ubifs_sig_node) (UBIFS_SIG_NODE_SZ,
64 bytes) into the node, so the payload is at most

	snod->len - sizeof(struct ubifs_sig_node)

bytes long. Adding the header size instead of subtracting it accepts a
declared length up to 2 * UBIFS_SIG_NODE_SZ larger than the node actually
holds -- past the end of c->sbuf, which is vmalloc(c->leb_size).
verify_pkcs7_signature() -> pkcs7_parse_message() -> asn1_ber_decoder()
is then handed that inflated length and reads beyond the allocation while
walking the DER headers. The node length comes straight from the mounted
image, so a crafted signed UBIFS image reaches this via
ubifs_read_superblock() before the signature is cryptographically checked.

snod->len is guaranteed to be >= UBIFS_SIG_NODE_SZ by the node scanner
(c->ranges[UBIFS_SIG_NODE].min_len == UBIFS_SIG_NODE_SZ), so the corrected
subtraction cannot underflow. Legitimately signed images are unaffected: a
correct superblock never declares a signature longer than the node it is
embedded in.

## Which versions are affected by CVE-2026-89720?


## How do I fix CVE-2026-89720?

Apply the vendor's patch.

## Is CVE-2026-89720 being exploited?

There is no known exploitation of CVE-2026-89720 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89720 and fix it if it is affected.

What it is: CVE-2026-89720 is a high-severity vulnerability in Linux, published on 11 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-89720 in the wild and no public exploit code.

Affected product: Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-89720/ (Markdown: https://insights.sbomify.com/CVE-2026-89720/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89720
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89720
- OSV: https://osv.dev/vulnerability/CVE-2026-89720

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89720/
Summaries are generated and may contain mistakes; the official description above is the record.