# CVE-2021-47636: ubifs: Fix read out-of-bounds in ubifs_wbuf_write_nolock()

> A read out-of-bounds vulnerability in the Linux kernel's UBIFS filesystem allows attackers to cause a denial of service or disclose sensitive memory information.

- **Severity:** High 7.1 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 26 Feb 2025
- **Updated:** 5 Aug 2026
- **Also known as:** DEBIAN-CVE-2021-47636, GHSA-v3v7-3vrh-65v3, UBUNTU-CVE-2021-47636

## What is CVE-2021-47636?

CVE-2021-47636 is a high-severity memory corruption vulnerability in Linux kernel, published on 26 Feb 2025. 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 read out-of-bounds in ubifs_wbuf_write_nolock()

Function ubifs_wbuf_write_nolock() may access buf out of bounds in
following process:

ubifs_wbuf_write_nolock():
  aligned_len = ALIGN(len, 8);   // Assume len = 4089, aligned_len = 4096
  if (aligned_len <= wbuf->avail) ... // Not satisfy
  if (wbuf->used) {
    ubifs_leb_write()  // Fill some data in avail wbuf
    len -= wbuf->avail;   // len is still not 8-bytes aligned
    aligned_len -= wbuf->avail;
  }
  n = aligned_len >> c->max_write_shift;
  if (n) {
    n <<= c->max_write_shift;
    err = ubifs_leb_write(c, wbuf->lnum, buf + written,
                          wbuf->offs, n);
    // n > len, read out of bounds less than 8(n-len) bytes
  }

, which can be catched by KASAN:
  =========================================================
  BUG: KASAN: slab-out-of-bounds in ecc_sw_hamming_calculate+0x1dc/0x7d0
  Read of size 4 at addr ffff888105594ff8 by task kworker/u8:4/128
  Workqueue: writeback wb_workfn (flush-ubifs_0_0)
  Call Trace:
    kasan_report.cold+0x81/0x165
    nand_write_page_swecc+0xa9/0x160
    ubifs_leb_write+0xf2/0x1b0 [ubifs]
    ubifs_wbuf_write_nolock+0x421/0x12c0 [ubifs]
    write_head+0xdc/0x1c0 [ubifs]
    ubifs_jnl_write_inode+0x627/0x960 [ubifs]
    wb_workfn+0x8af/0xb80

Function ubifs_wbuf_write_nolock() accepts that parameter 'len' is not 8
bytes aligned, the 'len' represents the true length of buf (which is
allocated in 'ubifs_jnl_xxx', eg. ubifs_jnl_write_inode), so
ubifs_wbuf_write_nolock() must handle the length read from 'buf' carefully
to write leb safely.

Fetch a reproducer in [Link].

Weakness: CWE-125 (Out-of-bounds Read)

## Which versions are affected by CVE-2021-47636?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2021-47636?

Apply the vendor's patch.

## Is CVE-2021-47636 being exploited?

There is no known exploitation of CVE-2021-47636 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-47636 and fix it if it is affected.

What it is: CVE-2021-47636 is a high-severity memory corruption vulnerability in Linux kernel, published on 26 Feb 2025. 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-2021-47636 in the wild and no public exploit code.

Affected product: Linux kernel. 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-2021-47636/ (Markdown: https://insights.sbomify.com/CVE-2021-47636/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-47636
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-47636
- OSV: https://osv.dev/vulnerability/CVE-2021-47636

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-47636/
Summaries are generated and may contain mistakes; the official description above is the record.