# CVE-2026-89778: isofs: fix out-of-bounds page array access on empty zisofs block

> A local attacker can trigger an out-of-bounds read in the Linux kernel's isofs module by mounting a crafted ISO9660 image, potentially causing a crash or leaking memory.

- **Severity:** Critical 9.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 16 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-101652, BELL-CVE-2026-89778, DEBIAN-CVE-2026-89778, GHSA-vfmj-8pgc-h88m, UBUNTU-CVE-2026-89778

## What is CVE-2026-89778?

CVE-2026-89778 is a critical memory corruption vulnerability in Linux, published on 16 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

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

isofs: fix out-of-bounds page array access on empty zisofs block

zisofs_uncompress_block()'s empty-block fast path returns
pcount << PAGE_SHIFT, ignoring the incoming poffset, unlike the
decompression path which returns bytes produced relative to poffset.
zisofs_fill_pages() uses that return to advance its page cursor, so when
the zisofs block size is below PAGE_SIZE and a sub-page block leaves
poffset partway into a page, a following empty block over-counts and
advances pages[] one element past its end, after which
"if (poffset && *pages)" reads pages[1] out of bounds.  rock.c only
rejects a block-size shift > 17, so a crafted "ZF" Rock Ridge record can
set it below PAGE_SHIFT; the bug is reached by an ordinary read() of a
compressed file on such a mounted ISO9660 image.

Return the byte count relative to poffset and zero only
[poffset, PAGE_SIZE) of the first page, matching the decompression path.
The page-aligned case (poffset == 0) is unaffected.

  BUG: KASAN: slab-out-of-bounds in zisofs_read_folio (fs/isofs/compress.c:290)
  Read of size 8 at addr ffff88800f5eac48 by task exploit/142
   zisofs_read_folio (fs/isofs/compress.c:290)
   read_pages (mm/readahead.c:184)
   ...
   filemap_read (mm/filemap.c:2814)
   vfs_read (fs/read_write.c:574)
   __x64_sys_pread64 (fs/read_write.c:769)
   do_syscall_64 (arch/x86/entry/syscall_64.c:94)
   entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
  The buggy address is located 0 bytes to the right of the
  allocated 8-byte region in the kmalloc-8 cache

## Which versions are affected by CVE-2026-89778?


## How do I fix CVE-2026-89778?

Apply the vendor's patch.

## Is CVE-2026-89778 being exploited?

There is no known exploitation of CVE-2026-89778 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89778 and fix it if it is affected.

What it is: CVE-2026-89778 is a critical memory corruption vulnerability in Linux, published on 16 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-89778 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-2026-89778/ (Markdown: https://insights.sbomify.com/CVE-2026-89778/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89778
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89778
- OSV: https://osv.dev/vulnerability/CVE-2026-89778

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89778/
Summaries are generated and may contain mistakes; the official description above is the record.