# CVE-2026-5928: Potential buffer under-read in ungetwc

> A buffer under-read in glibc's ungetwc function may disclose heap data or crash programs when using specific character encodings with byte overlaps.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/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:** 20 Apr 2026
- **Updated:** 15 Oct 2026
- **Also known as:** AZL-83087, DEBIAN-CVE-2026-5928, ECHO-c634-adb4-1dbe, GHSA-829p-mc9m-7xhr, MINI-4hx7-r8fj-4v45, MINI-f656-9m8q-m8hj, MINI-xp75-j88r-426r, ROOT-OS-DEBIAN-11-CVE-2026-5928, ROOT-OS-DEBIAN-12-CVE-2026-5928, UBUNTU-CVE-2026-5928

## What is CVE-2026-5928?

CVE-2026-5928 is a high-severity vulnerability in Gnu Glibc, published on 20 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Calling the ungetwc function on a FILE stream with wide characters encoded in a character set that has overlaps between its single byte and multi-byte character encodings, in the GNU C Library version 2.43 or earlier, may result in an attempt to read bytes before an allocated buffer, potentially resulting in unintentional disclosure of neighboring data in the heap, or a program crash.

A bug in the wide character pushback implementation (_IO_wdefault_pbackfail in libio/wgenops.c) causes ungetwc() to operate on the regular character buffer (fp->_IO_read_ptr) instead of the actual wide-stream read pointer (fp->_wide_data->_IO_read_ptr). The program crash may happen in cases where fp->_IO_read_ptr is not initialized and hence points to NULL. The buffer under-read requires a special situation where the input character encoding is such that there are overlaps between single byte representations and multibyte representations in that encoding, resulting in spurious matches. The spurious match case is not possible in the standard Unicode character sets.

Weakness: CWE-127 (Buffer Under-read)

## Which versions are affected by CVE-2026-5928?

It affects 1 product.


Products: Gnu Glibc.

## How do I fix CVE-2026-5928?

Apply the vendor's patch.

## Is CVE-2026-5928 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-5928 exists.

## Other vulnerabilities in Gnu Glibc

- [CVE-2026-5450](https://insights.sbomify.com/CVE-2026-5450/): scanf %mc off-by-one heap buffer overflow (critical 9.8)
- [CVE-2023-25139](https://insights.sbomify.com/CVE-2023-25139/): Memory corruption in n/a (critical 9.8)
- [CVE-1999-0199](https://insights.sbomify.com/CVE-1999-0199/): Other in n/a (critical 9.8)
- [CVE-2019-1010022](https://insights.sbomify.com/CVE-2019-1010022/): Security feature bypass in glibc (critical 9.8)
- [CVE-2005-3590](https://insights.sbomify.com/CVE-2005-3590/): Memory corruption in n/a (critical 9.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-5928 and fix it if it is affected.

What it is: CVE-2026-5928 is a high-severity vulnerability in Gnu Glibc, published on 20 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-5928 exists.

Affected product: Gnu Glibc. 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-2026-5928/ (Markdown: https://insights.sbomify.com/CVE-2026-5928/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-5928
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-5928
- OSV: https://osv.dev/vulnerability/CVE-2026-5928

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-5928/
Summaries are generated and may contain mistakes; the official description above is the record.