---
id: CVE-2026-5928
title: Potential buffer under-read in ungetwc
summary: >-
  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 at…
severity: high
cvss: 7.5
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H'
cvssSource: adp
cwe:
  - CWE-127
vendor: The GNU C Library
product: glibc
affected:
  - glibc >= 2.1.1-89 <= 2.43
ssvc:
  exploitation: poc
  automatable: 'yes'
  technicalImpact: partial
  timestamp: '2026-04-21T16:10:57.731635Z'
exploitAvailable: true
published: '2026-04-20'
updated: '2026-10-15'
sourceUpdated: '2026-10-15T01:12:14.900Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-5928'
references:
  - url: 'https://sourceware.org/bugzilla/show_bug.cgi?id=33998'
tags:
  - cve.org
  - exploit-available
epss: 0.00493
epssPercentile: 0.40855
ingestedAt: '2026-10-15T05:52:46.048Z'
---

## Overview

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.

## Affected

- `glibc >= 2.1.1-89 <= 2.43`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.