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CVE-2026-5928— Potential buffer under-read in ungetwc

Quick assessment

Affected
The GNU C Library glibc
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

GNU C Library是GNU社区的一种按照LGPL许可协议发布的开源免费的C语言编译程序。 GNU C Library 2.43及之前版本存在安全漏洞，该漏洞源于在具有单字节和多字节编码重叠的字符集上对FILE流调用ungetwc函数，可能导致读取已分配缓冲区之前的字节，造成数据泄露或程序崩溃。

AI Predicted 7.5 Difficulty: Hard EPSS 0.49% · P40

Possible ATT&CK Techniques 1 AI

T1005 · Data from Local System

Affected Version Matrix 1

VendorProduct Version RangeStatus
The GNU C Library glibc 2.1.1-89≤ 2.43 affected
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I. Basic Information for CVE-2026-5928

Vulnerability Information
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Vulnerability Title
Potential buffer under-read in ungetwc
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
N/A
Source: CVE Program / CVE List V5
Vulnerability Type
缓冲区下溢读取
Source: CVE Program / CVE List V5
Vulnerability Title
GNU C Library 安全漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
GNU C Library是GNU社区的一种按照LGPL许可协议发布的开源免费的C语言编译程序。 GNU C Library 2.43及之前版本存在安全漏洞，该漏洞源于在具有单字节和多字节编码重叠的字符集上对FILE流调用ungetwc函数，可能导致读取已分配缓冲区之前的字节，造成数据泄露或程序崩溃。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
The GNU C Library glibc 2.1.1-89 ~ 2.43 -

II. Public POCs for CVE-2026-5928

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III. Intelligence Information for CVE-2026-5928

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IV. Related Vulnerabilities

Same product: glibc
Same vendor: The GNU C Library
Same weakness: CWE-127

V. Comments for CVE-2026-5928

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