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

Potential buffer under-read in ungetwc

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.

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缓冲区下溢读取

GNU C Library 安全漏洞

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。

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