glibc <=2.43 ungetwc Buffer Under-Read via _IO_wdefault_pbackfail
CVE-2026-5928 Published on April 20, 2026
Static buffer overflow in deprecated nis_local_principal
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 Type
Buffer Under-read
The software reads from a buffer using buffer access mechanisms such as indexes or pointers that reference memory locations prior to the targeted buffer. This typically occurs when the pointer or its index is decremented to a position before the buffer, when pointer arithmetic results in a position before the beginning of the valid memory location, or when a negative index is used. This may result in exposure of sensitive information or possibly a crash.
Products Associated with CVE-2026-5928
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Affected Versions
The GNU C Library glibc:- Version 2.1.1-89, <= 2.43 is affected.