CVE-2026-5928
Received
Received - Intake
Buffer Under-Read in GNU C Library ungetwc Causes Data Leak
Publication date: 2026-04-20
Last updated on: 2026-04-23
Assigner: GNU C Library
Description
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.
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Meta Information
Affected Vendors & Products
| Vendor | Product | Version / Range |
|---|---|---|
| gnu | glibc | to 2.43 (inc) |
Helpful Resources
Exploitability
CWE
KEV
| CWE ID | Description |
|---|---|
| CWE-127 | The product reads from a buffer using buffer access mechanisms such as indexes or pointers that reference memory locations prior to the targeted buffer. |
