CVE-2026-5928 Potential buffer under-read in ungetwc
A buffer under-read in glibc's ungetwc function may disclose heap data or crash programs when using specific character encodings with byte overlaps.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-5928?
CVE-2026-5928 is a high-severity vulnerability in Gnu Glibc, published on 20 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-127
Which versions are affected by CVE-2026-5928?
It affects 1 product.
Products include Gnu Glibc.
How do I fix CVE-2026-5928?
Apply the vendor's patch.
Is CVE-2026-5928 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-5928 exists.
Other vulnerabilities in Gnu Glibc
Sources
Also known as AZL-83087, DEBIAN-CVE-2026-5928, ECHO-c634-adb4-1dbe, GHSA-829p-mc9m-7xhr, MINI-4hx7-r8fj-4v45, MINI-f656-9m8q-m8hj and 4 more.