CVE-2026-5928

Name	CVE-2026-5928
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 (at NVD; CERT, ENISA, LWN, oss-sec, fulldisc, Debian ELTS, Red Hat, Ubuntu, Gentoo, SUSE bugzilla/CVE, GitHub advisories/code/issues, web search, more)

Vulnerable and fixed packages

The table below lists information on source packages.

Source Package	Release	Version	Status
glibc (PTS)	bullseye	2.31-13+deb11u11	vulnerable
	bullseye (security)	2.31-13+deb11u13	vulnerable
	bookworm	2.36-9+deb12u13	vulnerable
	bookworm (security)	2.36-9+deb12u7	vulnerable
	trixie	2.41-12+deb13u2	vulnerable
	forky	2.42-14	vulnerable
	sid	2.42-15	vulnerable

The information below is based on the following data on fixed versions.

Package	Type	Release	Fixed Version	Urgency	Origin	Debian Bugs
glibc	source	(unstable)	(unfixed)

Notes

[trixie] - glibc <no-dsa> (Minor issue)
[bookworm] - glibc <no-dsa> (Minor issue)
https://sourceware.org/bugzilla/show_bug.cgi?id=33998
https://sourceware.org/git/?p=glibc.git;a=blob_plain;f=advisories/GLIBC-SA-2026-0010