Security Advisory: CVE-2026-5928
ubi-micro-dev Opinion:
This vulnerability requires calling
ungetwc() on a
wide-character stream with specific overlapping single-byte and
multi-byte encodings. None of the runtimes in these container
images use glibc's
ungetwc function: the JVM handles
character encoding via its own converters, Node.js uses V8/libuv,
and Python uses its own codec system. Container inspection
confirms no shared library in the image links against
ungetwc. Additionally, the
ungetwc
symbol can be localized in
libc.so.6 via ELF
.dynsym patching to prevent any future dynamic
linking without breaking glibc internals.
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.
Locations:
- glibc-2.28-251.el8_10.37
- glibc-common-2.28-251.el8_10.37
- glibc-langpack-en-2.28-251.el8_10.37
- glibc-minimal-langpack-2.28-251.el8_10.37
References: