CVE-2026-5928: GNU Glibc Buffer Overflow Vulnerability

CVE-2026-5928 Overview

A buffer under-read vulnerability exists in the GNU C Library (glibc) version 2.43 and earlier affecting the wide character pushback implementation. When the ungetwc function is called on a FILE stream with wide characters encoded in a character set that has overlaps between single byte and multi-byte character encodings, the vulnerability may result in an attempt to read bytes before an allocated buffer. This can lead to unintentional disclosure of neighboring heap data or cause a program crash.

Critical Impact

This vulnerability can cause denial of service through program crashes or potentially expose sensitive heap memory contents to attackers in applications processing specially crafted input streams.

Affected Products

• GNU glibc version 2.43 and earlier
• Applications using wide character stream processing with ungetwc()
• Systems using non-Unicode character sets with single/multi-byte encoding overlaps

Discovery Timeline

• 2026-04-20 - CVE CVE-2026-5928 published to NVD
• 2026-04-23 - Last updated in NVD database

Technical Details for CVE-2026-5928

Vulnerability Analysis

The vulnerability is classified as CWE-127 (Buffer Under-read), affecting the wide character pushback mechanism in glibc. The issue resides in the _IO_wdefault_pbackfail function located in libio/wgenops.c. The core problem stems from a pointer confusion bug where the implementation incorrectly operates on the regular character buffer instead of the wide-stream read pointer.

The attack can be conducted over the network against applications that process user-supplied character streams. While no authentication is required to trigger the vulnerability, exploitation depends on specific conditions being met related to character encoding configurations.

Root Cause

The bug in _IO_wdefault_pbackfail causes the ungetwc() function to operate on fp->_IO_read_ptr (the regular character buffer pointer) instead of fp->_wide_data->_IO_read_ptr (the actual wide-stream read pointer). This pointer mismatch leads to incorrect memory access operations.

When fp->_IO_read_ptr is not initialized and points to NULL, calling ungetwc() will cause the program to crash. In cases where the pointer contains a valid address but points to the wrong memory region, the function may attempt to read memory before the allocated buffer boundaries.

Attack Vector

The buffer under-read condition requires a specific scenario where the input character encoding has overlaps between single byte representations and multibyte representations. This overlap causes spurious matches during character processing, leading to incorrect buffer positioning.

It is important to note that the spurious match condition that enables the buffer under-read is not possible when using standard Unicode character sets (such as UTF-8). Exploitation therefore requires applications to be configured with non-standard character encodings that exhibit single/multi-byte encoding overlaps.

The vulnerability can manifest in two ways:

1. Program Crash: When fp->_IO_read_ptr is uninitialized (NULL), the application will crash upon calling ungetwc().
2. Heap Information Disclosure: When fp->_IO_read_ptr points to valid but incorrect memory, adjacent heap data may be exposed through the buffer under-read.

Technical details and the original bug report can be found at Sourceware Bug Report #33998.

Detection Methods for CVE-2026-5928

Indicators of Compromise

• Unexpected crashes in applications using wide character stream functions, particularly ungetwc()
• Segmentation faults originating from _IO_wdefault_pbackfail in glibc
• Anomalous memory access patterns in applications processing character-encoded input streams
• Core dumps showing NULL pointer dereference in libio/wgenops.c functions

Detection Strategies

• Monitor for segmentation faults and crashes in applications linked against glibc that process wide character streams
• Implement application-level logging to track ungetwc() calls on streams with non-Unicode character encodings
• Use memory sanitizers (AddressSanitizer, MemorySanitizer) during application testing to detect buffer under-reads
• Audit applications that use character sets with potential single/multi-byte encoding overlaps

Monitoring Recommendations

• Deploy crash monitoring and reporting for applications using glibc wide character functions
• Implement heap integrity monitoring in security-critical applications
• Review system logs for recurring crashes in applications processing internationalized text streams
• Consider runtime memory protection tools that can detect out-of-bounds read operations

How to Mitigate CVE-2026-5928

Immediate Actions Required

• Inventory all systems running GNU glibc version 2.43 or earlier
• Identify applications that use ungetwc() with non-Unicode character encodings
• Where possible, migrate applications to use Unicode (UTF-8) character encoding which is not affected by the spurious match condition
• Apply vendor patches when available from your Linux distribution

Patch Information

Refer to Sourceware Bug Report #33998 for the latest patch status and technical discussion. Monitor your Linux distribution's security advisories for glibc updates that address this vulnerability. The fix requires correcting the pointer usage in _IO_wdefault_pbackfail to properly reference fp->_wide_data->_IO_read_ptr instead of fp->_IO_read_ptr.

Workarounds

• Configure applications to use Unicode (UTF-8) character encoding where possible, as this encoding does not trigger the spurious match condition
• Avoid using non-standard character encodings that have overlaps between single byte and multi-byte representations
• Implement input validation to ensure streams are properly initialized before calling ungetwc()
• Consider using alternative wide character handling methods that do not rely on ungetwc() until patches are applied

# Check glibc version on your system
ldd --version

# Identify processes using glibc
lsof | grep libc.so

# Monitor for crashes related to this vulnerability
dmesg | grep -i "segfault"