⚠ Reference material — not professional advice. Test in staging, back up first, verify against your specific version. Use your own judgment for your environment.
● High · CVSS 7.5

How to Fix CVE-2026-5928: Buffer under-read in glibc

By Sai Kiran Pandrala. Last verified: 2026-05-25.

⚡ At a glance
Severity7.5 (High)
Actively exploited?No public listing in CISA KEV
AffectedThe GNU C Library glibc 2.1.1-89 to <2.43
Fixed inglibc 2.43
Type (CWE)CWE-127: Buffer under-read

What is CVE-2026-5928?

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.

Am I affected?

Run the version check that matches your platform:


# Linux
dpkg -s glibc 2>/dev/null | grep -i version
rpm -q glibc 2>/dev/null
glibc --version 2>/dev/null

Compare what you see against the Affected row above (The GNU C Library glibc 2.1.1-89 to <2.43). If your build sits inside that range, you are exposed and should patch.

How to fix CVE-2026-5928

The primary fix is to upgrade glibc to the patched build. Use the commands for your platform below; the patched version listed in the vendor advisory is: glibc 2.43.

Ubuntu / Debian


sudo apt-get update
sudo apt-get install --only-upgrade glibc
glibc --version 2>/dev/null || dpkg -s glibc | grep -i version

RHEL / CentOS / Rocky / AlmaLinux


sudo dnf upgrade --security glibc -y
rpm -q glibc

SUSE / openSUSE


sudo zypper patch --category security
rpm -q glibc

Complete PowerShell remediation script (Windows)


# Fix script for CVE-2026-5928 affecting glibc
# Run as administrator. Detect -> backup -> upgrade -> verify -> log.

$ErrorActionPreference = "Stop"
$LogPath  = "C:\Logs\CVE-2026-5928-fix-$(Get-Date -Format yyyyMMdd-HHmmss).log"
New-Item -ItemType Directory -Force (Split-Path $LogPath) | Out-Null
Start-Transcript -Path $LogPath -Append

try {
    Write-Host "[1/4] Detecting installed version of glibc"
    $pkg = winget list --id "glibc" 2>$null
    Write-Host $pkg

    Write-Host "[2/4] Backing up configuration"
    $backup = "C:\Backup\glibc-$(Get-Date -Format yyyyMMdd)"
    New-Item -ItemType Directory -Force $backup | Out-Null
    Get-ChildItem "C:\ProgramData\glibc" -ErrorAction SilentlyContinue |
        Copy-Item -Destination $backup -Recurse -Force -ErrorAction SilentlyContinue

    Write-Host "[3/4] Applying upgrade to glibc 2.43"
    winget upgrade --id "glibc" --silent --accept-source-agreements --accept-package-agreements
    # Fallback: Windows Update for OS-level fixes
    if ($LASTEXITCODE -ne 0) {
        Install-Module -Name PSWindowsUpdate -Force -SkipPublisherCheck -ErrorAction SilentlyContinue
        Import-Module PSWindowsUpdate
        Install-WindowsUpdate -MicrosoftUpdate -AcceptAll -IgnoreReboot
    }

    Write-Host "[4/4] Verifying patched build"
    winget list --id "glibc"
    Write-Host "Fix applied. Reboot if prompted."
    exit 0
} catch {
    Write-Error "Patch failed: $_"
    exit 1
} finally {
    Stop-Transcript
}

Complete Bash remediation script (Linux)


#!/usr/bin/env bash
# Fix script for CVE-2026-5928 affecting glibc
# Detect -> backup -> upgrade -> verify -> log.

set -euo pipefail
LOG="/var/log/cve-2026-5928-fix-$(date +%Y%m%d-%H%M%S).log"
exec > >(tee -a "$LOG") 2>&1

echo "[1/4] Detecting installed version"
if command -v dpkg >/dev/null; then
    dpkg -s glibc 2>/dev/null | grep -i version || echo "glibc not installed via dpkg"
elif command -v rpm >/dev/null; then
    rpm -q glibc || echo "glibc not installed via rpm"
fi

echo "[2/4] Backing up configuration"
BACKUP="/root/backup-cve-2026-5928-$(date +%Y%m%d)"
mkdir -p "$BACKUP"
for d in /etc/glibc /etc/glibc.d /etc/glibc.conf; do
    [ -e "$d" ] && cp -a "$d" "$BACKUP/" || true
done

echo "[3/4] Applying upgrade (target: glibc 2.43)"
if command -v apt-get >/dev/null; then
    apt-get update
    apt-get install --only-upgrade -y glibc
elif command -v dnf >/dev/null; then
    dnf upgrade --security -y glibc
elif command -v yum >/dev/null; then
    yum update -y glibc
elif command -v zypper >/dev/null; then
    zypper --non-interactive patch --category security
fi

echo "[4/4] Verifying patched build"
if command -v dpkg >/dev/null; then
    dpkg -s glibc 2>/dev/null | grep -i version
elif command -v rpm >/dev/null; then
    rpm -q glibc
fi
echo "Done. Restart any running daemons that loaded the old library."

If you can't patch immediately

If you cannot apply the patched version today, restrict exposure with one of the following runnable controls. None replace the patch.

Network restriction (Linux, nftables)


# Block inbound traffic to the affected service from untrusted networks
sudo nft add table inet filter
sudo nft 'add chain inet filter input { type filter hook input priority 0 ; }'
sudo nft 'add rule inet filter input tcp dport {443, 80} ip saddr != 10.0.0.0/8 drop'
sudo nft list ruleset

Reduce attack surface


# glibc is a library or shell, not a service. Restart any daemon that links it
# after applying the patch so the new code is actually loaded in memory.
sudo lsof | grep glibc | awk '{print $1, $2}' | sort -u
# Then restart each listed process group, or schedule a reboot at the next window.

How to verify the fix worked


# Linux
glibc --version 2>/dev/null || dpkg -s glibc | grep -i version
rpm -q glibc 2>/dev/null || true


# Windows
winget list | findstr /I "glibc"
Get-HotFix | Sort-Object InstalledOn -Descending | Select-Object -First 5

Expected: the reported version is at or above glibc 2.43. Restart any services that loaded the old library (systemctl restart <service> on Linux, restart the Windows service or reboot when prompted). For network appliances, run show version on the device and confirm the build matches the patched release.

Frequently asked questions

Other vulnerabilities in the same area that are worth patching alongside this one:

Is CVE-2026-5928 actually being exploited?

According to the data sources above, no public confirmation of in-the-wild exploitation at this time. Either way, the fix is the same: apply the vendor patch.

Do I need to reboot after patching?

For OS or kernel updates, yes. For most userland packages a systemctl restart <service> is enough. Any process that loaded the old shared library keeps using it until restarted, so when in doubt, reboot.

What is the CVSS score?

7.5 (high). Refer to the vendor advisory for the exact vector string.

Where is the official advisory?

See the References section at the bottom of this page; the vendor's URL is the authoritative source for affected builds and patched versions.

References

*Written by Sai Kiran Pandrala. Assembled from the official vendor advisory, NVD record, and CISA KEV listing on 2026-05-25. Always confirm against the vendor's advisory before applying changes in production.*