A heap-based buffer overflow was found in the DHCPv6 and TFTP response builders of libslirp.

CVE-2026-95508 is a heap-based buffer overflow in libslirp, the user-mode TCP/IP emulation library consumed by QEMU (-netdev user) and slirp4netns (rootless container networking). A malicious guest VM or container can send a crafted DHCPv6 CLIENTID option or TFTP blksize option that overflows the reply buffer in the host-side libslirp process, potentially achieving denial of service or arbitrary code execution on the host. The bug is triggered only when the host has configured a non-default small interface MTU for the slirp network; the default MTU is not affected. Affected versions span the entire libslirp 4.x line through 4.9.4; the fix landed in 4.9.5 via two GitLab commits addressing the DHCPv6 and TFTP response builders respectively.

The vendor severity of HIGH 7.4 overstates the real-world risk for enterprises. The CVSS vector marks AV:N (network), but the actual attacker position is *inside a guest VM or container* using slirp networking — that is post-compromise of the guest, not a true unauthenticated-remote-from-internet posture. More importantly, production QEMU/KVM deployments universally use tap/bridge or virtio networking, never slirp user-mode. Rootless Podman has defaulted to pasta since v5.0 and removed slirp4netns entirely in v6. The remaining population running slirp in anger is developer laptops, CI runners, and test labs — none of which are fleet-critical. On top of that, the trigger requires a non-default small MTU, further narrowing exposure. The vendor score is technically defensible in a vacuum but practically misleading for anyone managing production infrastructure.

1. MEDIUM 5.3→IGNORE 0.0
   Switch VMs from slirp to tap/bridge networking — Reconfigure any QEMU instances using -netdev user to use -netdev tap or bridge networking instead. This completely removes libslirp from the attack surface. No noisgate mitigation SLA applies at MEDIUM; proceed within the 365-day remediation window, but prioritize if you have known slirp usage.
2. MEDIUM 5.3→IGNORE 0.0
   Migrate rootless Podman to pasta networking — Set default_rootless_network_cmd = \"pasta\" in containers.conf or upgrade to Podman 5+. This removes slirp4netns from the container networking path entirely. If running Podman 6+, slirp4netns is already removed.
3. MEDIUM 5.3→LOW 2.0
   Audit and remove non-default small MTU configurations — Search QEMU command lines, libvirt XML, and container runtime configs for explicit small MTU settings on slirp interfaces. Revert to default MTU. Even if slirp remains in use, default MTU is not vulnerable.
4. MEDIUM 5.3→IGNORE 0.0
   Update libslirp to 4.9.5 — Apply the vendor patch when your distro ships it. Debian has not yet backported as of 2026-09-24; Red Hat is tracking via Bugzilla #2537748. This is the definitive fix within the 365-day remediation window.

Run this script on any host running QEMU or slirp4netns. No special privileges needed — it checks the installed libslirp version and scans for running QEMU processes using slirp networking. Invoke with: bash check_cve_2026_95508.sh

#!/usr/bin/env bash
# check_cve_2026_95508.sh — CVE-2026-95508 libslirp heap overflow checker
# Checks: (1) installed libslirp version, (2) running QEMU with slirp networking
# Output: VULNERABLE / PATCHED / UNKNOWN
# Exit codes: 0=PATCHED, 1=VULNERABLE, 2=UNKNOWN

set -euo pipefail

FIXED_VERSION="4.9.5"
RESULT="UNKNOWN"

version_gte() {
  printf '%s\n%s' "$1" "$2" | sort -V | head -n1 | grep -qx "$2"
}

# 1. Check installed libslirp version
INSTALLED=""
if command -v dpkg-query &>/dev/null; then
  INSTALLED=$(dpkg-query -W -f='${Version}' libslirp0 2>/dev/null | grep -oP '[0-9]+\.[0-9]+\.[0-9]+' || true)
elif command -v rpm &>/dev/null; then
  INSTALLED=$(rpm -q --qf '%{VERSION}' libslirp 2>/dev/null || true)
elif command -v pacman &>/dev/null; then
  INSTALLED=$(pacman -Q libslirp 2>/dev/null | awk '{print $2}' | grep -oP '[0-9]+\.[0-9]+\.[0-9]+' || true)
fi

if [ -n "$INSTALLED" ]; then
  echo "[*] libslirp installed version: $INSTALLED"
  if version_gte "$INSTALLED" "$FIXED_VERSION"; then
    echo "[+] Version >= $FIXED_VERSION — patched."
    RESULT="PATCHED"
  else
    echo "[-] Version < $FIXED_VERSION — library is vulnerable."
    RESULT="VULNERABLE"
  fi
else
  echo "[?] libslirp not found via package manager."
  # Try finding shared library
  SOFILE=$(find /usr/lib* /lib* -name 'libslirp.so*' 2>/dev/null | head -1 || true)
  if [ -n "$SOFILE" ]; then
    echo "[?] Found libslirp at $SOFILE but cannot determine version."
    RESULT="UNKNOWN"
  else
    echo "[+] libslirp not installed on this host."
    RESULT="PATCHED"
  fi
fi

# 2. Check for running QEMU with slirp (user-mode) networking
SLIRP_PROCS=$(pgrep -a qemu 2>/dev/null | grep -E '(-netdev\s+user|type=user)' || true)
if [ -n "$SLIRP_PROCS" ]; then
  echo ""
  echo "[!] WARNING: Running QEMU processes using slirp (user-mode) networking:"
  echo "$SLIRP_PROCS"
  if [ "$RESULT" = "VULNERABLE" ]; then
    echo "[-] ACTIVE EXPOSURE: vulnerable libslirp + slirp networking in use."
  fi
else
  echo ""
  echo "[+] No running QEMU processes using slirp networking detected."
fi

# 3. Check for slirp4netns
if command -v slirp4netns &>/dev/null; then
  S4N_PROCS=$(pgrep -a slirp4netns 2>/dev/null || true)
  if [ -n "$S4N_PROCS" ]; then
    echo "[!] WARNING: slirp4netns processes running (rootless container networking)."
    if [ "$RESULT" = "VULNERABLE" ]; then
      echo "[-] ACTIVE EXPOSURE: vulnerable libslirp + slirp4netns in use."
    fi
  else
    echo "[+] slirp4netns installed but no active processes."
  fi
fi

echo ""
echo "=== $RESULT ==="
case "$RESULT" in
  PATCHED)  exit 0 ;;
  VULNERABLE) exit 1 ;;
  *)        exit 2 ;;
esac

1. NixOS Security Tracker - Issue #566199
2. OSV - DEBIAN-CVE-2026-95508
3. Red Hat CVE Portal
4. Red Hat Bugzilla #2537748
5. Debian Package Tracker - libslirp
6. Strix.ai CVE-2026-95508 Analysis
7. ThreatInt CVE-2026-95508
8. libslirp GitLab DHCPv6 Fix Commit