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Out-of-bounds ReadWeakness · CWE-125

CVE-2026-9539

MEDIUM · 6.5 CVSS v3.1 Published 2026-06-24
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
67/100
Remediation priority · Elevated
Zero-click 8 weeks old

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
An out-of-bounds heap read and integer underflow in the TCP urgent data handling (sosendoob) in freedesktop.org libslirp version before v4.9.2 on hypervisor host environments (e.g., QEMU) allows a privileged guest VM attacker (root or CAP_NET_RAW) to leak gigabytes of sensitive host-process heap memory via sending crafted TCP segments with manipulated URG flags and urgent pointers (ti_urp).

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

An integer underflow and out-of-bounds heap read vulnerability exists in libslirp's TCP urgent data handling (sosendoob function) before version v4.9.2. A privileged guest VM attacker with root or CAP_NET_RAW capabilities can send crafted TCP segments with manipulated URG flags and urgent pointers (ti_urp), enabling gigabytes of sensitive host-process heap memory to be leaked.

MitigationUpgrade libslirp to v4.9.2 or later on hypervisor hosts. If upgrading is not immediately feasible, restrict VM privileges and network capabilities to prevent untrusted or non-privileged VMs from having CAP_NET_RAW access.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Local
Complexity
Low
Privileges
Low
User interaction
None
Scope
Changed
Confidentiality
High
Integrity
None
Availability
None

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify if libslirp is in use
    Search for libslirp library files on the system or within hypervisor installations. Common locations include /usr/lib/, /usr/local/lib/, or within hypervisor binary directories. Look for files named libslirp* or use package management commands (dpkg -l, rpm -qa) to list installed packages.
    Affected if libslirp library is present on the host system or bundled with a hypervisor installation.
  2. Determine the installed libslirp version
    Use commands appropriate to the system: for package managers, run 'dpkg -l | grep slirp' or 'rpm -qa | grep slirp'; for binary inspection, check the library file metadata or version strings within the file. Also check hypervisor documentation or 'qemu-img --version' output if QEMU is used.
    Affected if The installed version is any version earlier than v4.9.2, or the version cannot be determined and libslirp is present.
  3. Verify hypervisor usage of libslirp
    Identify if the host runs a hypervisor that uses libslirp for network isolation (such as QEMU with user-mode networking or libvirt configurations). Review hypervisor configurations to confirm slirp-based networking is enabled.
    Affected if The hypervisor uses libslirp for VM network connectivity, particularly with user-mode networking (slirp/port redirection).
  4. Assess VM privilege levels
    Review VM configurations and access controls to determine whether any VMs operate with elevated privileges, specifically root access or CAP_NET_RAW capability. Check VM definitions (libvirt XML, QEMU command lines) for security settings.
    Affected if Untrusted or less-trusted VMs have root privileges or CAP_NET_RAW capability granted, allowing them to send raw TCP segments.

The environment is affected if libslirp versions earlier than v4.9.2 are in use by the hypervisor and untrusted VMs have elevated network privileges (root or CAP_NET_RAW), enabling crafted TCP urgent data to trigger the heap read vulnerability.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Upgrade libslirp to v4.9.2 or later on hypervisor hosts. If upgrading is not immediately feasible, restrict VM privileges and network capabilities to prevent untrusted or non-privileged VMs from having CAP_NET_RAW access.

Recommended fix Moderate confidence

libslirp v4.9.2 or later

  1. 1. Identify all systems running libslirp in hypervisor host environments (e.g., QEMU)
  2. 2. Check the current installed version of libslirp using package manager or build system (e.g., rpm -qa | grep slirp, dpkg -l, or reviewing build files)
  3. 3. Verify if the current version is earlier than v4.9.2
  4. 4. Update the libslirp package to version 4.9.2 or later using the distribution's package manager (e.g., apt-get update && apt-get upgrade, yum update, or dnf update)
  5. 5. If using a compiled-from-source integration, obtain libslirp v4.9.2 or later from https://gitlab.freedesktop.org/slirp/libslirp/-/releases
  6. 6. Rebuild and redeploy any affected hypervisor/QEMU binaries that link against the updated libslirp
  7. 7. Restart affected virtual machine services or hosts to ensure the patched library is loaded
  8. 8. Verify the patched version is now running (e.g., ldd on the hypervisor binary or checking package version)
Caveat Security fix in a library; test in non-production environment first to ensure hypervisor functionality remains intact

Generated from the published advisory — verify against the referenced sources before acting.

Have this fixed Scoped from the published advisory
  • Consultation3.0 h
  • Implementation6.0 h
  • Testing4.0 h
  • Review / QA2.0 h
15.0 hours of engineering $2,640
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-9539 in production — separate from our analysis above.

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What this is

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What belongs here
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  • Version or environment caveats, and links to real fixes
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