dbcve.org
Integer OverflowWeakness · CWE-190

CVE-2026-39042

HIGH · 7.5 CVSS v3.1 Published 2026-07-13
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
84/100
Remediation priority · High
Remotely reachable No privileges Zero-click 5 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 issue in MikroTIk (SIA Mikrotikls, Latvia) RouterOS 7.21.x before v.7.21.4 and 7.22.x before v.7.22.2 allows a remote attacker to cause a denial of service via the unflatten() function in libumsg.so.

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 · moderate confidence

A denial of service vulnerability in MikroTIk RouterOS versions 7.21.x before 7.21.4 and 7.22.x before 7.22.2 allows remote attackers to crash the device via a flaw in the unflatten() function within libumsg.so.

MitigationUpgrade RouterOS to version 7.21.4 or 7.22.2 or later to patch the vulnerable unflatten() function in libumsg.so.

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
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

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

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. Check RouterOS version
    Run '/system resource print' in the RouterOS CLI or check the System > Resources page in WebFig/WinBox to view the current RouterOS version
    Affected if The version displayed is 7.21.x before 7.21.4 (such as 7.21.3, 7.21.2, 7.21.1, or 7.21) or 7.22.x before 7.22.2 (such as 7.22.1 or 7.22)
  2. Confirm libumsg.so is in use
    Verify the RouterOS installation includes the libumsg.so library by checking '/file print' for libumsg.so or using '/system package print' to confirm messaging packages are installed
    Affected if The libumsg.so library exists on the device and the messaging subsystem is active
  3. Identify exposure to network attack surface
    Review firewall rules with '/ip firewall print' and NAT configuration with '/ip nat print' to determine if the device's WAN interface or relevant network paths allow external traffic to reach the messaging service that uses libumsg.so
    Affected if The device is directly accessible from untrusted networks on ports or protocols that exercise the unflatten() function in libumsg.so
  4. Check for crash logs
    Run '/log print' and filter for entries around the time of any unexplained device reboots or crashes, looking for libumsg.so related errors
    Affected if Recent crash logs contain references to libumsg.so, unflatten, or memory corruption events

A device is affected if it runs RouterOS version 7.21.x before 7.21.4 or 7.22.x before 7.22.2, has the libumsg.so library present, and is reachable by remote attackers who can trigger the unflatten() function flaw causing denial of service.

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 RouterOS to version 7.21.4 or 7.22.2 or later to patch the vulnerable unflatten() function in libumsg.so.

Recommended fix Moderate confidence

RouterOS 7.21.4 or 7.22.2 (or later stable release)

  1. 1. Identify current RouterOS version using '/system resource print' or '/system routerboard print'
  2. 2. If running 7.21.x before 7.21.4, or 7.22.x before 7.22.2, plan for upgrade
  3. 3. Download the fixed RouterOS version (7.21.4 or 7.22.2 or later stable) from mikrotik.com/download
  4. 4. Upload the .npk file to the router via Files or use '/system package update' if using the beta/channel
  5. 5. Reboot the router to apply the new RouterOS version
  6. 6. After reboot, verify the new version with '/system resource print' to confirm upgrade success
Caveat MikroTik RouterOS upgrades typically preserve configuration but always backup config before upgrading; test in lab if possible

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

Have this fixed Scoped from the published advisory
  • Consultation2.0 h
  • Implementation4.0 h
  • Testing2.0 h
  • Review / QA1.0 h
9.0 hours of engineering $1,600
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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-2026-39042 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

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