dbcve.org
FnetApplication · Butok

CVE-2020-27633

CRITICAL · 9.1 CVSS v3.1 Published 2023-10-10
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
100/100
Remediation priority · Urgent
Remotely reachable No privileges Zero-click

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
In FNET 4.6.3, TCP ISNs are improperly random.

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

FNET 4.6.3 contains a vulnerability where TCP Initial Sequence Numbers (ISNs) are not properly randomized, making them predictable. This allows attackers to potentially predict ISNs and perform TCP sequence number prediction attacks, enabling session hijacking or man-in-the-middle attacks on TCP connections.

MitigationUpgrade FNET to a version that implements cryptographically secure TCP ISN generation. If no patched version is available, consider network-level controls such as firewalls or IPS to mitigate TCP-based injection attacks.

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

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
FnetApplication
Affected:= 4.6.3

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
High
Integrity
High
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/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 FNET is in use
    Search your codebase, firmware, or software Bill of Materials (BOM) for references to 'FNET', 'Butok Fnet', or the FNET library files. Inspect any third-party or networking stack components used in your application or device firmware.
    Affected if FNET or Butok Fnet is present in your environment
  2. Confirm the FNET version
    Locate the version information for the FNET library in use. This may be in source files (often in header files like fnet.h or in version-related source files), documentation, or build configuration. Compare your installed version against the affected version 4.6.3.
    Affected if The installed FNET version is exactly 4.6.3
  3. Verify TCP protocol is enabled
    Inspect your FNET configuration files or build settings to determine if TCP/IP networking is enabled. Look for FNET configuration options such as FNET_CFG_TCP or similar TCP-related compile-time flags in your project.
    Affected if TCP protocol is enabled and in use
  4. Inspect ISN generation implementation
    Examine the FNET source code files responsible for TCP sequence number generation, typically found in the TCP implementation source files. Look for the function or code that generates the Initial Sequence Number for new TCP connections.
    Affected if The ISN generation uses a predictable or non-randomized method (such as a simple counter or time-based value without cryptographic randomization)

Your environment is affected if you are using Butok Fnet version 4.6.3 with TCP enabled, and the TCP ISN generation does not use cryptographically secure randomization.

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 FNET to a version that implements cryptographically secure TCP ISN generation. If no patched version is available, consider network-level controls such as firewalls or IPS to mitigate TCP-based injection attacks.

Recommended fix Moderate confidence

Latest stable FNET release newer than 4.6.3 (check official FNET repository for current version)

  1. 1. Check the current FNET version in your project by examining the version header file (typically fnet.h or version.h)
  2. 2. Navigate to the official FNET project repository or release page to identify the latest stable release
  3. 3. Review the release notes and changelog for versions newer than 4.6.3 to confirm the ISN randomization vulnerability is addressed
  4. 4. If a newer version exists, update the FNET dependency in your project to that version
  5. 5. Rebuild and test the application to ensure the upgrade does not introduce regressions
  6. 6. Verify the fix by examining the TCP sequence number generation code in the updated version to confirm cryptographic random number generation is implemented
Caveat Minor - verify API compatibility if using specific FNET networking APIs; embedded TCP/IP stack upgrades typically have low breaking change risk

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

Fix this in Fnet Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing6.0 h
  • Review / QA4.0 h
22.0 hours of engineering $3,860
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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-2020-27633 in production — separate from our analysis above.

No notes yet

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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
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data