dbcve.org
FastnetmonApplication · Pavel Odintsov

CVE-2026-48686

CRITICAL · 9.8 CVSS v3.1 Published 2026-05-26
Fix available
A fix is available. Upgrade to after 1.2.9 or later.
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
FastNetMon Community Edition through 1.2.9 contains a stack-based buffer overflow in the BGP NLRI (Network Layer Reachability Information) decoder. The function decode_bgp_subnet_encoding_ipv4_raw() in src/bgp_protocol.cpp reads prefix_bit_length directly from the BGP packet (line 99) without validating it is <= 32 for IPv4 prefixes. This value is passed to how_much_bytes_we_need_for_storing_certain_subnet_mask() which computes ceil(prefix_bit_length / 8), returning up to 32 bytes for a prefix_bit_length of 255. The result is used as the length argument to memcpy() (line 106), which copies into a 4-byte uint32_t stack variable (prefix_ipv4). This causes a stack buffer overflow of up to 28 bytes, which can be exploited for arbitrary code execution. Additionally, the unvalidated prefix_bit_length is passed to convert_cidr_to_binary_netmask_local_function_copy() (line 111), where a shift of (32 - cidr) with cidr > 32 causes undefined behavior.

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

FastNetMon Community Edition through 1.2.9 has a stack-based buffer overflow in the BGP NLRI decoder. The function decode_bgp_subnet_encoding_ipv4_raw() reads prefix_bit_length from the BGP packet without validating it is <= 32, then uses this value to compute a memcpy size that can be up to 32 bytes while copying into a 4-byte stack variable, causing up to 28 bytes of overflow exploitable for arbitrary code execution. Additionally, unvalidated prefix_bit_length passed to a shift operation causes undefined behavior when > 32.

MitigationImplement strict validation of prefix_bit_length to ensure it is <= 32 for IPv4 before any processing; use dynamically-sized buffers or validate buffer sizes to prevent overflow; add bounds checking before shift operations.

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
FastnetmonApplication
Affected:<= 1.2.9

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
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/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 FastNetMon installed version
    Run 'fastnetmon --version' or 'dpkg -l | grep fastnetmon' (Debian) / 'rpm -qi fastnetmon' (RHEL)
    Affected if Version is 1.2.9 or lower
  2. Verify BGP processing is enabled
    Inspect fastnetmon.conf for 'bgp_enable' or 'process_bgp' setting set to 'yes' or 'true'
    Affected if BGP processing is enabled in the configuration
  3. Check for configured BGP neighbors
    Look for 'bgp_neighbors' or 'bgp_peer' entries in fastnetmon.conf
    Affected if BGP peers are configured and being processed
  4. Inspect BGP traffic handling
    Monitor if FastNetMon is receiving BGP packets on port 179 or parsing BGP updates
    Affected if FastNetMon is actively parsing BGP NLRI data

You are affected if FastNetMon version is 1.2.9 or earlier AND BGP processing is enabled with active BGP neighbors sending traffic to the instance.

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
Upgrade available Upgrade to a release after 1.2.9
Interim mitigation

Implement strict validation of prefix_bit_length to ensure it is <= 32 for IPv4 before any processing; use dynamically-sized buffers or validate buffer sizes to prevent overflow; add bounds checking before shift operations.

Recommended fix Moderate confidence

Latest stable release after 1.2.9 (check GitHub releases for version > 1.2.9)

  1. 1. Identify the current FastNetMon installation version using 'fastnetmon --version' or checking the package manager
  2. 2. Obtain the latest stable release from the official FastNetMon GitHub repository (github.com/pavel-odintsov/fastnetmon)
  3. 3. Review the release notes or commits to confirm the vulnerability in decode_bgp_subnet_encoding_ipv4_raw() has been addressed
  4. 4. Upgrade to the fixed release version (version higher than 1.2.9)
  5. 5. Verify the upgrade by checking 'fastnetmon --version' post-installation
  6. 6. Test BGP parsing functionality to ensure the fix does not break legitimate traffic processing
Caveat Review release notes for potential breaking changes between 1.2.9 and the target version, particularly in BGP handling functionality

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

Fix this in Fastnetmon Scoped from the published advisory
  • Consultation3.0 h
  • Implementation6.0 h
  • Testing5.0 h
  • Review / QA3.0 h
17.0 hours of engineering $2,970
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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-2026-48686 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
  • 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