# CVE-2026-48686: fastnetmon: FastNetMon Community Edition through 1.2.9 contains a stack-based buffer overflow in the BGP NLRI (Network Layer Reac...

CVE-2026-48686 is fixed in fastnetmon on 1 platform, first by fastnetmon 1.2.8+git20250911-2ubuntu0.1~esm1, released May 26, 2026. It is not reported as exploited. Patch Radar priority: Urgent.

- Web page: https://patchblox.com/security/cve/CVE-2026-48686
- Priority: Urgent
- CVSS: 9.8 (critical)
- EPSS: 0.0057
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: May 26, 2026

## Description

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.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | fastnetmon Debian 13.5 (trixie) | 1.2.9-0+deb13u1 | [DEBIAN-CVE-2026-48686](https://patchblox.com/security/data/fix/linux/debian-cve-2026-48686.md) | Jun 24, 2026 |
| Linux | fastnetmon Debian 14 | 1.2.9-1 | [DEBIAN-CVE-2026-48686](https://patchblox.com/security/data/fix/linux/debian-cve-2026-48686.md) | Jun 24, 2026 |
| Linux | fastnetmon Ubuntu 24.04.5 LTS (noble) | 1.2.6-1ubuntu0.1~esm1 | [fastnetmon 1.2.6-1ubuntu0.1~esm1](https://patchblox.com/security/data/fix/linux/ubuntu-noble-fastnetmon-1.2.6-1ubuntu0.1-esm1.md) | Jun 24, 2026 |
| Linux | fastnetmon Ubuntu 26.04.1 LTS (resolute) | 1.2.8+git20250911-2ubuntu0.1~esm1 | [fastnetmon 1.2.8+git20250911-2ubuntu0.1~esm1](https://patchblox.com/security/data/fix/linux/ubuntu-resolute-fastnetmon-1.2.8-git20250911-2ubuntu0.1-esm1.md) | May 26, 2026 |

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Data as of 2026-11-08T00:53:09Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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