# CVE-2026-48691: fastnetmon: FastNetMon Community Edition through 1.2.9 contains an integer overflow in the BGP AS_PATH attribute encoder

CVE-2026-48691 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-2026-48691
- Priority: Urgent
- CVSS: 9.8 (critical)
- EPSS: 0.0031
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: May 26, 2026

## Description

FastNetMon Community Edition through 1.2.9 contains an integer overflow in the BGP AS_PATH attribute encoder. In src/bgp_protocol.hpp, the IPv4UnicastAnnounce::get_attributes() function computes attribute_length as 'sizeof(bgp_as_path_segment_element_t) + this->as_path_asns.size() * sizeof(uint32_t)' and stores it in a uint8_t field (line 600-605). Since uint8_t can only hold values 0-255, an AS_PATH containing more than 63 ASNs (2 + 64*4 = 258 > 255) causes silent truncation. The truncated length is used for buffer sizing, while the actual data written is the full untruncated amount, resulting in a heap buffer overflow. Similarly, the path_segment_length field at line 621 is also uint8_t, truncating with more than 255 ASNs.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | fastnetmon Debian 13.5 (trixie) | 1.2.9-0+deb13u1 | [DEBIAN-CVE-2026-48691](https://patchblox.com/security/data/fix/linux/debian-cve-2026-48691.md) | Jun 24, 2026 |
| Linux | fastnetmon Debian 14 | 1.2.9-1 | [DEBIAN-CVE-2026-48691](https://patchblox.com/security/data/fix/linux/debian-cve-2026-48691.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 2026-11-09T01:18:40Z 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 Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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