# CVE-2026-48689: fastnetmon: FastNetMon Community Edition through 1.2.9 contains an off-by-one heap-based buffer overflow in the dynamic_binary_bu...

CVE-2026-48689 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-2026-48689
- Priority: Urgent
- CVSS: 9.8 (critical)
- EPSS: 0.0068
- 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 off-by-one heap-based buffer overflow in the dynamic_binary_buffer_t class (src/dynamic_binary_buffer.hpp). Five methods (append_dynamic_buffer, append_data_as_pointer, append_data_as_object_ptr, memcpy_from_ptr, memcpy_from_object_ptr) use an incorrect bounds check of the form 'if (offset + length > maximum_internal_storage_size + 1)' instead of the correct 'if (offset + length > maximum_internal_storage_size)'. This allows writing exactly one byte past the end of the heap-allocated buffer. The class is used pervasively in BGP message encoding/decoding, NetFlow template processing, and Flow Spec NLRI construction. An attacker who can send network traffic (NetFlow, sFlow, IPFIX, or BGP) to a FastNetMon instance can trigger this overflow, potentially achieving arbitrary code execution by corrupting heap metadata. Notably, the append_byte() method uses the correct bounds check, confirming the inconsistency.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | fastnetmon Debian 13.5 (trixie) | 1.2.9-0+deb13u1 | [DEBIAN-CVE-2026-48689](https://patchblox.com/security/data/fix/linux/debian-cve-2026-48689.md) | Jun 24, 2026 |
| Linux | fastnetmon Debian 14 | 1.2.9-1 | [DEBIAN-CVE-2026-48689](https://patchblox.com/security/data/fix/linux/debian-cve-2026-48689.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 2026-11-08T13:06:02Z 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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