CVE-2026-48689 is a critical off by one heap buffer overflow (CVSS 9.8) in FastNetMon Community Edition through 1.2.9. Five methods of the dynamic_binary_buffer_t class use a wrong bounds check. The researcher's related pages name FastNetMon 2.0.380 as the fix release, so upgrade and confirm in the vendor notes.
What's affected
FastNetMon Community Edition through 1.2.9, in src/dynamic_binary_buffer.hpp. Five methods share the flawed check.
The class is used for BGP messages, NetFlow template processing and Flow Spec construction, so several protocols can reach it. The researcher's pages for sibling CVEs say 2.0.380 contains the vendor fix, so confirm that your release notes list this CVE too.
How to check if you're exposed
List FastNetMon installs and read the version. Community Edition through 1.2.9 is the stated range.
dpkg -l | grep -i fastnetmon
rpm -qa | grep -i fastnetmon
Also list who can send BGP, NetFlow, sFlow or IPFIX traffic to the collector.
What to do now
- List FastNetMon Community Edition installs and versions.
- Upgrade to 2.0.380 or later, after confirming the vendor notes cover this CVE.
- Accept NetFlow, sFlow, IPFIX and BGP only from known exporters and peers.
- Monitor the service for crashes and restarts, and keep core dumps off shared storage.
Why it matters
The classes are CWE-122, heap based buffer overflow, CWE-193, off by one error, and CWE-787, out of bounds write. The record describes a check of the form offset plus length greater than the maximum plus one, where it should compare to the maximum. That lets one byte be written past the end of the heap buffer.
One byte sounds small, but it can corrupt heap bookkeeping data, and the record says this may lead to code execution. The number of code paths using the class makes it easy to reach. The record does not mark this CVE as exploited, and it is not on the CISA Known Exploited Vulnerabilities list.
There is no package manifest to scan for this one, so inventory comes first. List every host, instance or device running an affected build. The Gold directory page for CVE-2026-48689 is a handy place to follow later updates.
Quick check
Why does one byte matter?
Heap allocators store metadata next to data. A single byte written over it can change how memory is later managed.
Sources
Last reviewed: 19 October 2026.
