---
id: "CVE-2026-122129"
kind: "vulnerability"
aliases: ["CVE-2026-122129"]
title: "ipfixcol2: unbounded LZ4 buffer lets remote attackers exhaust collector memory"
severity: "medium"
cvss: 6.9
epss: null
exploited: false
status: "open"
category: "other"
products: []
ecosystems: []
tags: ["ipfixcol2", "linux", "network-monitoring", "other"]
not_affected_if: ["file-input-only", "firewalled-collector", "no-tcp-listener"]
published_at: "2026-11-04T21:18:08+00:00"
first_seen_at: "2026-11-04T21:49:19+00:00"
updated_at: "2026-11-04T21:51:03+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-122129", "first_seen_at": "2026-11-04T21:49:19+00:00"}]
exploit_sources: []
---

# ipfixcol2: unbounded LZ4 buffer lets remote attackers exhaust collector memory

CESNET ipfixcol2 before 2.9.0 is an IPFIX flow collector whose LZ4 decoder trusts a declared ring buffer size from the start header. An unauthenticated attacker who opens TCP connections with very large sizes can force up to 4 GiB of allocation per connection and deny collector service.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Check the installed ipfixcol2 version, for example with `ipfixcol2 --version` or the package manager, and see whether it is below `2.9.0`.
- Look for an LZ4-compressed TCP IPFIX listener in the collector configuration.

## Fix

- ipfixcol2: move to `2.9.0` or later

## Timeline

- 2026-11-04T21:49:19+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-122129

## Documents

- [advisory] Third-party advisory: www.vulncheck.com https://www.vulncheck.com/advisories/cesnet-ipfixcol2-2.7.1-before-2.9.0-memory-exhaustion-via-tcp-lz4-decoder
- [fix] Patch: github.com https://github.com/CESNET/ipfixcol2/commit/1a199033eff847941353c5c039cb806a9ad8aa6f
- [fix] Release notes: github.com https://github.com/CESNET/ipfixcol2/releases/tag/v2.9.0

## From the sources

CESNET ipfixcol2 2.7.1 before 2.9.0 contains a memory allocation vulnerability in Lz4Decoder::read_start_header() that allows unauthenticated attackers to exhaust memory by declaring unbounded ring buffer sizes. Attackers can open TCP connections sending LZ4 start headers with very large buffer sizes, forcing up to 4 GiB allocations per connection and denying collector service.

---

_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_