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The vulnerability resides in the `process_netflow_v9_options_template()` function within the `src/netflow_plugin/netflow_v9_collector.cpp` file. When FastNetMon receives a NetFlow v9 Options Template FlowSet, it enters two parsing loops that lack essential bounds checking. The first loop (lines 224-229) processes “scopes” for the option template. It continues to iterate until an index named `scopes_offset` reaches a value supplied by the attacker, option_scope_length. At each iteration, it attempts to read a `netflow9_template_flowset_record_t` structure. Before performing this read, there is no verification to ensure that `(zone_address + scopes_offset + sizeof(record))` remains within the allocated UDP packet buffer. A second, identical vulnerability exists in the “options field” loop (lines 241-257), which uses another attacker-controlled value, option_length. An attacker can set either `option_scope_length` or `option_length` to a value that forces the loop to continue beyond the packet’s end. Furthermore, the code does not verify that `option_scope_length` is a multiple of the structure’s size. A misaligned value can cause the parser to read from arbitrary, unaligned memory offsets, leading to undefined behavior. By crafting a malicious NetFlow v9 Options Template FlowSet with inflated length values and sending it to the collector’s default UDP port (2055), an attacker can force the application to read memory past the end of the packet buffer. This out-of-bounds read may leak sensitive information or cause the application to crash.
DailyCVE Form
Platform: FastNetMon Community
Version: 1.2.9 and earlier
Vulnerability: Out-of-bounds read
Severity: Medium (CVSS 6.5)
Date: 2026-05-26
Prediction: Patch expected 2026-06-30
What Undercode Say:
Monitor for malformed NetFlow v9 packets sudo tcpdump -i eth0 udp port 2055 -vv -X Log current memory maps of fastnetmon process sudo cat /proc/$(pidof fastnetmon)/maps Check for crash logs after suspected exploitation sudo journalctl -u fastnetmon -f --since "2026-05-26"
Exploit:
!/usr/bin/env python3 import socket def exploit(target_ip): Craft a NetFlow v9 Options Template FlowSet Set option_scope_length and option_length to induce OOB read sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM) packet = b"\x00\x01" Placeholder crafted payload sock.sendto(packet, (target_ip, 2055)) sock.close()
Protection from this CVE
- Allowlist the NetFlow port (2055/udp) at the firewall to only trusted sources.
- Bind the collector to a management interface instead of
0.0.0.0. - Run the daemon under `systemd-restart` with
Restart=on-failure. - Disable NetFlow v9 collection entirely if not strictly required.
- Restrict the collector to trusted, authenticated peers only.
Impact
- Information Disclosure: Enables an attacker to read heap memory two bytes at a time, potentially revealing sensitive data like internal state or other traffic flows.
- Denial of Service (DoS): Triggering the out-of-bounds read can cause the application to crash, disrupting DDoS detection capabilities.
- Type Confusion: The parser may interpret out-of-bounds data as legitimate NetFlow fields, leading to unpredictable behavior and data corruption.
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Sources:
Reported By: nvd.nist.gov
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