EUVD-2026-31840
GHSA-6jf7-3j4w-4v3m
Severity by source
AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:L
Primary rating from NVD · only source for this CVE.
CVSS VectorNVD
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:L
Lifecycle Timeline
3DescriptionCVE.org
FastNetMon Community Edition through 1.2.9 contains an out-of-bounds read in the NetFlow v9 options template parser. In process_netflow_v9_options_template() (src/netflow_plugin/netflow_v9_collector.cpp), the scope parsing loop (lines 224-229) iterates until scopes_offset reaches the attacker-controlled option_scope_length value, reading netflow9_template_flowset_record_t structures at each step. No bounds check validates that (zone_address + scopes_offset + sizeof(record)) stays within the flowset. The same issue affects the options field loop (lines 241-257) with option_length. Furthermore, option_scope_length is not validated to be a multiple of sizeof(netflow9_template_flowset_record_t), potentially causing misaligned reads. An attacker can trigger reads past the end of the UDP packet buffer.
AnalysisAI
Out-of-bounds read in FastNetMon Community Edition through 1.2.9 exposes network monitoring infrastructure to unauthenticated remote attack via malformed NetFlow v9 UDP packets. The vulnerability resides in the options template parser (process_netflow_v9_options_template()), where attacker-controlled length fields - option_scope_length and option_length - drive iteration loops with no bounds validation, enabling reads past the end of the UDP packet buffer. With a CVSS vector of AV:N/AC:L/PR:N/UI:N and SSVC automatable:yes, mass exploitation is technically feasible, though EPSS sits at 0.03% (9th percentile) and no public exploit or KEV listing has been identified at time of analysis.
Technical ContextAI
FastNetMon is an open-source DDoS detection and traffic analysis tool that ingests network flow data, including Cisco NetFlow v9. The vulnerable code in src/netflow_plugin/netflow_v9_collector.cpp implements the NetFlow v9 options template record format (RFC 3954), which uses variable-length scope and option field descriptors. CWE-125 (Out-of-bounds Read) is the root cause: the scope parsing loop (lines 224-229) advances scopes_offset by sizeof(netflow9_template_flowset_record_t) on each iteration, terminating only when scopes_offset equals the attacker-supplied option_scope_length value, with no check that (zone_address + scopes_offset + sizeof(record)) remains within the allocated packet buffer. A second, identical flaw exists in the options field loop (lines 241-257) controlled by option_length. An additional misalignment issue arises because option_scope_length is not validated as a multiple of sizeof(netflow9_template_flowset_record_t), which can cause the parser to read at misaligned memory addresses, compounding the out-of-bounds risk. CPE data returned by NVD is listed as n/a, so affected version scope is taken from the description (through 1.2.9) and the project's GitHub repository at https://github.com/pavel-odintsov/fastnetmon.
RemediationAI
No vendor-released patch identified at time of analysis - no fix version is referenced in any of the available advisories or the upstream GitHub repository at time of writing. Operators should monitor the FastNetMon GitHub repository (https://github.com/pavel-odintsov/fastnetmon) for a patched release. As immediate compensating controls: restrict UDP access to the FastNetMon NetFlow v9 collection port (typically UDP/2055 or operator-configured) to trusted flow-exporting devices only, using firewall ACLs or host-based iptables rules - this directly eliminates the network attack surface since the flaw requires delivery of a crafted UDP packet. If NetFlow v9 options template processing is not required in the deployment, evaluate whether it can be disabled in FastNetMon configuration to reduce parser exposure. Running FastNetMon under a dedicated low-privilege service account and applying process sandboxing (e.g., systemd's MemoryDenyWriteExecute, NoNewPrivileges) limits the impact of a successful read-past-end exploit. The security research blog post at https://lorikeetsecurity.com/blog/fastnetmon-cve-2026-48684-netflow-v9-options-oob may contain additional technical detail relevant to workaround selection.
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Same weakness CWE-125 – Out-of-bounds Read
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External POC / Exploit Code
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