CVE-2026-93424
Description
A flaw was found in FastNetMon. An unauthenticated remote attacker with access to the NetFlow or IPFIX listening ports can trigger a race condition during concurrent template updates and traffic processing. This issue causes the application to access memory after it has been freed (a use-after-free condition), leading to an application crash and resulting in a Denial of Service (DoS).
Statement
This vulnerability is rated as Important severity because unauthenticated remote attackers capable of reaching the collector listener ports can crash the monitoring service without requiring privileges, disrupting denial-of-service detection across the network. In typical Red Hat Enterprise Linux environments, FastNetMon is not included in standard package repositories and monitoring collectors are intended to reside on restricted management segments. The issue is further constrained to configurations defining multiple listening ports where parallel flow processing can trigger memory corruption. Neither data integrity nor system confidentiality is compromised, as the flaw exclusively causes daemon termination.
Mitigation
Restrict network access to the NetFlow and IPFIX collector ports to only trusted flow exporters, or configure FastNetMon to listen on a single port.
1. Restrict incoming traffic on the collector UDP port using firewalld to authorized exporter IP addresses:
firewall-cmd --permanent --zone=public --add-rich-rule='rule family="ipv4" source address="<trusted_router_ip>" port port="<collector_port>" protocol="udp" accept'
firewall-cmd --reload
2. Alternatively, adjust `/etc/fastnetmon.conf` to declare only a single listening port in `netflow_ports` rather than multiple ports, and restart the daemon:
systemctl restart fastnetmon
Caveats: Limiting ingress packets to explicit IP addresses requires defining all flow-exporting network devices. Restricting the configuration to a single port may constrain environments relying on multi-port flow ingestion.
Warning: Executing `firewall-cmd --reload` reloads active firewall rules. Restarting the service via `systemctl restart fastnetmon` will temporarily interrupt traffic flow processing.
--- CME Mitigation Guidance (2026-11-06) ---
Disclaimer: The following mitigation solutions are provided as a courtesy, pending the application of an official patch. This content was generated utilizing AI tools in conjunction with data from https://cmetaxonomy.org. Customers are advised to independently evaluate and test these temporary mitigations within their own infrastructure.
The highest-value action is to populate NetFlow and IPFIX template lookups by deep copy into local storage under mutex protection rather than returning raw pointers to shared cached template memory: upgrade to the fixed Fastnetmon builds, which pass a local template storage object into peer_find_template by reference to populate a deep copy rather than returning a raw pointer to cached template memory, closing concurrent UDP flow packets redefining and querying NetFlow/IPFIX templates across multiple listener ports. Until patched, this is only a concern on Linux deployments of FastNetMon configured with two or more listening ports in netflow_ports accessible over UDP — FastNetMon configurations operating with only a single listening port are unaffected — so Configure FastNetMon to declare a single listening port in netflow_ports or restrict incoming collector UDP port access exclusively to trusted flow exporter IP addresses. These controls limit blast radius and contain exploitation, but they do not fix the underlying heap-use-after-free race condition defect — apply the upstream patch to fully remediate.
Understanding the Weakness (CWE)
Confidentiality
Technical Impact: Read Memory
If the expired pointer is used in a read operation, an attacker might be able to control data read in by the application.
Availability
Technical Impact: DoS: Crash, Exit, or Restart
If the expired pointer references a memory location that is not accessible to the product, or points to a location that is "malformed" (such as NULL) or larger than expected by a read or write operation, then a crash may occur.
Integrity,Confidentiality,Availability
Technical Impact: Execute Unauthorized Code or Commands
If the expired pointer is used in a function call, or points to unexpected data in a write operation, then code execution may be possible.
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