Description

FastNetMon is a high-performance DDoS detector and sensor. In version 1.2.9 and earlier, the NetFlow v9 collector calls  strlen()  on an attacker-controlled interface-description field without bounding the scan to the field’s declared length. Although fields longer than 128 bytes are excluded from this operation, an accepted field without a null terminator can cause reads beyond the field and packet buffer. An unauthenticated remote attacker who can send crafted UDP packets to the collector can trigger an out-of-bounds read and potentially crash the process, causing denial of service. No fixed release is available as of this review.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "7.5",
  "pubDate": "2026-11-05T23:17:02.460Z",
  "pubdate": "2026-11-05T23:17:02.460Z",
  "executiveSummary": "FastNetMon versions 1.2.9 and earlier are susceptible to an out-of-bounds read vulnerability within its NetFlow v9 collector component. This vulnerability stems from improper handling of the interface-description field during packet parsing.\nAn unauthenticated remote attacker can exploit this flaw by sending specifically crafted UDP packets containing an interface-description field that lacks a null terminator. By manipulating these fields, the attacker can force the application to perform memory reads beyond the boundaries of the designated packet buffer.\nThe primary impact of this vulnerability is a denial-of-service (DoS) condition, as triggering an out-of-bounds access attempt typically results in the process crashing. Because the attack is triggered via the network-facing NetFlow collector, it requires no prior authentication, exposing the system to potential disruption from any remote source capable of reaching the collector via the UDP protocol.\nAs of the current assessment, no security patch has been released by the vendor to address this flaw. Organizations relying on FastNetMon should treat this as a high-risk operational concern, particularly if the sensor is deployed in environments where network traffic sources cannot be strictly filtered.",
  "technicalDetails": "The vulnerability resides in the NetFlow v9 collector implementation of FastNetMon, specifically within the logic responsible for processing the interface-description field of incoming flow packets. The root cause is the unsafe invocation of the strlen() function on data sourced directly from an attacker-controlled packet field without enforcing a strict boundary based on the field's declared length.\nUnder normal operating conditions, the NetFlow v9 collector expects a null-terminated string within the interface-description field. However, the implementation fails to account for malformed input where a valid string terminator is omitted. While the software includes a safeguard that excludes fields exceeding 128 bytes from further processing, it does not validate the presence of a null byte within that window before passing the pointer to strlen().\nThe exploitation flow proceeds as follows: An unauthenticated remote attacker crafts a malicious UDP packet adhering to the NetFlow v9 protocol specifications. Within this packet, the attacker populates the interface-description field with a sequence of non-null bytes that occupy the memory space up to the 128-byte exclusion threshold but fails to include a null termination character. When the FastNetMon collector receives this packet, it invokes strlen() on the buffer. Because the string is not null-terminated, strlen() continues reading memory sequentially past the allocated packet buffer until it inadvertently encounters a null byte in adjacent memory space.\nThis out-of-bounds read operation poses significant risks. If the read accesses unmapped memory regions or sensitive memory pages that trigger a segmentation fault or memory protection violation, the FastNetMon process will terminate abruptly. This leads to a denial of service, rendering the DDoS detection capabilities of the product unavailable until the service is manually restarted. In more complex scenarios, if the memory layout allows, the out-of-bounds read could potentially leak information from the process heap or stack; however, the primary observed impact is process instability and service crash. The attack is easily repeatable as the collector is inherently designed to ingest high volumes of incoming UDP traffic, making the detection of a single malformed packet amidst legitimate traffic challenging for standard signature-based intrusion detection systems."
}