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CVE-2026-48691 FastNetMon CVE debrief

FastNetMon Community Edition through 1.2.9 contains an integer overflow vulnerability in the BGP AS_PATH attribute encoder. The vulnerability exists in src/bgp_protocol.hpp within the IPv4UnicastAnnounce::get_attributes() function. The code computes attribute_length as 'sizeof(bgp_as_path_segment_element_t) + this->as_path_asns.size() * sizeof(uint32_t)' and stores this value in a uint8_t field. Since uint8_t can only represent values 0-255, an AS_PATH containing more than 63 ASNs (2 + 64*4 = 258 > 255) causes silent truncation. The truncated length is subsequently used for buffer sizing, while the actual data written uses the full untruncated amount, resulting in a heap buffer overflow. A secondary truncation issue affects the path_segment_length field, which also uses uint8_t and truncates with more than 255 ASNs.

Vendor
FastNetMon
Product
FastNetMon Community Edition
CVSS
CRITICAL 9.8
CISA KEV
Not listed in stored evidence
Original CVE published
2026-05-26
Original CVE updated
2026-05-27
Advisory published
2026-05-26
Advisory updated
2026-05-27

Who should care

Network operators using FastNetMon Community Edition for DDoS detection and BGP flowspec; security teams managing BGP infrastructure; organizations relying on FastNetMon for traffic analysis and automated response

Technical summary

The vulnerability stems from improper integer type selection in BGP protocol handling. The attribute_length calculation uses uint8_t (8-bit unsigned integer) to store a value that can exceed 255 when processing AS_PATH attributes with 64 or more ASNs. The sizeof(bgp_as_path_segment_element_t) is 2 bytes, and each ASN requires 4 bytes (uint32_t). With 64 ASNs: 2 + (64 * 4) = 258, which truncates to 2 in uint8_t. This causes heap buffer overflow when the truncated size is used for memory allocation but the full 258 bytes are written. The path_segment_length field at line 621 similarly truncates at 255 ASNs. Attack vectors require control over BGP announcements with crafted AS_PATH attributes.

Defensive priority

high

Recommended defensive actions

  • Upgrade FastNetMon Community Edition to a version newer than 1.2.9 when available
  • Review and restrict BGP session configurations to limit AS_PATH lengths
  • Monitor for unusual BGP announcement patterns that may indicate exploitation attempts
  • Apply network segmentation to limit exposure of BGP-speaking infrastructure
  • Review source code modifications if maintaining custom FastNetMon builds

Evidence notes

Vulnerability confirmed in FastNetMon Community Edition through version 1.2.9. Root cause identified in src/bgp_protocol.hpp lines 600-605 and 621. Integer overflow occurs when AS_PATH contains more than 63 ASNs due to uint8_t storage limitations.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-48691 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-48691

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2026-48691 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-48691

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.