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The vulnerability exists in the BGP NLRI decoder of FastNetMon Community Edition versions up to 1.2.9. The function decode_bgp_subnet_encoding_ipv4_raw() inside src/bgp_protocol.cpp reads the prefix_bit_length field directly from a malicious BGP packet at line 99. This value is not validated to ensure it is ≤ 32 for IPv4 prefixes. It is then passed to how_much_bytes_we_need_for_storing_certain_subnet_mask(), which computes ceil(prefix_bit_length / 8). For a prefix_bit_length of 255, the function returns 32 bytes. This computed length is used as the third argument to memcpy() at line 106. The memcpy() copies the raw prefix data into a 4-byte uint32_t stack variable named prefix_ipv4. Since the destination buffer is only 4 bytes, copying up to 32 bytes causes a stack buffer overflow of up to 28 extra bytes. An attacker can overwrite adjacent stack memory, including return addresses and saved frame pointers. Additionally, at line 111, the same unvalidated prefix_bit_length is passed to convert_cidr_to_binary_netmask_local_function_copy(), where a shift operation (32 – cidr) with cidr > 32 triggers undefined behavior, potentially leading to further memory corruption. This overflow can be exploited to achieve arbitrary code execution by controlling the overwritten return address. The attack requires the ability to send crafted BGP packets to the FastNetMon daemon, typically over TCP port 179.
Platform: FastNetMon Community
Version: 1.2.9 and below
Vulnerability : Stack buffer overflow
Severity: Critical
date: 2026-05-26
Prediction: Patch within 30 days
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Check FastNetMon version fastnetmon --version Monitor BGP traffic on port 179 for suspicious NLRI sudo tcpdump -i eth0 'tcp port 179' -vv Test with malformed BGP packet (example using Scapy) from scapy.all import packet = IP(dst="target")/TCP(dport=179)/BGPHeader()/BGPUpdate(NLRI=Raw(b'\xff'32)) send(packet)
Exploit:
Craft BGP UPDATE message with IPv4 NLRI prefix_bit_length > 32 (e.g., 255). The decoder computes ceil(255/8)=32 bytes, memcpy into 4-byte buffer. Overwrite return address with shellcode pointer. Achieve RCE.
Protection from this CVE
Upgrade to patched version (>1.2.9) when available. Use BGP MD5 authentication and prefix filtering. Deploy network IPS to drop anomalous NLRI lengths. Run fastnetmon with non-root user and ASLR.
Impact:
Remote code execution via unauthenticated BGP session. Attacker can completely compromise the monitoring host, inject false routes, or disrupt network visibility.
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Sources:
Reported By: nvd.nist.gov
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