CVE-2026-48686: fastnetmon Buffer Overflow (CVSS 9.8)

Description

FastNetMon Community Edition through 1.2.9 contains a stack-based buffer overflow in the BGP NLRI (Network Layer Reachability Information) decoder. The function decode_bgp_subnet_encoding_ipv4_raw() in src/bgp_protocol.cpp reads prefix_bit_length directly from the BGP packet (line 99) without validating it is <= 32 for IPv4 prefixes. This value is passed to how_much_bytes_we_need_for_storing_certain_subnet_mask() which computes ceil(prefix_bit_length / 8), returning up to 32 bytes for a prefix_bit_length of 255. The result is used as the length argument to memcpy() (line 106), which copies into a 4-byte uint32_t stack variable (prefix_ipv4). This causes a stack buffer overflow of up to 28 bytes, which can be exploited for arbitrary code execution. Additionally, the unvalidated prefix_bit_length is passed to convert_cidr_to_binary_netmask_local_function_copy() (line 111), where a shift of (32 - cidr) with cidr > 32 causes undefined behavior.

Is your site exposed to CVE-2026-48686?

Run a free security scan — no signup, results in seconds.

CVSS v3.1 Score

9.8

CRITICAL

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

EPSS — Exploit Prediction

0.0057

Probability of exploitation

0.43%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-120 CWE-120

CWE-787 Out-of-bounds Write

Affected Products

Vendor	Product	Versions
pavel-odintsov	fastnetmon	< 1.2.9

References

Other References

https://github.com/pavel-odintsov/fastnetmon https://github.com/pavel-odintsov/fastnetmon/blob/master/src/bgp_protocol.cpp https://lorikeetsecurity.com/blog/fastnetmon-cve-2026-48686-bgp-nlri-stack-overflow

Frequently Asked Questions

What is CVE-2026-48686? +

FastNetMon Community Edition through 1.2.9 contains a stack-based buffer overflow in the BGP NLRI (Network Layer Reachability Information) decoder. The function decode_bgp_subnet_encoding_ipv4_raw() in src/bgp_protocol.cpp reads prefix_bit_length directly from the BGP packet (line 99) without validating it is <= 32 for IPv4 prefixes. This value is passed to how_much_bytes_we_need_for_storing_certain_subnet_mask() which computes ceil(prefix_bit_length / 8), returning up to 32 bytes for a prefix_bit_length of 255. The result is used as the length argument to memcpy() (line 106), which copies into a 4-byte uint32_t stack variable (prefix_ipv4). This causes a stack buffer overflow of up to 28 bytes, which can be exploited for arbitrary code execution. Additionally, the unvalidated prefix_bit_length is passed to convert_cidr_to_binary_netmask_local_function_copy() (line 111), where a shift of (32 - cidr) with cidr > 32 causes undefined behavior. It has a CVSS v3.1 base score of 9.8 (CRITICAL).

How severe is CVE-2026-48686? +

CVE-2026-48686 has a CVSS v3.1 score of 9.8 out of 10, rated CRITICAL. This is a critical vulnerability that should be patched immediately. The EPSS score is 0.0057, placing it in the 0th percentile for exploitation probability.

What products are affected by CVE-2026-48686? +

CVE-2026-48686 affects products from pavel-odintsov, specifically: fastnetmon. Check the affected products table above for specific version ranges.

How do I check if I'm vulnerable to CVE-2026-48686? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

Related Vulnerabilities

CVE-2026-27820

zlib is a Ruby interface for the zlib compression/decompression library. Versions 3.0.0 and below, 3.1.0, 3.1.1, 3.2.0 and 3.2.1 contain …

CVE-2025-11698

A denial-of-service issue exists in 5380/5480/5580 controllers boot firmware lower than version 1.072. This vulnerability could potentially allow a malicious …

CVE-2025-12012

A denial-of-service issue exists in 5380/5480/5580 controllers. This vulnerability could potentially allow a malicious user to write invalid file data …

CVE-2025-12011

A denial-of-service issue exists in 5370/5570 controllers. This vulnerability could potentially allow a remote user to load an invalid project, …

CVE-2024-13503

Buffer Copy without Checking Size of Input ('Classic Buffer Overflow') vulnerability in Newtec NTC2218, NTC2250, NTC2299 on Linux, PowerPC, ARM …

CVE-2024-6199

An unauthenticated attacker on the WAN interface, with the ability to intercept Dynamic DNS (DDNS) traffic between DDNS services and …

CVE-2026-48686