CVE-2026-48685

Description

FastNetMon Community Edition through 1.2.9 has out-of-bounds memory access because it incorrectly parses BGP path attributes with the extended length flag set. In src/bgp_protocol.hpp, the parse_raw_bgp_attribute() function correctly identifies when extended_length_bit is set and sets length_of_length_field to 2, but then reads only a single byte for the attribute value length (attribute_value_length = value[2] at line 173). Per RFC 4271 Section 4.3, when the Extended Length bit is set, the Attribute Length field is two octets and the value should be read as a 16-bit big-endian integer from value[2] and value[3]. As a result, any attribute longer than 255 bytes has its length silently truncated to the low byte (e.g., 300 bytes = 0x012C is read as 0x2C = 44 bytes). The remaining 256 bytes are then misinterpreted as subsequent attributes, causing cascading parse failures and potential out-of-bounds memory access.

Predictions

Heuristic predictions, AS-IS, for prioritization only.

Mitigations

References

• https://github.com/pavel-odintsov/fastnetmon
• https://github.com/pavel-odintsov/fastnetmon/blob/master/src/bgp_protocol.hpp
• https://lorikeetsecurity.com/blog/fastnetmon-cve-2026-48685-bgp-extended-length
• https://security-tracker.debian.org/tracker/CVE-2026-48685

CWEs

CWE-130