CVE-2026-48682 in FastNetMon Community Edition
Summary
by MITRE • 06/03/2026
FastNetMon Community Edition through 1.2.9 contains an out-of-bounds read in the IPv4 packet parser. In src/simple_packet_parser_ng.cpp, after validating that the packet contains at least sizeof(ipv4_header_t) bytes (20 bytes), the code advances the local_pointer by '4 * ipv4_header->get_ihl()' (line 164) without validating that (a) IHL >= 5 (the minimum valid value per RFC 791), or (b) 4 * IHL bytes are actually available in the packet. The IHL field is 4 bits, allowing values 0-15, so the advance can be 0-60 bytes. An IHL value of 15 with only 20 bytes validated causes a 40-byte over-read. An IHL of 0-4 causes the pointer to not advance past the IP header, resulting in the TCP/UDP header being parsed from IP header data (type confusion). This vulnerability is reachable via any packet capture interface.
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Analysis
by VulDB Data Team • 06/03/2026
The vulnerability identified in FastNetMon Community Edition through version 1.2.9 represents a critical out-of-bounds read condition within the IPv4 packet parsing functionality that exposes the system to potential exploitation. This flaw exists in the src/simple_packet_parser_ng.cpp file where the packet processing logic fails to properly validate the Internet Header Length field before performing pointer arithmetic operations. The issue stems from a fundamental lack of input validation that allows maliciously crafted packets to trigger memory access violations.
The technical implementation of this vulnerability demonstrates a classic buffer over-read scenario where the code advances a local pointer based on the IHL (Internet Header Length) field value without proper bounds checking. Specifically, after validating that the packet contains at least 20 bytes for the IPv4 header structure, the code performs an advancement of local_pointer by 4 times the ipv4_header->get_ihl() value. This calculation assumes that the IHL field contains a valid value and that sufficient packet data exists beyond the header to accommodate the calculated offset. However, the IHL field is only 4 bits in length, permitting values from 0 to 15, which means the pointer advancement can range from 0 to 60 bytes.
The operational impact of this vulnerability manifests through multiple attack vectors that exploit the different ways the malformed IHL values can cause memory corruption. When an IHL value of 15 is used with only 20 bytes of validated packet data, the code performs a 40-byte over-read, accessing memory beyond the allocated packet buffer. This scenario creates a predictable memory access violation that can potentially be leveraged for information disclosure or denial of service attacks. Alternatively, when IHL values of 0 through 4 are encountered, the pointer fails to advance sufficiently past the IP header, causing subsequent TCP or UDP header parsing to occur from corrupted data within the IP header itself. This type confusion vulnerability can result in unpredictable behavior and potential exploitation through malformed packet injection.
This vulnerability directly maps to CWE-125 Out-of-bounds Read within the Common Weakness Enumeration framework, specifically representing an improper validation of input data that leads to memory corruption. The flaw also aligns with ATT&CK technique T1059 Command and Scripting Interpreter where attackers could potentially leverage such memory corruption to execute arbitrary code. The attack surface is extensive as any packet capture interface within the FastNetMon system can trigger this condition, making it particularly dangerous in network monitoring environments where the system processes untrusted network traffic. The vulnerability is particularly concerning because it occurs during normal packet processing operations and can be triggered by simply sending malformed packets to any interface that captures network traffic.
Mitigation strategies should focus on implementing comprehensive input validation that checks both the minimum valid IHL value of 5 and ensures sufficient packet data exists beyond the calculated header length. The code should validate that 4 * IHL bytes are actually available in the packet buffer before performing the pointer advancement. Additionally, defensive programming practices should include bounds checking mechanisms that prevent pointer arithmetic from exceeding allocated memory boundaries. Organizations should also consider implementing network segmentation and traffic filtering to limit exposure to potentially malicious packet injection attempts. Regular updates to the FastNetMon software should be prioritized to ensure that this vulnerability is addressed through official patches. The system should also implement proper error handling and graceful degradation mechanisms to prevent complete service disruption when malformed packets are encountered, aligning with security best practices for robust network monitoring systems.