CVE-2026-31911 in libpcap
Summary
by MITRE • 09/05/2026
libpcap BPF interpreter calls abort() if it encounters a BPF instruction that has an invalid opcode. In particular uncommon use cases a crafted filter program can terminate the OS process.
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Analysis
by VulDB Data Team • 09/05/2026
The libpcap library, which serves as the foundational packet capture interface for numerous network monitoring and security tools including Wireshark, tcpdump, and various intrusion detection systems, contains a critical implementation flaw within its Berkeley Packet Filter interpreter. This vulnerability arises from insufficient validation of BPF instruction opcodes during the compilation or execution phase of filter programs. Specifically, when the interpreter encounters an opcode that is not recognized as valid according to the defined BPF specification, it triggers a call to the standard C library function abort(). While this behavior might seem like a reasonable safeguard against malformed code in some contexts, it results in an immediate and unhandled termination of the host process running libpcap. This design choice fails to account for scenarios where input data may be maliciously crafted by an attacker aiming to disrupt service availability or potentially escalate privileges through subsequent exploitation vectors.
From a technical perspective, this flaw represents a classic example of improper error handling leading to denial of service conditions. The BPF interpreter is designed to parse and execute filter programs that determine which packets are captured from the network interface. These filters can be provided by users or applications dynamically at runtime. If an attacker can inject or influence the content of these filter programs, they can supply a sequence of instructions containing invalid opcodes. Upon processing this crafted input, the interpreter does not return an error code to allow graceful degradation or logging; instead, it invokes abort(), which sends a SIGABRT signal to the process. This causes the application to terminate abruptly without performing cleanup operations such as releasing file descriptors, closing network sockets, or flushing buffers. In environments where libpcap is used in long-running daemons like packet sniffers, log collectors, or real-time security monitoring appliances, this results in a complete service outage until the affected process is manually restarted.
The operational impact of this vulnerability extends beyond simple application crashes. Many enterprise security solutions rely on continuous network visibility to detect threats and anomalies. A successful exploitation allows an attacker with the ability to supply BPF filter programs—such as through web-based packet capture interfaces, compromised configuration files, or maliciously crafted pcap file headers—to cause a denial of service against critical infrastructure components. Furthermore, in systems where multiple users share access to network monitoring tools, this vulnerability can be leveraged by lower-privileged users to disrupt the services of higher-privileged processes if those processes utilize libpcap for packet filtering. The abrupt termination also complicates forensic analysis because crash dumps may not capture sufficient state information depending on system configuration and signal handling policies, potentially obscuring evidence of the attack vector used.
This vulnerability aligns with CWE-617: Reachable Assertion, which describes situations where an assertion or abort call is reachable by external input rather than being triggered only during development or testing phases. It also relates to CWE-20: Improper Input Validation, as the library fails to adequately validate the opcodes within the BPF instruction stream before execution. In terms of the MITRE ATT&CK framework, this flaw facilitates Denial of Service (T1499) and potentially serves as a precursor for further exploitation if the crash state can be manipulated or if it triggers secondary vulnerabilities in dependent libraries that do not handle process termination gracefully. The lack of robust error recovery mechanisms means that even minor input anomalies lead to catastrophic failure, highlighting a significant gap in defensive coding practices within the packet processing pipeline.
Mitigation strategies must focus on both immediate remediation and long-term architectural improvements. For system administrators and developers currently using affected versions of libpcap, upgrading to a patched version is the primary recommendation. The maintainers have addressed this issue by modifying the interpreter logic to handle invalid opcodes more gracefully, typically by returning an error code or skipping the instruction rather than terminating the process. Additionally, organizations should implement input validation layers at the application level before passing filter programs to libpcap, ensuring that only syntactically correct and expected opcode sequences are processed. Deploying network monitoring tools within containerized environments with automatic restart policies can mitigate availability impacts by quickly recovering from crashes while patches are applied. Regular security audits of packet capture implementations should also include fuzz testing specifically targeting BPF instruction streams to identify similar validation gaps in other components of the stack.