CVE-2026-31911: libpcap BPF Interpreter DOS Vulnerability

CVE-2026-31911 Overview

CVE-2026-31911 is a denial of service vulnerability in the libpcap Berkeley Packet Filter (BPF) interpreter. The interpreter invokes abort() when it encounters a BPF instruction containing an invalid opcode. In uncommon use cases where an attacker can supply a crafted filter program, this behavior terminates the host operating system process that loaded libpcap. The flaw is categorized under [CWE-617] Reachable Assertion.

Because libpcap underpins packet capture tooling such as tcpdump and many network monitoring agents, an unexpected abort() disrupts availability of security and observability workloads on the affected host.

Critical Impact
A local user able to supply a crafted BPF filter program can force the libpcap-linked process to terminate, causing loss of packet capture and monitoring availability.

Affected Products

libpcap (the-tcpdump-group) — versions prior to the fix commit a715bcd
Downstream distributions and applications statically or dynamically linking the vulnerable libpcap
Network monitoring and capture tooling built on libpcap (for example, tcpdump)

Discovery Timeline

2026-09-05 - CVE-2026-31911 published to NVD
2026-09-08 - Last updated in NVD database

Technical Details for CVE-2026-31911

Vulnerability Analysis

The libpcap BPF interpreter walks a program of pseudo-machine instructions and dispatches on each instruction's code field. When the interpreter encountered an opcode it did not recognize, it fell through the switch statement to a default branch that called abort(). Calling abort() raises SIGABRT and terminates the process holding the BPF program.

An attacker who can influence the BPF program supplied to a libpcap consumer can craft an instruction with an unrecognized opcode. Loading and executing this program forces the interpreter into the default branch, terminating the process and denying service to any capture, filter, or monitoring functionality that depended on it.

The issue affects confidentiality and integrity minimally, but produces a high availability impact on the affected process. Exploitation requires local access and low privileges, aligned with the local attack vector.

Root Cause

The root cause is unsafe error handling in bpf_filter.c. The interpreter treated an unknown opcode as an unrecoverable programming error and called abort() rather than returning a safe value. Any user-supplied filter with an invalid instruction therefore turned a data validation failure into a process-terminating signal.

Attack Vector

Exploitation requires a local attacker with the ability to submit a BPF filter program to a process using libpcap. Typical exposure surfaces include tools that accept user-provided BPF expressions or applications that load compiled filters from untrusted files or IPC channels.

		switch (pc->code) {

		default:
-			abort();
+			return 0;
		case BPF_RET|BPF_K:
			return (u_int)pc->k;

Source: the-tcpdump-group/libpcap commit a715bcd

Detection Methods for CVE-2026-31911

Indicators of Compromise

Unexpected SIGABRT termination of processes linked against libpcap (tcpdump, monitoring agents, IDS sensors).
Core dumps referencing bpf_filter_with_aux_data or the interpreter dispatch loop with a default/abort frame.
Repeated crash-restart cycles for capture services after ingesting filter programs from untrusted input.

Detection Strategies

Correlate process crash events (SIGABRT, exit code 134) for libpcap-consuming binaries with recent filter changes or user activity.
Audit application logs for filter compilation or load operations that immediately precede a service crash.
Inventory hosts running libpcap versions predating commit a715bcd and flag any that accept BPF programs from non-administrative users.

Monitoring Recommendations

Enable auditd or equivalent kernel auditing to record SIGABRT signals delivered to capture and monitoring daemons.
Forward crash telemetry and coredump metadata to a centralized log platform for anomaly analysis.
Alert on capture services restarting more frequently than baseline, which can indicate repeated exploitation attempts.

How to Mitigate CVE-2026-31911

Immediate Actions Required

Update libpcap to a version containing commit a715bcd ("Fail opcodes safely in the BPF interpreter").
Rebuild or repackage downstream tools that statically link libpcap so they pick up the fixed interpreter.
Restrict which local users can supply BPF filter programs to privileged capture services.

Patch Information

The fix replaces the abort() call in the interpreter's default opcode branch with return 0, causing the filter to safely reject the packet instead of terminating the process. Apply the upstream patch from the libpcap security commit a715bcd or install the vendor-supplied package update once your distribution publishes it.

Workarounds

Limit execution of tools that consume untrusted BPF programs to trusted administrators only.
Where feasible, validate or reject BPF programs sourced from non-administrative users before passing them to libpcap.
Run capture services under a supervisor that rate-limits restarts to prevent crash-loop amplification.

bash

# Verify installed libpcap version and rebuild consumers after patching
tcpdump --version
ldd $(which tcpdump) | grep libpcap
# On Debian/Ubuntu
sudo apt-get update && sudo apt-get install --only-upgrade libpcap0.8 tcpdump
# On RHEL/Fedora
sudo dnf upgrade libpcap tcpdump