CVE-2026-4247: FreeBSD TCP Information Disclosure Flaw

CVE-2026-4247 Overview

A memory leak vulnerability exists in the FreeBSD TCP stack's handling of challenge ACK responses. When the tcp_respond() function constructs and sends a challenge ACK, it properly consumes the mbuf (memory buffer) that is passed in. However, when no challenge ACK should be sent, the function returns without freeing the mbuf, resulting in a memory leak.

An attacker who is either on-path with an established TCP connection or can establish their own TCP connection to an affected FreeBSD machine can craft and send packets that trigger the challenge ACK criteria, causing the FreeBSD host to leak an mbuf for each crafted packet that exceeds the configured rate limit settings. With default settings, crafted packets in excess of the first 5 sent within a 1-second period will leak an mbuf. Off-path attackers can also potentially exploit this vulnerability by guessing IP addresses, TCP port numbers, and sequence numbers, though this approach is significantly more difficult to execute effectively.

Critical Impact

This memory leak vulnerability allows remote attackers to exhaust system memory resources through repeated crafted TCP packets, potentially leading to denial of service conditions on affected FreeBSD systems.

Affected Products

• FreeBSD (affected versions detailed in FreeBSD-SA-26:06.tcp)

Discovery Timeline

• 2026-03-26 - CVE CVE-2026-4247 published to NVD
• 2026-03-26 - Last updated in NVD database

Technical Details for CVE-2026-4247

Vulnerability Analysis

This vulnerability is classified as CWE-401 (Missing Release of Memory after Effective Lifetime), a memory leak condition in the FreeBSD kernel's TCP implementation. The vulnerability resides in the tcp_respond() function, which is responsible for handling TCP challenge ACK responses.

The core issue stems from an inconsistent code path in memory management. When a challenge ACK is legitimately sent, the function correctly consumes (frees) the mbuf passed to it. However, in scenarios where the function determines that no challenge ACK should be sent, it returns early without deallocating the mbuf, leaving allocated memory unreferenced and unrecoverable.

The network-accessible nature of this vulnerability allows remote exploitation without requiring authentication or user interaction. The impact is primarily on system availability, as accumulated memory leaks can lead to resource exhaustion and system instability.

Root Cause

The root cause is improper resource management in the tcp_respond() function. The function has two code paths: one for sending challenge ACKs (which properly frees memory) and one for not sending them (which fails to free memory). This asymmetric handling of the mbuf resource violates proper memory management practices and creates a leak condition that can be triggered remotely.

The FreeBSD kernel uses mbufs (memory buffers) as the primary data structure for storing network packets. Each leaked mbuf represents allocated kernel memory that cannot be reclaimed, progressively reducing available system resources.

Attack Vector

The attack can be executed by remote attackers via the network without requiring any privileges or user interaction. The exploitation mechanism involves:

1. Establishing a TCP connection to the target FreeBSD system, or being positioned on-path with existing connections
2. Crafting TCP packets that meet the challenge ACK criteria but trigger the code path that bypasses the ACK response
3. Sending these packets at rates exceeding the configured rate limit (default: 5 packets per second)
4. Each packet beyond the rate limit causes one mbuf to leak, gradually exhausting system memory

Off-path attackers face additional challenges as they must guess connection parameters (IP addresses, ports, sequence numbers) to successfully spoof packets, making this attack vector less reliable but still theoretically possible.

The vulnerability mechanism relates to TCP challenge ACK handling, which is part of TCP's protection against blind in-window attacks. For detailed technical information, refer to the FreeBSD Security Advisory.

Detection Methods for CVE-2026-4247

Indicators of Compromise

• Unusual increase in kernel memory usage without corresponding legitimate workload
• Growing mbuf allocation counts without deallocation over time
• System performance degradation or instability on FreeBSD servers handling TCP traffic
• Network traffic patterns showing high volumes of TCP packets designed to trigger challenge ACK conditions

Detection Strategies

• Monitor kernel memory statistics using vmstat -m to track mbuf pool usage and detect abnormal growth patterns
• Implement network-level monitoring for unusual TCP packet patterns targeting FreeBSD systems
• Configure system alerts for memory pressure conditions in the kernel's network subsystem
• Review TCP connection logs for anomalous connection patterns or high packet rates from single sources

Monitoring Recommendations

• Deploy continuous monitoring of mbuf allocation statistics on FreeBSD systems
• Establish baseline memory usage patterns to quickly identify deviations indicating potential exploitation
• Implement rate-based alerting for TCP packets that could trigger challenge ACK conditions
• Consider network flow analysis to detect potential attack traffic patterns

How to Mitigate CVE-2026-4247

Immediate Actions Required

• Apply the security patch from FreeBSD as detailed in FreeBSD-SA-26:06.tcp immediately
• Review and potentially lower rate limit settings to reduce the attack surface
• Monitor affected systems for signs of memory exhaustion
• Consider implementing additional network-level filtering for suspicious TCP traffic patterns

Patch Information

FreeBSD has released a security advisory (FreeBSD-SA-26:06.tcp) addressing this vulnerability. System administrators should apply the corresponding patches to their FreeBSD installations immediately. The patch corrects the memory management in tcp_respond() to ensure proper mbuf deallocation in all code paths.

For detailed patch information and instructions, see the FreeBSD Security Advisory.

Workarounds

• Implement strict network access controls to limit exposure of FreeBSD systems to untrusted networks
• Configure firewall rules to rate-limit incoming TCP connections from individual sources
• Monitor and set alerts for kernel memory usage to enable early detection of exploitation attempts
• Consider network segmentation to isolate critical FreeBSD systems from potential attackers

# Example: Monitor mbuf usage on FreeBSD
# Run periodically to detect memory leak indicators
netstat -m | grep "mbufs in use"
vmstat -m | grep mbuf