Executive Summary

This vulnerability occurs in the FreeBSD TCP stack when handling challenge ACK TCP segments. Normally, when a challenge ACK is sent, the system properly consumes the associated memory buffer (mbuf). However, if the challenge ACK is not sent due to rate limiting, the function returns without freeing the mbuf, causing a memory leak.

An attacker who is on the network path or can establish a TCP connection to the affected FreeBSD machine can send specially crafted TCP packets that trigger this behavior. Packets exceeding the configured rate limit (default is 5 packets per second) cause the system to leak one mbuf per excess packet.

Off-path attackers can also exploit this by guessing IP addresses, TCP ports, and sequence numbers to spoof packets, but this is more difficult.

Useful 0 Not Useful 0

Impact Analysis

The vulnerability can lead to resource exhaustion on the affected FreeBSD system because each crafted packet exceeding the rate limit causes a memory buffer (mbuf) leak.

This resource exhaustion can result in a denial-of-service (DoS) condition, potentially making the system unresponsive or unstable.

Useful 0 Not Useful 0

Detection Guidance

This vulnerability involves a memory buffer (mbuf) leak in the FreeBSD TCP stack triggered by crafted TCP packets that cause challenge ACKs to be rate limited. Detection involves monitoring for unusual TCP challenge ACK behavior or mbuf resource exhaustion on affected FreeBSD systems.

While the advisory does not provide explicit detection commands, you can monitor system resource usage related to mbufs and TCP connections using standard FreeBSD commands such as:

• netstat -m (to monitor mbuf usage and detect leaks)
• netstat -s tcp (to view TCP statistics including challenge ACKs)
• tcpdump or packet capture tools to analyze TCP packets and identify unusual challenge ACK traffic patterns

Additionally, reviewing the sysctl parameters related to challenge ACK rate limiting can help understand if the system is under attack:

• sysctl net.inet.tcp.ack_war_timewindow
• sysctl net.inet.tcp.ack_war_cnt

Useful 0 Not Useful 0

Mitigation Strategies

The immediate mitigation step is to disable the rate limiting of challenge ACKs, which prevents the mbuf leak by ensuring all challenge ACKs are sent and mbufs are properly consumed.

This can be done by setting the following sysctl parameter:

• sysctl net.inet.tcp.ack_war_timewindow=0

Disabling the 1-second window causes challenge ACKs to be sent for all qualifying packets, eliminating the leak but potentially increasing CPU and resource usage.

To make this change persistent across reboots, add the setting to /etc/sysctl.conf.

The long-term solution is to upgrade FreeBSD to a version with the fix applied, using one of the following methods depending on your installation:

• For base system packages: pkg upgrade -r FreeBSD-base, then reboot
• For binary distribution sets: freebsd-update fetch and install, then reboot
• For source-based systems: download and apply the patch, recompile the kernel, and reboot

Useful 0 Not Useful 0

Compliance Impact

The provided information does not specify any direct impact of this vulnerability on compliance with common standards and regulations such as GDPR or HIPAA.

Useful 0 Not Useful 0