CVE-2026-101307
Undergoing Analysis Undergoing Analysis - In Progress

Out-of-Bounds Write in RACK Stack

Vulnerability report for CVE-2026-101307, including description, CVSS score, EPSS score, affected products, exploitability, helpful resources, and attack-flow context.

Publication date: 2026-10-15

Last updated on: 2026-10-15

Assigner: FreeBSD

Description

When the PCM sample array needs to grow, the RACK stack doubles a 16-bit allocation counter. Repeated doublings caused the counter to wrap to zero, resulting in a near-zero-length allocation followed by a copy of the existing samples into the undersized buffer. A remote peer can trigger the expansion by sending a sufficient number of ACKs on an established connection. A remote attacker who can establish a TCP connection to a system using the RACK stack can trigger an out-of-bounds write of approximately 512 KB in the kernel heap possibly resulting in a kernel panic.

CVSS Scores

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Meta Information

Published
2026-10-15
Last Modified
2026-10-15
Generated
2026-10-19
AI Q&A
2026-10-15
EPSS Evaluated
2026-10-18
NVD
EUVD

Affected Vendors & Products

Showing 1 associated CPE
Vendor Product Version / Range
FreeBSD FreeBSD 15.1-RELEASE

Helpful Resources

Exploitability

CWE
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KEV
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CWE ID Description
CWE-122 A heap overflow condition is a buffer overflow, where the buffer that can be overwritten is allocated in the heap portion of memory, generally meaning that the buffer was allocated using a routine such as malloc().
CWE-787 The product writes data past the end, or before the beginning, of the intended buffer.

Attack-Flow Graph

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Executive Summary

This vulnerability is a remotely triggerable buffer overflow in FreeBSD's tcp_rack kernel module, which implements the Recent Acknowledgment (RACK) loss detection algorithm for TCP connections. The issue occurs when the Path Capacity Measurement (PCM) module attempts to grow a dynamically allocated array. A 16-bit counter used for tracking array size can wrap around to zero after repeated doublings, leading to an undersized buffer allocation. A remote attacker can exploit this by sending many ACK packets on an established TCP connection, causing an out-of-bounds write of about 512 KB in the kernel heap, potentially resulting in a kernel panic.

Detection Guidance

Check if the tcp_rack kernel module is loaded using the command kldstat -m tcp_rack. If it is loaded, the system is potentially vulnerable.

Impact Analysis

If you are using FreeBSD 15.1 with the tcp_rack module loaded, a remote attacker could exploit this vulnerability by sending a large number of ACK packets to your system. This could cause a kernel panic or system crash, leading to denial of service. Systems not using the tcp_rack module are unaffected.

Compliance Impact

This vulnerability does not directly affect compliance with GDPR, HIPAA, or similar standards as it pertains to a technical flaw in the FreeBSD kernel's TCP stack. Compliance impacts would depend on whether the system is used in environments subject to these regulations and whether the vulnerability leads to unauthorized data access or system downtime.

Mitigation Strategies

Unload the tcp_rack module using kldunload tcp_rack. Alternatively, upgrade the system to a patched version dated after October 14, 2026, using pkg upgrade, freebsd-update, or source code modification, then reboot.

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