CVE-2026-4247 | FreeBSD ACK tcp_respond memory leak

VDB-353563 · CVE-2026-4247 · EUVD-2026-16128 FREEBSD ACK TCP_RESPOND MEMORY LEAK HISTORYDIFFRELATEJSONXMLCTI CVSS Meta Temp Score Current Exploit Price (≈) CTI Interest Score 3.6 $0-$5k 2.08+ Summaryinfo A vulnerability was found in FreeBSD. It has been rated as problematic. Affected by this issue is the function tcp_respond of the component ACK Handler. This manipulation causes memory leak. This vulnerability appears as CVE-2026-4247. The attack may be initiated remotely. There is no available exploit. To fix this issue, it is recommended to deploy a patch. Detailsinfo A vulnerability was found in FreeBSD (version unknown). It has been declared as problematic. Affected by this vulnerability is the function tcp_respond of the component ACK Handler. The manipulation with an unknown input leads to a memory leak vulnerability. The CWE definition for the vulnerability is CWE-401. The product does not sufficiently track and release allocated memory after it has been used, which slowly consumes remaining memory. As an impact it is known to affect availability. The summary by CVE is: When a challenge ACK is to be sent tcp_respond() constructs and sends the challenge ACK and consumes the mbuf that is passed in. When no challenge ACK should be sent the function returns and leaks the mbuf. If an attacker is either on path with an established TCP connection, or can themselves establish a TCP connection, to an affected FreeBSD machine, they can easily craft and send packets which meet the challenge ACK criteria and cause the FreeBSD host to leak an mbuf for each crafted packet in excess of the configured rate limit settings i.e. with default settings, crafted packets in excess of the first 5 sent within a 1s period will leak an mbuf. Technically, off-path attackers can also exploit this problem by guessing the IP addresses, TCP port numbers and in some cases the sequence numbers of established connections and spoofing packets towards a FreeBSD machine, but this is harder to do effectively. The weakness was presented by Michael Tuexen. It is possible to read the advisory at security.freebsd.org. This vulnerability is known as CVE-2026-4247 since 03/16/2026. The exploitation appears to be difficult. The attack can be launched remotely. The exploitation doesn't need any form of authentication. Technical details of the vulnerability are known, but there is no available exploit. The pricing for an exploit might be around USD $0-$5k at the moment (estimation calculated on 03/26/2026). The attack technique deployed by this issue is T1499 according to MITRE ATT&CK. Applying a patch is able to eliminate this problem. The vulnerability is also documented in the vulnerability database at EUVD (EUVD-2026-16128). Productinfo Type Operating System Name FreeBSD License open-source Website Product: https://www.freebsd.org/ CPE 2.3info 🔒 CPE 2.2info 🔒 CVSSv4info VulDB Vector: 🔒 VulDB Reliability: 🔍 CVSSv3info VulDB Meta Base Score: 3.7 VulDB Meta Temp Score: 3.6 VulDB Base Score: 3.7 VulDB Temp Score: 3.6 VulDB Vector: 🔒 VulDB Reliability: 🔍 CVSSv2info Vector Complexity Authentication Confidentiality Integrity Availability Unlock Unlock Unlock Unlock Unlock Unlock Unlock Unlock Unlock Unlock Unlock Unlock Unlock Unlock Unlock Unlock Unlock Unlock VulDB Base Score: 🔒 VulDB Temp Score: 🔒 VulDB Reliability: 🔍 Exploitinginfo Class: Memory leak CWE: CWE-401 / CWE-404 CAPEC: 🔒 ATT&CK: 🔒 Physical: No Local: No Remote: Yes Availability: 🔒 Status: Not defined Price Prediction: 🔍 Current Price Estimation: 🔒 0-Day Unlock Unlock Unlock Unlock Today Unlock Unlock Unlock Unlock Threat Intelligenceinfo Interest: 🔍 Active Actors: 🔍 Active APT Groups: 🔍 Countermeasuresinfo Recommended: Patch Status: 🔍 0-Day Time: 🔒 Timelineinfo 03/16/2026 CVE reserved 03/26/2026 +10 days Advisory disclosed 03/26/2026 +0 days VulDB entry created 03/26/2026 +0 days VulDB entry last update Sourcesinfo Product: freebsd.org Advisory: security.freebsd.org Researcher: Michael Tuexen Status: Confirmed CVE: CVE-2026-4247 (🔒) GCVE (CVE): GCVE-0-2026-4247 GCVE (VulDB): GCVE-100-353563 EUVD: 🔒 Entryinfo Created: 03/26/2026 07:46 Updated: 03/26/2026 09:06 Changes: 03/26/2026 07:46 (55), 03/26/2026 09:06 (1) Complete: 🔍 Cache ID: 99:CC1:101 Discussion No comments yet. Languages: en. Please log in to comment. ◂ PreviousOverviewNext ▸