CVE-2026-4247 TCP: remotely exploitable DoS vector (mbuf leak)
Remote attackers can cause a denial of service on FreeBSD systems by leaking memory through crafted TCP packets that trigger a challenge ACK logic error.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-4247?
CVE-2026-4247 is a high-severity denial of service vulnerability in Freebsd Freebsd, published on 26 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
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.
Weakness: CWE-401
Which versions are affected by CVE-2026-4247?
It affects 1 product.
Products include Freebsd Freebsd.
How do I fix CVE-2026-4247?
No fix has been published for CVE-2026-4247 yet. Limit exposure of the affected software until one is available.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-4247 being exploited?
There is no known exploitation of CVE-2026-4247 in the wild and no public exploit code.
Other vulnerabilities in Freebsd Freebsd
Sources
Also known as GHSA-374j-38xh-4wr2.