CVE-2026-72502 tcp: ipv6: clamp default adverting MSS to avoid GSO_BY_FRAGS (0xFFFF)
A local attacker can trigger a NULL pointer dereference in the Linux kernel TCP stack by exploiting large MTU values, causing a system crash.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-72502?
CVE-2026-72502 is a high-severity vulnerability in Linux, published on 15 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
tcp: ipv6: clamp default adverting MSS to avoid GSO_BY_FRAGS (0xFFFF)
When MTU is large, ip6_default_advmss() can return IPV6_MAXPLEN (65535). This is interpreted by TCP as mss_clamp, allowing the MSS to reach 65535.
However, 0xFFFF is also used as a magic value GSO_BY_FRAGS in the kernel. If a TCP packet with gso_size=0xFFFF is passed to skb_segment(), it will be mistakenly treated as GSO_BY_FRAGS, leading to a NULL pointer dereference because local TCP packets do not use frag_list.
Fix this by returning min(IPV6_MAXPLEN, GSO_BY_FRAGS - 1) (65534) from ip6_default_advmss() when MTU is large.
Also update the stale comment in ip6_default_advmss() which suggested that IPV6_MAXPLEN is returned to mean "any MSS".
Which versions are affected by CVE-2026-72502?
How do I fix CVE-2026-72502?
Apply the vendor's patch.
Is CVE-2026-72502 being exploited?
There is no known exploitation of CVE-2026-72502 in the wild and no public exploit code.
Sources
Also known as BELL-CVE-2026-72502, DEBIAN-CVE-2026-72502, GHSA-cg8c-jghv-84p9, UBUNTU-CVE-2026-72502.