Look up another CVE · open CVE-2026-72502 in the interactive tracker
CVE-2026-72502
tcp: ipv6: clamp default adverting MSS to avoid GSO_BY_FRAGS (0xFFFF)
Published 2026-08-15 (2 months ago) by the Linux kernel CNA. Subsystem net/ipv6. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-72502 in every release this site tracks, and what each one means in practice. Every status below is the one the Debian Security Team publishes for this CVE. source
| Release | Status | Version | What this means |
|---|---|---|---|
| Debian 12bookworm | fixed | 6.1.180-1 | Update the kernel and reboot. Fixed in 6.1.180-1. A running kernel keeps the old code until the machine restarts. |
| Debian 13trixie | fixed | 6.12.100-1 | Update the kernel and reboot. Fixed in 6.12.100-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.1.5-1 | Update the kernel and reboot. Fixed in 7.1.5-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.1.5-1 | Update the kernel and reboot. Fixed in 7.1.5-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
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".
Where it lives
The fix touches these files. If the subsystem is one you do not use, the practical exposure is lower, though the package is still the vulnerable one.
net/ipv6/route.c
- What this is
- NETWORKING [IPv4/IPv6]MAINTAINERS section matched on net/ipv6/route.c · netdev@vger.kernel.org · Maintained
Check it on the machine
grep -w CONFIG_IPV6 /boot/config-$(uname -r)Reads the booted kernel's own configuration, on the machine and nowhere else. =y is built in, =m is a module, and an is not set line means the code is not there.
Show whether Debian builds this, per release
Debian builds this code. Every release below compiles the affected file into the kernels it ships.
- bookworm, trixie, forky, sidy
- y
- built into the kernel image on every flavour - only a patch removes it
- Build switch
- CONFIG_IPV6
From the configuration Debian publishes for every kernel flavour it builds. It covers only the architectures Debian ships a linux-config package for, and nothing about a kernel you built yourself. Debian's own status above stays authoritative about the source package.MAINTAINERS kbuild Makefiles Debian linux-config packages
Signals
- Published
- 2026-08-15 (2 months ago)source
- CVSS
- 7.5 high
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:Hsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.72%, higher than 52.9% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- net/ipv6
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 4.8 | 5.10.261 | 560b33b434e9 |
| 4.8 | 5.15.212 | 21f69ac1879b |
| 4.8 | 6.1.178 | c0db3dc2ac32 |
| 4.8 | 6.6.145 | 572fff10819d |
| 4.8 | 6.12.97 | 8e6214a530c0 |
| 4.8 | 6.18.40 | d774cdbda663 |
| 4.8 | 7.1.5 | a210791f3334 |
| 4.8 | 7.2 | 2bf43d0e2e6a |
Why the CNA scored it this way
Quoted from the kernel CNA's own scoring file, word for word. A highlighted metric carries the most severe value CVSS v3.1 defines for it, which is what pushes the score up. source
- AV:NAttack vector, network
- Reachable from the network via IPv6 TCP: a remote peer sends a crafted SYN (MSS=65535) or connects over a large-MTU IPv6 route, causing outbound TCP GSO skbs with gso_size=0xFFFF to be software-segmented in skb_segment() during normal packet transmit.
- AC:LAttack complexity, low
- The attacker controls the TCP handshake MSS option and can induce >64KB outbound segments; syzbot reproduced this reliably, and software GSO paths (loopback, veth, NETIF_F_GSO_SOFTWARE) are common in cloud/container deployments where skb_segment() is invoked.
- PR:NPrivileges required, none
- No privileges are required; exploitation occurs on the pre-authentication passive-open path where the kernel accepts a remote TCP SYN and MSS option before any application-level authentication.
- UI:NUser interaction, none
- No victim user interaction is needed beyond the target system normally accepting IPv6 TCP connections and transmitting response data to the attacker-controlled peer.
- S:UScope, unchanged
- The vulnerability causes a kernel crash on the attacked host itself; it does not cross a security boundary such as a VM escape, container sandbox breakout, or IOMMU/DMA isolation bypass.
- C:NConfidentiality impact, none
- Impact is a NULL pointer dereference in skb_segment() when gso_size equals GSO_BY_FRAGS; there is no out-of-bounds read, use-after-free, or other memory disclosure primitive.
- I:NIntegrity impact, none
- The bug does not corrupt or modify attacker-controlled memory; it dereferences a NULL frag_list pointer during segmentation, yielding a crash rather than a write or code-execution primitive.
- A:HAvailability impact, high
- The NULL pointer dereference in skb_segment() triggers a kernel oops/panic, causing complete loss of availability for the affected system when the malicious IPv6 TCP traffic path is exercised.