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CVE-2026-63920
ipv6: validate extension header length before copying to cmsg
Published 2026-07-19 (3 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-2026-63920 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 | 6.1.176-1 | Update the kernel and reboot. Fixed in 6.1.176-1. A running kernel keeps the old code until the machine restarts. |
| Debian 13trixie | fixed | 6.12.94-1 | Update the kernel and reboot. Fixed in 6.12.94-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.0.12-1 | Update the kernel and reboot. Fixed in 7.0.12-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.0.12-1 | Update the kernel and reboot. Fixed in 7.0.12-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
ip6_datagram_recv_specific_ctl() builds IPV6_{HOPOPTS,DSTOPTS,RTHDR} cmsgs (and their IPV6_2292* legacy counterparts) by trusting the on-wire hdrlen byte (ptr[1]) when computing the put_cmsg() length. The length was validated only at parse time (ipv6_parse_hopopts(), etc.). An nftables payload-write expression can rewrite hdrlen after parsing and before the skb reaches recvmsg; the write itself is in-bounds but put_cmsg() then reads up to ((hdrlen+1) << 3) = 2040 bytes from an 8-byte header. nftables is reachable from an unprivileged user namespace, so this is an unprivileged slab-out-of-bounds read: BUG: KASAN: slab-out-of-bounds in put_cmsg+0x3ac/0x540 put_cmsg+0x3ac/0x540 udpv6_recvmsg+0xca0/0x1250 sock_recvmsg+0xdf/0x190 ____sys_recvmsg+0x1b1/0x620 Add ipv6_get_exthdr_len() which validates that at least two bytes are accessible before reading the hdrlen field, then checks the computed length against skb_tail_pointer(skb), returning 0 on failure. Extension headers are kept in the linear skb area by pskb_may_pull() during input, so skb_tail_pointer() is the correct bound. Use ipv6_get_exthdr_len() at all non-AH call sites: the five standalone cmsg blocks (HbH, 2292HbH, 2292DSTOPTS x2, 2292RTHDR) and the three standard cases in the extension-header walk loop (DSTOPTS, ROUTING, default). AH retains an inline bounds check because its length formula differs ((ptr[1]+2)<<2). The walk loop also gets a pre-read bounds check at the top to validate ptr before any case accesses ptr[0] or ptr[1]. When the walk loop detects a corrupted header, return from the function instead of continuing to process later socket options.
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/datagram.c
- What this is
- NETWORKING [GENERAL]MAINTAINERS section matched on net/ · 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
- 2026-07-19 (3 months ago)source
- CVSS
- 7.1 high
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:Hsource recompute it - Attack vector
- local (AV:L)source
- EPSS
- 0.17%, higher than 6.1% 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 |
|---|---|---|
| 2.6.12 | 5.10.259 | 81394827dfb7 |
| 2.6.12 | 5.15.210 | 931b4a1f1340 |
| 2.6.12 | 6.1.176 | 08464413e628 |
| 2.6.12 | 6.6.143 | a29768d56eb3 |
| 2.6.12 | 6.12.93 | 0d330eff318c |
| 2.6.12 | 6.18.35 | a35daeabb433 |
| 2.6.12 | 7.0.12 | eb18a1b1644e |
| 2.6.12 | 7.1 | dd433671fef3 |
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:LAttack vector, local
- Exploitation requires local syscalls (nftables netlink to install a payload-set rule, setsockopt for IPV6_RECV* options, and recvmsg). Although the trigger packet is received over IPv6, the mandatory nftables corruption step is classified as Local per kernel guidance.
- AC:LAttack complexity, low
- The attacker fully controls the nftables payload-set rule, the on-wire extension header, socket receive options, and the recvmsg call, making the slab out-of-bounds read reliably reproducible without races or external conditions.
- PR:LPrivileges required, low
- An unprivileged local user can create a user+network namespace, obtain CAP_NET_ADMIN there, configure nftables, open a UDPv6/raw socket with extension-header cmsg options, and trigger the bug without real root in the initial namespace.
- UI:NUser interaction, none
- No victim interaction is required; the attacker can send the crafted IPv6 datagram and call recvmsg on their own socket to trigger the out-of-bounds read.
- S:UScope, unchanged
- Impact is confined to kernel memory disclosure and potential kernel instability within the same kernel security domain; it does not cross a VM/host or IOMMU boundary.
- C:HConfidentiality impact, high
- put_cmsg() can copy up to 2040 bytes past the actual extension header into userspace ancillary data, leaking adjacent slab/kernel memory (confirmed by KASAN slab-out-of-bounds in put_cmsg).
- I:NIntegrity impact, none
- The bug is an out-of-bounds read that copies kernel data to userspace; it does not modify kernel memory, corrupt control structures, or provide a write primitive.
- A:HAvailability impact, high
- The out-of-bounds slab read can provoke a KASAN fault/kernel oops during put_cmsg(), and repeated triggering can deny availability of the affected system.