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CVE-2026-63912
xfrm: esp: restore combined single-frag length gate
Published 2026-07-19 (3 months ago) by the Linux kernel CNA. Subsystem net/ipv4. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-63912 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
The ESP out-of-place fast path appends the trailer in esp_output_head() before esp_output_tail() allocates the destination page frag. The head-side gate currently checks skb->data_len and tailen separately, but the tail code allocates a single destination frag from the combined post-trailer skb->data_len. Reject the page-frag fast path when the combined aligned length exceeds a page. Otherwise skb_page_frag_refill() may fall back to a single page while the destination sg still spans the combined skb->data_len. Restore this combined-length page gate for both IPv4 and IPv6.
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/ipv4/esp4.c
net/ipv6/esp6.c
- What this is
- NETWORKING [IPSEC]MAINTAINERS section matched on net/ipv4/esp4* · netdev@vger.kernel.org · Maintained
- Kernel module
- esp4
Check it on the machine
grep -wE 'CONFIG_INET|CONFIG_INET_ESP' /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.
lsmod | grep -w esp4Prints a line if that module is loaded right now, and nothing if it is not. One that is not loaded can still be loaded later by anything that needs it.
Show whether Debian builds this, per release
Debian builds this as a loadable module. Every release below compiles it as a module on every flavour, so whether the code is in the running kernel depends on whether that module is loaded.
- bookworm, trixie, forky, sidm
- m
- a loadable module on every flavour - check lsmod, can be blacklisted
- Build switches
- CONFIG_INET, CONFIG_INET_ESP
- Reads as
- all on means the file is compiled; any one off means it is not.
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
- 9.8 critical
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:Hsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.71%, higher than 52.8% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- net/ipv4
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 5.10.113 | 5.10.259 | 566295735530 |
| 5.15.36 | 5.15.210 | 5d7ab86e2b6b |
| 5.18 | 6.1.176 | 36519e3d941f |
| 5.18 | 6.6.143 | 322e48187e02 |
| 5.18 | 6.12.93 | b84091ceddc9 |
| 5.18 | 6.18.35 | c093468aea82 |
| 5.18 | 7.0.12 | 65f3b3fc2347 |
| 5.18 | 7.1 | dfa0d7b0ff1e |
| 4.14.288 | 0 | 0 |
| 4.19.252 | 0 | 0 |
| 5.4.205 | 0 | 0 |
| 5.17.5 | 0 | 0 |
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
- The bug is in the ESP outbound encryption path reached when IPsec-protected traffic is sent through xfrm (e.g., from an internet-facing VPN/tunnel gateway encrypting responses or forwarded flows to a remote peer). A network peer can influence outbound skb fragment sizes to hit the vulnerable fast path without local shell access.
- AC:LAttack complexity, low
- An attacker can reliably craft payload sizes so ALIGN(skb->data_len) and ALIGN(tailen) each pass the broken per-field PAGE_SIZE gate while their combined aligned length exceeds PAGE_SIZE, repeatedly probing until the out-of-place page-frag path is taken.
- PR:NPrivileges required, none
- Exploitation does not require configuring xfrm or host admin rights; it only needs the target to already run ESP and the attacker to send traffic that is encrypted on output (e.g., as a remote VPN/site-to-site peer), which is network-layer access rather than local OS privilege.
- UI:NUser interaction, none
- Triggering the vulnerable ESP output path requires only attacker-controlled network traffic and does not depend on any victim user action such as mounting a filesystem or opening a file.
- S:UScope, unchanged
- Impact is confined to kernel memory corruption and privilege escalation within the same kernel security authority, not a cross-boundary escape such as guest-to-host VM breakout or IOMMU bypass.
- C:HConfidentiality impact, high
- The flaw is a heap out-of-bounds write during AEAD encryption over an undersized page frag; such memory corruption can expose adjacent kernel objects and be leveraged for arbitrary memory disclosure, not merely a bounded leak.
- I:HIntegrity impact, high
- Writing past the allocated page-frag buffer corrupts adjacent heap memory and can be developed into arbitrary write or control-flow hijack primitives, matching kernel guidance for out-of-bounds writes.
- A:HAvailability impact, high
- Corrupting kernel heap memory during crypto output can cause immediate kernel oops/panic or persistent instability, and repeated triggering can deny service on VPN gateways and other IPsec endpoints.