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CVE-2026-63992
tunnels: do not assume transport header in iptunnel_pmtud_check_icmp()
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-2026-63992 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
In some cases, iptunnel_pmtud_check_icmp() can be called while skb transport header is not set. This triggers an out-of-bound access, because (typeof(skb->transport_header))~0U is 65535. Access the icmp header based on IPv4 network header, after making sure icmp->type is present in skb linear part. Note that iptunnel_pmtud_check_icmpv6()) is fine.
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/ip_tunnel_core.c
- What this is
- NETWORKING [IPv4/IPv6]MAINTAINERS section matched on net/ipv4/ip* · netdev@vger.kernel.org · Maintained
Check it on the machine
grep -w CONFIG_INET /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_INET
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
- 9.1 critical
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:Hsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.51%, higher than 42.5% 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 |
|---|---|---|
| 5.9 | 5.10.259 | 5a92cb45e347 |
| 5.9 | 5.15.210 | c7b7ec3e69e6 |
| 5.9 | 6.1.176 | 7f4f7efe7f30 |
| 5.9 | 6.6.143 | e917d0c69f01 |
| 5.9 | 6.12.93 | a096b6e34f60 |
| 5.9 | 6.18.35 | 43368636c663 |
| 5.9 | 7.0.12 | cb549df9ce4e |
| 5.9 | 7.1 | 509323077ef7 |
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 vulnerable code runs on the VXLAN/GENEVE transmit path when processing remotely received UDP-encapsulated tunnel traffic (or L2 frames forwarded through a bridge port), which is reachable by sending crafted packets over the network to the victim VTEP.
- AC:LAttack complexity, low
- The attacker fully controls the inner IPv4 header fields (DF bit, length, addresses) and can reliably craft oversized packets that exceed the tunnel-adjusted PMTU on bridge-port tunnel devices without depending on race timing or uncontrollable system state.
- PR:NPrivileges required, none
- Exploitation requires only the ability to send network packets to a host with VXLAN/GENEVE bridged; no local account, authentication, or capabilities on the victim are needed.
- UI:NUser interaction, none
- No victim user action is required; the kernel processes attacker-supplied packets automatically through the tunnel receive, bridge forward, and re-encapsulation path.
- S:UScope, unchanged
- The out-of-bounds read and resulting kernel oops occur within the kernel network stack on the same host; there is no crossing of a VM, container, or IOMMU security boundary beyond standard kernel memory safety failure.
- C:HConfidentiality impact, high
- Reading through `icmp_hdr(skb)` when `transport_header` is unset causes an out-of-bounds read at `skb->head + 65535`, potentially disclosing adjacent kernel memory beyond the skb buffer before any bounds checking occurs.
- I:NIntegrity impact, none
- The flaw is an out-of-bounds read of the ICMP type field; it does not directly modify or write kernel memory, though it may cause a kernel fault.
- A:HAvailability impact, high
- The invalid pointer dereference at offset 65535 can trigger a kernel oops or panic when accessing unmapped memory, causing denial of service on the affected host.