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CVE-2026-74705
udp: fix potential use-after-free in tunnel segmentation
Published 2026-08-22 (2 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-2026-74705 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 | fixedDLA-4777-1 · 2026-09-08 · text | 6.1.187-1 | Update the kernel and reboot. DLA-4777-1 shipped the fix on 2026-09-08, in 6.1.187-1. A running kernel keeps the old code until the machine restarts. |
| Debian 13trixie | fixedDSA-6466-1 · 2026-08-25 · text | 6.12.105-1 | Update the kernel and reboot. DSA-6466-1 shipped the fix on 2026-08-25, in 6.12.105-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.1.9-1 | Update the kernel and reboot. Fixed in 7.1.9-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.1.9-1 | Update the kernel and reboot. Fixed in 7.1.9-1. A running kernel keeps the old code until the machine restarts. |
| Debian 12bookworm · linux-6.12 | fixedDLA-4788-1 · 2026-09-19 · text | 6.12.107-1~deb12u1 | Update the kernel and reboot. DLA-4788-1 shipped the fix on 2026-09-19, in 6.12.107-1~deb12u1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
__skb_udp_tunnel_segment() gets the UDP header before ensuring the tunnel header is in the skb head. If the pull reallocates skb->head, the saved UDP header pointer is no longer valid. Get the UDP header after the pull to avoid a potential use-after-free.
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/udp_offload.c
- What this is
- NETWORKING [GENERAL]MAINTAINERS section matched on net/ · 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, sid, bookworm · linux-6.12y
- 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
- 2026-08-22 (2 months ago)source
- CVSS
- 10.0 critical
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:Hsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.50%, higher than 41.5% 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 |
|---|---|---|
| 4.6 | 5.10.265 | 6a733a38b983 |
| 4.6 | 5.15.216 | 19d89b13a436 |
| 4.6 | 6.1.183 | b3df61bb745e |
| 4.6 | 6.6.152 | 1ae134c012e1 |
| 4.6 | 6.12.104 | 5161e67c561c |
| 4.6 | 6.18.45 | 64d322c28857 |
| 4.6 | 7.1.9 | 588d4a6795d9 |
| 4.6 | 7.2 | d0f86fb36eb2 |
Debian advisories
| Advisory | Date | Shipped to | Debian |
|---|---|---|---|
| DLA-4788-1 | 2026-09-19 | bookworm 6.12.107-1~deb12u1 | tracker |
| DLA-4777-1 | 2026-09-08 | bookworm 6.1.187-1 | tracker |
| DSA-6466-1 | 2026-08-25 | trixie 6.12.105-1 | tracker |
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
- Remote peers reach __skb_udp_tunnel_segment on egress when GRO-coalesced VXLAN/Geneve/FOU GSO superpackets are forwarded or re-encapsulated through dev_queue_xmit, and malicious virtio-net/cloud tenants can inject VIRTIO_NET_HDR_GSO_UDP_TUNNEL frames the host later segments on physical or overlay NICs.
- AC:LAttack complexity, low
- Triggering is deterministic once a GSO UDP-tunnel skb has tunnel headers beyond skb_headlen so pskb_may_pull reallocates skb->head; attackers control that layout via fragmented or GRO-built outer/encap headers and can repeat crafted sends until segmentation dereferences the stale UDP pointer.
- PR:NPrivileges required, none
- No host authentication or credentials are required to send UDP tunnel traffic to listening overlay ports on cloud/DC nodes, and co-resident virtio-net guests or tenant VMs can emit GSO UDP-tunnel frames into the host datapath without CAP_NET_ADMIN on the victim host.
- UI:NUser interaction, none
- Exploitation needs only crafted encapsulated GSO traffic into an already-running overlay, bridge, IPVS, OVS, or virtio forwarding path; no victim file open, mount, link click, or other interactive action beyond normal network or VM I/O.
- S:CScope, changed
- In cloud/Kubernetes and virtio-net multitenant deployments, remote or guest/tenant senders can corrupt hypervisor kernel heap during host-side UDP-tunnel GSO segmentation on egress, crossing the VM/container-to-host security boundary rather than staying within the attacker's own namespace.
- C:HConfidentiality impact, high
- After pskb_may_pull reallocates skb->head, the pre-pull udp_hdr pointer is stale yet uh->len and uh->check are read for checksum adjustment, yielding a classic heap use-after-free read primitive that can disclose kernel memory and support further exploitation.
- I:HIntegrity impact, high
- UAF on the UDP header allows attacker-influenced heap reuse and corruption of header fields and checksum state during tunnel GSO segmentation, providing a write/control primitive suitable for kernel code execution rather than a benign checksum miscalculation.
- A:HAvailability impact, high
- Dereferencing the freed UDP header during GSO segmentation can immediately oops or panic the kernel, and repeated exploitation attempts against this UAF commonly crash or hang the host even when full control is not achieved.