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CVE-2026-74663
net/sched: reject overly deep qdisc hierarchies
Published 2026-08-22 (2 months ago) by the Linux kernel CNA. Subsystem include/net. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-74663 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
Deep qdisc hierarchies can lead to excessive recursion in qdisc tree walkers and exhaust the kernel stack. The existing loop check does not cover the create-and-graft path, so a hierarchy can still be extended by creating a new child qdisc below an already deep parent. Store the hierarchy depth in struct Qdisc and update it when qdiscs are grafted. Reject new child qdiscs once the parent is already at the maximum allowed depth.
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.
include/net/sch_generic.h
net/sched/sch_api.c
- What this is
- NETWORKING [GENERAL]MAINTAINERS section matched on include/net/ · netdev@vger.kernel.org · Maintained
Check it on the machine
grep -w CONFIG_NET_SCHED /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_NET_SCHED
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
- 7.8 high
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:Hsource recompute it - Attack vector
- local (AV:L)source
- EPSS
- 0.13%, higher than 2.3% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- include/net
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 | 5.10.267 | 08dc49df1527 |
| 2.6.12 | 5.15.218 | a627d36c2a94 |
| 2.6.12 | 6.1.183 | 9f69bb9fdaa2 |
| 2.6.12 | 6.6.152 | 8ca8cdb74939 |
| 2.6.12 | 6.12.104 | 2759acf08a34 |
| 2.6.12 | 6.18.45 | a4b14a4df29d |
| 2.6.12 | 7.1.9 | e2d658c64278 |
| 2.6.12 | 7.2 | dedd34b0f231 |
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:LAttack vector, local
- The bug is reached only through RTM_NEWQDISC/tc netlink operations that build or graft qdiscs and invoke recursive qdisc tree walkers such as check_loop(); per kernel CNA guidance, tc/netlink qdisc configuration is Local and is not on any remote packet-receive path.
- AC:LAttack complexity, low
- The create-and-graft path bypasses the existing depth-7 check_loop() guard, so an attacker can deterministically extend a linear classful qdisc chain (e.g., repeated HTB/HFSC grafts) and then trigger check_loop() with a graft/replace to recurse until the kernel stack is exhausted; no race or uncontrollable layout is required.
- PR:LPrivileges required, low
- RTM_NEWQDISC is gated by netlink_net_capable(skb, CAP_NET_ADMIN) in rtnetlink_rcv_msg(), evaluated against the socket netns user_ns; an unprivileged local user routinely obtains CAP_NET_ADMIN via user+network namespaces (unshare -Urn) or containers with NET_ADMIN, not init-namespace root.
- UI:NUser interaction, none
- Exploitation requires only the attacker's own tc/netlink qdisc setup and a follow-on graft/replace that walks the deep hierarchy; no victim must open files, mount filesystems, click prompts, or perform any cooperative action.
- S:UScope, unchanged
- Impact is kernel-stack exhaustion and crash/panic within the same kernel security domain; it does not cross a VM, IOMMU, or sandbox boundary to another security authority such as guest-to-host escape.
- C:HConfidentiality impact, high
- Unbounded recursion in qdisc tree walkers overflows the kernel task stack and corrupts stack memory before the guard-page fault; under overestimation guidance, such stack corruption is treated as plausibly enabling kernel memory disclosure rather than a pure crash with no data exposure.
- I:HIntegrity impact, high
- Recursive stack exhaustion overwrites saved return addresses and adjacent kernel stack frames, providing a plausible path to control-flow hijack or arbitrary kernel modification even though the immediate failure mode is oops/panic, not a bounded integrity change.
- A:HAvailability impact, high
- Excessive recursion in qdisc tree walkers exhausts the kernel stack and causes a kernel oops/panic, denying all system availability on the host until reboot; any kernel stack guard fault or panic from this path scores Availability High.