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CVE-2026-74704
net/sched: sch_cake: drop WARN_ON(1) for malformed packets in ACK filter
Published 2026-08-22 (2 months ago) by the Linux kernel CNA. Subsystem net/sched. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-74704 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
The sch_cake ACK filter parses packets to find the TCP header and filter duplicated ACKs if the flow is backlogged. The parsing code contains a WARN_ON(1) which can be triggered by a malformed IP header in certain cases. Depending on the system configuration, this leads either to either spamming dmesg with warnings, or a panic if panic_on_warn is set. The code already correctly skips the offending packet in the branch that triggers the warning, so the WARN_ON itself doesn't really serve any purpose. So just drop it altogether to avoid the inconvenient side effects.
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/sched/sch_cake.c
- What this is
- CAKE QDISCMAINTAINERS section matched on net/sched/sch_cake.c · cake@lists.bufferbloat.net · Maintained
- Kernel module
- sch_cake
Check it on the machine
grep -w CONFIG_NET_SCH_CAKE /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 sch_cakePrints 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, sid, bookworm · linux-6.12m
- m
- a loadable module on every flavour - check lsmod, can be blacklisted
- Build switch
- CONFIG_NET_SCH_CAKE
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
- 8.2 high
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:Hsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.44%, higher than 37.1% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- net/sched
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 4.19 | 5.10.265 | c1693b7844a6 |
| 4.19 | 5.15.216 | a4b52612004a |
| 4.19 | 6.1.183 | ae1b2f8e21a4 |
| 4.19 | 6.6.152 | 0c4882bff345 |
| 4.19 | 6.12.104 | 2504a76e5c06 |
| 4.19 | 6.18.45 | cd2f1d9fe8a5 |
| 4.19 | 7.1.9 | a1ae353d8355 |
| 4.19 | 7.2 | 2a33516f9ef5 |
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
- The bug fires in cake_ack_filter() during cake_enqueue() on the packet datapath; remotely sourced traffic through an internet-facing SQM/OpenWrt gateway with CAKE ingress/egress and ack-filter enabled reaches the vulnerable ACK-filter parsing loop without any local syscall or netlink operation by the attacker.
- AC:LAttack complexity, low
- Once CAKE ack-filter is configured, an attacker reliably crafts malformed encapsulated IP headers (e.g., IPv4 carrying an inner header with invalid version) and floods matching TCP ACKs to keep the flow backlogged, deterministically hitting the unsupported-version branch on each qualifying enqueue.
- PR:NPrivileges required, none
- Installing CAKE with ack-filter requires CAP_NET_ADMIN, but triggering the bug on a preconfigured edge router or gateway needs no credentials on the target; an unauthenticated remote peer only sends crafted packets, consistent with CNA precedent for packet-driven tc issues (e.g., CVE-2026-72256).
- UI:NUser interaction, none
- Exploitation requires only attacker-generated packets delivered through the shaped interface; no victim login, file open, mount, or other interactive action is needed beyond normal network traffic reaching the CAKE-managed path.
- S:UScope, unchanged
- Impact is confined to kernel traffic-shaping code and warning/panic handling within the affected host; it does not cross VM, container, IOMMU, or other security-authority boundaries.
- C:LConfidentiality impact, low
- Each WARN_ON(1) emits a kernel backtrace that can disclose kernel text and module pointers via dmesg/syslog to the attacker, constituting limited information disclosure even though no out-of-bounds read or use-after-free occurs.
- I:NIntegrity impact, none
- The failure path only executes continue and skips the malformed queued packet; there is no memory corruption, type confusion, or attacker-controlled modification of kernel or network data beyond normal ACK-filter drop behavior.
- A:HAvailability impact, high
- Each matching malformed packet triggers WARN_ON(1) in cake_ack_filter(), flooding kernel warnings and, when panic_on_warn is enabled, invoking check_panic_on_warn() to kernel panic, yielding repeatable denial of service on CAKE-managed router and gateway paths.