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CVE-2026-74739
net/sched: cls_u32: skip hash tables in u32_bind_class()
Published 2026-08-26 (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-2026-74739 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 | fixed | 6.12.105-1 | Update the kernel and reboot. Fixed in 6.12.105-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.1.10-1 | Update the kernel and reboot. Fixed in 7.1.10-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.1.10-1 | Update the kernel and reboot. Fixed in 7.1.10-1. 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
u32_walk() enumerates both struct tc_u_hnode and struct tc_u_knode through the walker callback. u32_bind_class() unconditionally casts the passed fh to tc_u_knode and accesses &n->res, so when fh is actually a tc_u_hnode, which has no tcf_result member, this results in a slab-out-of-bounds read of res->classid in tc_cls_bind_class(). The issue can be reproduced with the following commands: tc qdisc add dev lo root handle 1: hfsc tc class add dev lo parent 1: classid 1:1 hfsc sc rate 1000kbit tc filter add dev lo parent 1:1 protocol ip prio 1 u32 match u32 0 0 flowid 1:1 tc class add dev lo parent 1: classid 1:2 hfsc sc rate 2000kbit Fix this by skipping hash tables via the TC_U32_KEY(handle) check.
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/cls_u32.c
- What this is
- TC subsystemMAINTAINERS section matched on net/sched/ · netdev@vger.kernel.org · Maintained
- Kernel module
- cls_u32
Check it on the machine
grep -w CONFIG_NET_CLS_U32 /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 cls_u32Prints 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_CLS_U32
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
- 2026-08-26 (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
- 0.14%, higher than 2.9% 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 |
|---|---|---|
| 4.14 | 6.1.184 | ec5f3005586a |
| 4.14 | 6.6.153 | 19d114b93c94 |
| 4.14 | 6.12.105 | 594a064d6032 |
| 4.14 | 6.18.46 | 31f26a95eeee |
| 4.14 | 7.1.10 | e71f8e9ed6f3 |
| 4.14 | 7.2 | 6d3724e616fa |
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 |
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 via RTM_NEWTCLASS rtnetlink (tc class add) in __tc_ctl_tclass(), which calls tc_bind_tclass() to walk u32 filters; per kernel CNA guidance, tc/qdisc/netlink classifier configuration is Local, not remotely reachable by packets.
- AC:LAttack complexity, low
- The reproducer is deterministic: install a u32 filter with flowid, then add another tc class so reverse binding walks hash nodes; the attacker controls every netlink step and needs no race or uncontrollable victim state.
- PR:LPrivileges required, low
- RTM_NEWTCLASS requires CAP_NET_ADMIN enforced by netlink_net_capable() in rtnetlink; this capability is obtainable by unprivileged users in a user+network namespace (unshare -Urn) or containers granted NET_ADMIN, not init-namespace root only.
- UI:NUser interaction, none
- Exploitation requires only the attacker's own tc/netlink operations (qdisc, filter, class add) inside a namespace they control; no separate victim action such as opening files or mounting media is needed.
- S:UScope, unchanged
- Impact is kernel slab memory read and possible adjacent-slab corruption during tc class binding within the same host kernel; this is standard local kernel compromise, not VM escape, IOMMU bypass, or another cross-authority boundary.
- C:HConfidentiality impact, high
- u32_bind_class() casts tc_u_hnode to tc_u_knode and reads &n->res.classid past the hnode allocation in tc_cls_bind_class(); per CNA guidance, slab out-of-bounds reads of kernel memory warrant High confidentiality impact.
- I:HIntegrity impact, high
- If the out-of-bounds classid read matches the newly created class, tc_cls_bind_class() calls __tcf_bind_filter(), which xchg-writes to &r->class at the bogus offset, enabling adjacent slab corruption and potential control-flow hijack per memory-corruption guidance.
- A:HAvailability impact, high
- The slab out-of-bounds access is reported as a KASAN violation on trigger, and mistaken reverse binding can corrupt adjacent kernel objects leading to oops/panic during later tc operations; per CNA guidance, kernel crashes and memory corruption score Availability High.