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CVE-2026-74544
net/sched: cls_u32: validate offshift to prevent shift-out-of-bounds
Published 2026-08-15 (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-2026-74544 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 | vulnerable | 6.1.190-1 | No fix published anywhere yet, neither in Debian nor upstream for the series bookworm tracks. Watch the Debian tracker page for this CVE. Advisories for bookworm have historically landed a median of 23 days after publication. |
| Debian 13trixie | vulnerable | 6.12.113-1 | No fix published anywhere yet, neither in Debian nor upstream for the series trixie tracks. Watch the Debian tracker page for this CVE. Advisories for trixie have historically landed a median of 11 days after publication. |
| Debian 14forky | fixed | 7.1.8-1 | Update the kernel and reboot. Fixed in 7.1.8-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.1.8-1 | Update the kernel and reboot. Fixed in 7.1.8-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
u32_change() copies the user-provided tc_u32_sel.offshift (unsigned char, 0-255) into the kernel knode object without bounds validation. When a packet later hits u32_classify() with TC_U32_VAROFFSET set, it evaluates `ntohs(offmask & *data) >> offshift` where the left operand is a 16-bit value promoted to a 32-bit int. Any offshift >= 32 is undefined behavior per C11 6.5.7p3, triggerable by an unprivileged user via user/network namespaces. UBSAN: shift-out-of-bounds in net/sched/cls_u32.c:236:43 shift exponent 32 is too large for 32-bit type int Fix this by rejecting offshift >= 16 during filter creation in u32_change().
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, sidm
- 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
- 2026-08-15 (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
- 0.17%, higher than 6.3% 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 |
|---|---|---|
| 2.6.12 | 6.18.57 | 9990cac513e8 |
| 2.6.12 | 7.1.8 | 313cb9ffc410 |
| 2.6.12 | 7.2 | aef96eead286 |
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 configured through RTM_NEWTFILTER rtnetlink (tc filter add) in u32_change(); per kernel CNA guidance, tc/qdisc/netlink classifier setup is Local even though subsequent packets invoke u32_classify() on ingress/egress paths.
- AC:LAttack complexity, low
- The attacker fully controls offshift (32-255), TC_U32_VAROFFSET selector flags, and matching packet bytes when installing the filter and sending traffic through the attached qdisc; no race or external victim state is required.
- PR:LPrivileges required, low
- Creating u32 filters requires CAP_NET_ADMIN, which an unprivileged local user obtains in a user/network namespace via unshare -Urn; rtnetlink enforces netlink_net_capable() against that namespace's user_ns, not only init-root.
- UI:NUser interaction, none
- The attacker installs the malicious u32 filter and injects triggering packets themselves within their namespace; no separate victim action (opening files, mounting media, clicking links) is needed.
- S:UScope, unchanged
- Corrupted classifier offset arithmetic and any resulting kernel crash stay within the host kernel's security authority; this is standard local kernel compromise, not a VM escape, IOMMU bypass, or sandbox boundary crossing.
- C:HConfidentiality impact, high
- Undefined-behavior right-shift (offshift>=32) on packet-derived values corrupts variable offset math in u32_classify(), driving attacker-influenced skb_header_pointer reads at unintended offsets; per CNA guidance, exploitable kernel memory-unsafe operations warrant High confidentiality impact.
- I:HIntegrity impact, high
- Invalid shift results corrupt off2 and unsigned off accumulation in the classifier hot path, derailing cls_u32 control flow; C11 undefined behavior in kernel packet classification is exploitable for integrity compromise and potential code execution per CNA memory-corruption guidance.
- A:HAvailability impact, high
- Hitting u32_classify() with offshift>=32 provokes documented UBSAN shift-out-of-bounds at cls_u32.c:236; undefined shifts can also trigger kernel oops/panic during packet classification, satisfying CNA guidance that kernel crashes are High availability impact.