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CVE-2026-53250
xsk: cache csum_start/csum_offset to fix TOCTOU in xsk_skb_metadata()
Published 2026-06-25 (4 months ago) by the Linux kernel CNA. Subsystem net/xdp. Debian urgency: unimportant.
What to do, per Debian release
Debian's own status for CVE-2026-53250 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 | not affected | 6.1.190-1 | Nothing to do. bookworm never shipped the vulnerable code. |
| Debian 13trixie | fixedDSA-6528-1 · 2026-09-29 · text | 6.12.111-1 | Update the kernel and reboot. DSA-6528-1 shipped the fix on 2026-09-29, in 6.12.111-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.0.13-1 | Update the kernel and reboot. Fixed in 7.0.13-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.0.13-1 | Update the kernel and reboot. Fixed in 7.0.13-1. A running kernel keeps the old code until the machine restarts. |
| Debian 12bookworm · linux-6.12 | fixedDLA-4817-1 · 2026-10-04 · text | 6.12.111-1~deb12u1 | Update the kernel and reboot. DLA-4817-1 shipped the fix on 2026-10-04, in 6.12.111-1~deb12u1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
The TX metadata area resides in the UMEM buffer which is memory-mapped and concurrently writable by userspace. In xsk_skb_metadata(), csum_start and csum_offset are read from shared memory for bounds validation, then read again for skb assignment. A malicious userspace application can race to overwrite these values between the two reads, bypassing the bounds check and causing out-of-bounds memory access during checksum computation in the transmit path. Fix this by reading csum_start and csum_offset into local variables once, then using the local copies for both validation and assignment. Note that other metadata fields (flags, launch_time) and the cached csum fields may be mutually inconsistent due to concurrent userspace writes, but this is benign: the only security-critical invariant is that each field's validated value is the same one used, which local caching guarantees.
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/xdp/xsk.c
- What this is
- XDP SOCKETS (AF_XDP)MAINTAINERS section matched on net/xdp/ · netdev@vger.kernel.org · Maintained
Check it on the machine
grep -w CONFIG_XDP_SOCKETS /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_XDP_SOCKETS
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
- 2026-06-25 (4 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
- 0.10%, higher than 1.0% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- unimportantsource
- Subsystem
- net/xdp
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 6.8 | 6.12.110 | eb778aed8a9b |
| 6.8 | 6.18.36 | 0dfe05b93843 |
| 6.8 | 7.0.13 | bfdfd2706d5f |
| 6.8 | 7.1 | 22ba97ea9cc1 |
Debian advisories
| Advisory | Date | Shipped to | Debian |
|---|---|---|---|
| DLA-4817-1 | 2026-10-04 | bookworm 6.12.111-1~deb12u1 | tracker |
| DSA-6528-1 | 2026-09-29 | trixie 6.12.111-1 | 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 vulnerable path is reached by a local process using AF_XDP socket setup, UMEM registration, TX ring submission, and send/poll-driven transmit; it is not reachable directly from remote network packets.
- AC:LAttack complexity, low
- The attacker controls the UMEM metadata and can trigger TX from the same process, so the TOCTOU race can be created with attacker-controlled concurrent writes.
- PR:LPrivileges required, low
- AF_XDP socket creation requires CAP_NET_RAW in the relevant network namespace, which is attainable by a low-privileged local user in user/network namespace deployments.
- UI:NUser interaction, none
- No victim action is needed after the attacker has local execution; the attacker sets up the socket, UMEM, metadata, and transmit operation directly.
- S:UScope, unchanged
- The vulnerability corrupts or reads kernel networking memory within the same kernel security authority; this is standard local kernel attack surface, not a VM or hardware boundary crossing.
- C:HConfidentiality impact, high
- Racing csum_start/csum_offset can bypass validation and drive out-of-bounds checksum reads in kernel transmit checksum handling, so potential kernel memory disclosure is high.
- I:HIntegrity impact, high
- The same unvalidated checksum offsets can cause checksum writes to an attacker-influenced out-of-bounds location relative to skb data, making kernel memory corruption plausible.
- A:HAvailability impact, high
- The out-of-bounds memory access in the kernel transmit path can trigger warnings, faults, or kernel crashes and can be repeatedly exercised by the local attacker.