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CVE-2026-31536
smb: server: let send_done handle a completion without IB_SEND_SIGNALED
Published 2026-04-24 (6 months ago) by the Linux kernel CNA. Subsystem fs/smb. Debian urgency: not yet assigned.
Debian does not build this code. The per-release build detail is under "Where it lives" below.
What to do, per Debian release
Debian's own status for CVE-2026-31536 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 | 6.18.12-1 | Update the kernel and reboot. Fixed in 6.18.12-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 6.18.12-1 | Update the kernel and reboot. Fixed in 6.18.12-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
With smbdirect_send_batch processing we likely have requests without IB_SEND_SIGNALED, which will be destroyed in the final request that has IB_SEND_SIGNALED set. If the connection is broken all requests are signaled even without explicit IB_SEND_SIGNALED.
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.
fs/smb/server/transport_rdma.c
- What this is
- SMBDIRECT (RDMA Stream Transport with Read/Write-Offload, MS-SMBD)MAINTAINERS section matched on fs/smb/server/transport_rdma.* · linux-cifs@vger.kernel.org · Maintained
- Kernel module
- none to look for. The file would have gone into ksmbd, and Debian does not compile it into that module, so finding ksmbd in lsmod would say nothing about this CVE.
Check it on the machine
grep -wE 'CONFIG_SMBFS|CONFIG_SMB_SERVER|CONFIG_SMB_SERVER_SMBDIRECT' /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 does not build this code. In 4 releases below, every kernel flavour Debian publishes has the switch off, so the vulnerable file is compiled into no kernel binary Debian ships.
- bookworm, trixie, forky, sidn
- n
- not enabled on any flavour - this release does not build the code
- Build switches
- CONFIG_SMBFS, CONFIG_SMB_SERVER, CONFIG_SMB_SERVER_SMBDIRECT
- Reads as
- all on means the file is compiled; any one off means it is not.
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
- 2026-04-24 (6 months ago)source
- CVSS
- 9.8 critical
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:Hsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.65%, higher than 50.1% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- fs/smb
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 5.15 | 6.18.11 | 24082642654f |
| 5.15 | 6.19.1 | e38b415c024b |
| 5.15 | 7.0 | 9da82dc73cb0 |
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 vulnerable code is in ksmbd SMB Direct, which accepts remote RDMA/SMB Direct connections on network ports 445 and 5445 and processes peer-controlled SMB Direct traffic.
- AC:LAttack complexity, low
- An attacker can negotiate a small SMB Direct max receive size to force batched multi-WR sends, then break the connection to make unsignaled sends complete with error. The disconnect/error condition is attacker-controlled rather than an external race.
- PR:NPrivileges required, none
- The path is reachable before SMB authentication: SMB Direct negotiation completes first, and SMB2 NEGOTIATE/SESSION_SETUP do not require a valid session. A pre-auth remote client can trigger outbound responses through the vulnerable RDMA send batching path.
- UI:NUser interaction, none
- No victim user action is required once ksmbd with SMB Direct is listening; the attacker only needs to connect and send protocol messages.
- S:UScope, unchanged
- The impact is within the kernel/ksmbd host security authority and does not cross a hypervisor, IOMMU, or separate sandbox boundary.
- C:HConfidentiality impact, high
- The bug is a use-after-free/double-free style memory corruption in RDMA send objects, and kernel UAFs can be shaped into disclosure primitives through heap reuse.
- I:HIntegrity impact, high
- The freed `smbdirect_send_io` objects contain list pointers, CQ callbacks, and work request state; corrupting or reusing them can plausibly support arbitrary write or control-flow hijack.
- A:HAvailability impact, high
- Even without full exploitation, the UAF/list corruption is expected to cause kernel warnings, crashes, or hangs during RDMA QP drain and completion handling.