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CVE-2026-43185
ksmbd: fix signededness bug in smb_direct_prepare_negotiation()
Published 2026-05-06 (5 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-2026-43185 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 | 6.19.6-1 | Update the kernel and reboot. Fixed in 6.19.6-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 6.19.6-1 | Update the kernel and reboot. Fixed in 6.19.6-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
smb_direct_prepare_negotiation() casts an unsigned __u32 value from sp->max_recv_size and req->preferred_send_size to a signed int before computing min_t(int, ...). A maliciously provided preferred_send_size of 0x80000000 will return as smaller than max_recv_size, and then be used to set the maximum allowed alowed receive size for the next message. By sending a second message with a large value (>1420 bytes) the attacker can then achieve a heap buffer overflow. This fix replaces min_t(int, ...) with min_t(u32)
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
- 2026-05-06 (5 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
- 0.70%, higher than 52.3% 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 | 6.18.16 | ceae058eb707 |
| 5.15 | 6.19.6 | 55abc475d096 |
| 5.15 | 7.0 | 6b4f875aac34 |
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 ksmbd SMB Direct negotiation is reached by a remote peer over the network via the RDMA listener on SMB Direct ports such as 445/5445.
- AC:LAttack complexity, low
- An attacker can trigger the bug with a crafted negotiation request followed by an oversized data-transfer message; no race or victim-controlled timing is required.
- PR:NPrivileges required, none
- The vulnerable path runs during SMB Direct transport negotiation before SMB authentication or session setup, so no credentials or prior privileges are required.
- UI:NUser interaction, none
- No victim user action is needed beyond the server exposing ksmbd SMB Direct; the attacker sends network protocol messages directly.
- S:UScope, unchanged
- The impact is kernel memory corruption within the same host security authority, not a cross-scope escape such as guest-to-host or IOMMU boundary bypass.
- C:HConfidentiality impact, high
- The signedness bug lets the attacker turn a small allocated receive buffer into a huge posted RDMA receive target, causing attacker-controlled kernel heap memory corruption that can plausibly be leveraged for information disclosure.
- I:HIntegrity impact, high
- The second SMB Direct message causes an out-of-bounds write/DMA overwrite into kernel heap memory, which is high-integrity impact and plausibly exploitable for control-flow or data corruption.
- A:HAvailability impact, high
- The heap buffer overflow is remotely triggerable pre-authentication and can crash or panic the kernel even without full code execution.