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CVE-2026-64257
smb: client: reject overlapping data areas in SMB2 responses
Published 2026-07-25 (3 months ago) by the Linux kernel CNA. Subsystem fs/smb. Debian urgency: unimportant.
What to do, per Debian release
Debian's own status for CVE-2026-64257 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 | not affected | 6.12.113-1 | Nothing to do. trixie never shipped the vulnerable code. |
| Debian 14forky | not affected | 7.2.11-1 | Nothing to do. forky never shipped the vulnerable code. |
| Debian unstablesid | not affected | 7.2.11-1 | Nothing to do. sid never shipped the vulnerable code. |
The vulnerability
What the bug is
Commit 53b7c271f06b ("smb: client: restrict implied bcc[0] exemption to responses without data area") restricted the implied bcc[0] length exception to responses without a data area. However, the overlap handling in __smb2_calc_size() clears data_length, which can make an invalid response appear to have no data area and so qualify for the exception. Track data area overlap separately and reject such responses before applying the length compatibility exceptions.
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/client/smb2misc.c
- What this is
- COMMON INTERNET FILE SYSTEM CLIENT (CIFS and SMB3)MAINTAINERS section matched on fs/smb/client/ · linux-cifs@vger.kernel.org · Supported
- Kernel module
- cifs
Check it on the machine
grep -wE 'CONFIG_CIFS|CONFIG_SMBFS' /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 cifsPrints 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 switches
- CONFIG_CIFS, CONFIG_SMBFS
- 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
- 2026-07-25 (3 months ago)source
- CVSS
- 9.1 critical
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/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
- unimportantsource
- 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 |
|---|---|---|
| 6.1.178 | 6.1.178 | 445ece263131 |
| 6.6.145 | 6.6.145 | 36bfa52459e4 |
| 6.12.96 | 6.12.96 | 4a9d2657d3e0 |
| 6.18.39 | 6.18.39 | fdafa1e68dc7 |
| 7.1.4 | 7.1.4 | 57cba95f0e97 |
| 7.2 | 7.2 | 8986c932905e |
| 5.10.261 | 0 | 0 |
| 5.15.212 | 0 | 0 |
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
- A malicious or compromised SMB server can deliver the crafted SMB2 response over TCP during client negotiation.
- AC:LAttack complexity, low
- The overlapping offset and oversized length are attacker-controlled and deterministically bypass validation; no race or uncontrolled memory layout is required to trigger the out-of-bounds read.
- PR:NPrivileges required, none
- The vulnerable NEGOTIATE response is processed before SMB authentication or signature verification, requiring no privileges or credentials on the target.
- UI:NUser interaction, none
- An already configured SMB mount can negotiate during automatic mounting or reconnection without a human opening crafted content.
- S:UScope, unchanged
- The vulnerable parser and the affected kernel memory remain within the Linux kernel's security authority.
- C:HConfidentiality impact, high
- The validation mismatch permits an attacker-controlled ASN.1 length to read far beyond the 448-byte receive object, potentially traversing tens of kilobytes of adjacent kernel memory.
- I:NIntegrity impact, none
- The investigated paths provide an out-of-bounds read but no out-of-bounds write, use-after-free, type confusion, or other arbitrary modification primitive.
- A:HAvailability impact, high
- The slab out-of-bounds access is repeatable and can produce a kernel fault or panic, particularly on hardened or memory-safety-instrumented deployments.