Bug 2454853 (CVE-2026-31392)

Summary:	CVE-2026-31392 kernel: smb: client: fix krb5 mount with username option
Product:	[Other] Security Response	Reporter:	OSIDB Bzimport <bzimport>
Component:	vulnerability	Assignee:	Product Security DevOps Team <prodsec-dev>
Status:	NEW ---	QA Contact:
Severity:	medium	Docs Contact:
Priority:	medium
Version:	unspecified	Keywords:	Security
Target Milestone:	---
Target Release:	---
Hardware:	All
OS:	Linux
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Fixed In Version:		Doc Type:	---
Doc Text:	A flaw was found in the Linux kernel's Server Message Block (SMB) client. A local attacker, by attempting to mount SMB shares using Kerberos (sec=krb5) with a specified username, could cause the client to incorrectly reuse an existing SMB session. This session reuse occurs even when a different username is provided for subsequent mounts, potentially leading to an authentication bypass where shares are accessed with unintended credentials.	Story Points:	---
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Regression:	---	Mount Type:	---
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Verified Versions:		Category:	---
oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
Embargoed:

Description OSIDB Bzimport 2026-04-03 16:04:21 UTC

In the Linux kernel, the following vulnerability has been resolved:

smb: client: fix krb5 mount with username option

Customer reported that some of their krb5 mounts were failing against
a single server as the client was trying to mount the shares with
wrong credentials.  It turned out the client was reusing SMB session
from first mount to try mounting the other shares, even though a
different username= option had been specified to the other mounts.

By using username mount option along with sec=krb5 to search for
principals from keytab is supported by cifs.upcall(8) since
cifs-utils-4.8.  So fix this by matching username mount option in
match_session() even with Kerberos.

For example, the second mount below should fail with -ENOKEY as there
is no 'foobar' principal in keytab (/etc/krb5.keytab).  The client
ends up reusing SMB session from first mount to perform the second
one, which is wrong.

```
$ ktutil
ktutil:  add_entry -password -p testuser -k 1 -e aes256-cts
Password for testuser:
ktutil:  write_kt /etc/krb5.keytab
ktutil:  quit
$ klist -ke
Keytab name: FILE:/etc/krb5.keytab
KVNO Principal
 ---- ----------------------------------------------------------------
   1 testuser (aes256-cts-hmac-sha1-96)
$ mount.cifs //w22-root2/scratch /mnt/1 -o sec=krb5,username=testuser
$ mount.cifs //w22-root2/scratch /mnt/2 -o sec=krb5,username=foobar
$ mount -t cifs | grep -Po 'username=\K\w+'
testuser
testuser
```

Comment 1 Jon Moroney 2026-04-03 17:37:08 UTC

Upstream advisory:
https://lore.kernel.org/linux-cve-announce/2026040324-CVE-2026-31392-7952@gregkh/T