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sambaSamba SecuritySAMBA:CVE-2021-43566
HistoryJan 10, 2022 - 12:00 a.m.

Symlink race error can allow directory creation

2022-01-1000:00:00
Samba Security
www.samba.org
20

2.5 Low

CVSS3

Attack Vector

LOCAL

Attack Complexity

HIGH

Privileges Required

LOW

User Interaction

NONE

Scope

UNCHANGED

Confidentiality Impact

NONE

Integrity Impact

LOW

Availability Impact

NONE

CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:N

1.2 Low

CVSS2

Access Vector

LOCAL

Access Complexity

HIGH

Authentication

NONE

Confidentiality Impact

NONE

Integrity Impact

PARTIAL

Availability Impact

NONE

AV:L/AC:H/Au:N/C:N/I:P/A:N

0.001 Low

EPSS

Percentile

38.7%

Description

All versions of Samba prior to 4.13.16 are vulnerable to a malicious
client using an SMB1 or NFS symlink race to allow a directory to be
created in an area of the server file system not exported under the
share definition. Note that SMB1 has to be enabled, or the share
also available via NFS in order for this attack to succeed.

Clients that have write access to the exported part of the file system
under a share via SMB1 unix extensions or NFS can create symlinks that
can race the server by renaming an existing path and then replacing it
with a symlink. If the client wins the race it can cause the server to
create a directory under the new symlink target after the exported
share path check has been done. This new symlink target can point to
anywhere on the server file system. The authenticated user must have
permissions to create a directory under the target directory of the
symlink.

This is a difficult race to win, but theoretically possible. Note that
the proof of concept code supplied wins the race only when the server
is slowed down and put under heavy load. Exploitation of this bug has
not been seen in the wild.

Patch Availability

Patches addressing this issue has been posted to:

https://www.samba.org/samba/security/

Samba 4.13.16 has been issued as a security releases to correct the
defect. Samba administrators are advised to upgrade to this release as
soon as possible.

CVSSv3.1 calculation

CVSS:2.2/AV:A/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N/E:P/RL:O/RC:C/CR:L/IR:L/AR:L/MAV:N/MAC:H/MPR:L/MUI:N/MS:U/MC:L/MI:N/MA:N

base score of 2.6.

Workaround and mitigating factors

Do not enable SMB1 (please note SMB1 is disabled by default in Samba
from version 4.11.0 and onwards). This prevents the creation of
symbolic links via SMB1. If SMB1 must be enabled for backwards
compatibility then add the parameter:

unix extensions = no

to the [global] section of your smb.conf and restart smbd. This
prevents SMB1 clients from creating symlinks on the exported file
system.

However, if the same region of the file system is also exported using
NFS, NFS clients can create symlinks that potentially can also hit the
race condition. For non-patched versions of Samba we recommend only
exporting areas of the file system by either SMB2 or NFS, not both.

Credits

Reported by Michael Hanselmann of Google.
Jeremy Allison of Google and the Samba Team provided the fix.

== Our Code, Our Bugs, Our Responsibility.
== The Samba Team

2.5 Low

CVSS3

Attack Vector

LOCAL

Attack Complexity

HIGH

Privileges Required

LOW

User Interaction

NONE

Scope

UNCHANGED

Confidentiality Impact

NONE

Integrity Impact

LOW

Availability Impact

NONE

CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:N

1.2 Low

CVSS2

Access Vector

LOCAL

Access Complexity

HIGH

Authentication

NONE

Confidentiality Impact

NONE

Integrity Impact

PARTIAL

Availability Impact

NONE

AV:L/AC:H/Au:N/C:N/I:P/A:N

0.001 Low

EPSS

Percentile

38.7%

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