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CVE-2021-23240

Published: 12 January 2021

selinux_edit_copy_tfiles in sudoedit in Sudo before 1.9.5 allows a local unprivileged user to gain file ownership and escalate privileges by replacing a temporary file with a symlink to an arbitrary file target. This affects SELinux RBAC support in permissive mode. Machines without SELinux are not vulnerable.

Notes

AuthorNote
sbeattie
selinux is not the default MAC in Ubuntu, though users can
boot into it.
rodrigo-zaiden
fs.protected_symlinks is '1' by default in Ubuntu,
and if not changed, can prevent this issue from being exploited.

There are backports available for version
1.8 but not straightforward for 1.8.16 (xenial baseline). Some of
the selinux specific code that needs to be patched was added later
in time and it seems to me that the backports for 1.8 are based on
1.8.32. So, in xenial, a fix would be very intrusive and likely
to introduce a regression.
The reproducer in sudo.ws reference does not reproduce
in xenial.
The fact that the reproducer does not reproduce, that
the kernel hardening fs.protected_symlinks is '1' by default (with
no clear reason to be turned off to '0'), that there is a very low
possibility to have Ubuntu running selinux without a working policy
and, mainly, that the risky of regression is high, we are marking
this CVE as ignored for xenial.

Priority

Negligible

Cvss 3 Severity Score

7.8

Score breakdown

Status

Package Release Status
sudo
Launchpad, Ubuntu, Debian
bionic Needs triage

focal Needs triage

groovy Ignored
(end of life)
hirsute Ignored
(end of life)
impish Not vulnerable
(1.9.5p2-3ubuntu1)
jammy Not vulnerable
(1.9.9-1ubuntu2)
kinetic Not vulnerable
(1.9.9-1ubuntu2)
lunar Not vulnerable
(1.9.9-1ubuntu2)
mantic Not vulnerable
(1.9.9-1ubuntu2)
trusty Needs triage

upstream
Released (1.9.5)
xenial Ignored

Patches:
upstream: https://www.sudo.ws/repos/sudo/rev/8fcb36ef422a
upstream: https://www.sudo.ws/repos/sudo/rev/b5d5bd506487
upstream: https://www.sudo.ws/repos/sudo/rev/593ce9fdf5dd
upstream: https://www.sudo.ws/repos/sudo/rev/645752e64b72

Severity score breakdown

Parameter Value
Base score 7.8
Attack vector Local
Attack complexity Low
Privileges required Low
User interaction None
Scope Unchanged
Confidentiality High
Integrity impact High
Availability impact High
Vector CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H