Bug 1539758 (CVE-2017-18078)

Summary:	CVE-2017-18078 systemd: Unsafe handling of hard links allowing privilege escalation
Product:	[Other] Security Response	Reporter:	Adam Mariš <amaris>
Component:	vulnerability	Assignee:	Red Hat Product Security <security-response-team>
Status:	CLOSED WONTFIX	QA Contact:
Severity:	medium	Docs Contact:
Priority:	medium
Version:	unspecified	CC:	lnykryn, msekleta, ssahani, s, systemd-maint-list, systemd-maint, zbyszek
Target Milestone:	---	Keywords:	Security
Target Release:	---
Hardware:	All
OS:	Linux
Whiteboard:
Fixed In Version:	systemd 237	Doc Type:	If docs needed, set a value
Doc Text:		Story Points:	---
Clone Of:		Environment:
Last Closed:	2018-01-30 00:42:20 UTC	Type:	---
Regression:	---	Mount Type:	---
Documentation:	---	CRM:
Verified Versions:		Category:	---
oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
Embargoed:
Bug Depends On:	1539977
Bug Blocks:	1539764

Description Adam Mariš 2018-01-29 14:47:36 UTC

systemd-tmpfiles in systemd before 237 attempts to support ownership/permission changes on hardlinked files even if the fs.protected_hardlinks sysctl is turned off, which allows local users to bypass intended access restrictions via vectors involving a hard link to a file for which the user lacks write access, as demonstrated by changing the ownership of the /etc/passwd file.

Upstream bug:

https://github.com/systemd/systemd/issues/7736

Upstream patch:

https://github.com/systemd/systemd/commit/5579f85663d10269e7ac7464be6548c99cea4ada

Comment 2 Doran Moppert 2018-01-30 00:37:46 UTC

Created systemd tracking bugs for this issue:

Affects: fedora-all [bug 1539977]

Comment 3 Doran Moppert 2018-01-30 00:41:28 UTC

Statement:

Red Hat Product Security has rated this issue as having Moderate security impact. This issue is not exploitable in the default configuration Red Hat Enterprise Linux. This issue is not currently planned to be addressed in future updates. For additional information, refer to the Issue Severity Classification:
https://access.redhat.com/security/updates/classification/.

Comment 4 Doran Moppert 2018-01-30 00:41:30 UTC

Mitigation:

This flaw, and many others like it, is mitigated by enabling hardlink and symlink protections. These protections are enabled by default in Red Hat Enterprise Linux and this vulnerability will only be exploitable if disabled.

To ensure your system is protected, check that `fs.protected_hardlinks` is enabled as in the following example:

  # sysctl fs.protected_hardlinks
  fs.protected_hardlinks = 1