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AUSCERT External Security Bulletin Redistribution
Denial of service and possible privilege escallation in filemon
18 December 2019
AusCERT Security Bulletin Summary
Operating System: NetBSD
Impact/Access: Create Arbitrary Files -- Existing Account
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NetBSD Security Advisory 2019-006
Topic: Denial of service and possible privilege escallation in filemon
Version: NetBSD-current: affected up to 9.99.17
NetBSD 8.1: affected
NetBSD 7.x: unaffected
Severity: Local users can crash the machine
Fixed: NetBSD-current: October 28, 2019
NetBSD-9 branch: October 28, 2019
NetBSD-8 branch: October 28, 2019
Please note that NetBSD releases prior to 7.1 are no longer supported.
It is recommended that all users upgrade to a supported release.
An unprivileged user can write filemon output to arbitrary files,
or crash the system.
The filemon module is experimental and not suitable for general
use. Unfortunately it can be inadvertently auto-loaded when
/dev/filemon is opened, and /dev/filemon is accessible to any user.
There was a missing write check in the filemon module thus permitting
any user to overwrite any file in the system. While we are not
currently aware of an exploit, it is conceivable that one can
overwrite a configuration file parsed by a privileged daemon that
does not abort on syntax errors.
Additionally the way filemon does filesystem interception is racy
and can lead to random crashes if the system calls are in use
while the module is unloaded.
Solutions and Workarounds
Unloading and removing the filemon module, either manually:
# modunload filemon
# rm -rf /stand/*/*/modules/filemon/
Or by updating the machine to a newer build and running
postinstall fix obsolete
Ilja Van Sprundel for reporting this vulnerability.
Advisories may be updated as new information becomes available.
The most recent version of this advisory (PGP signed) can be found at
Information about NetBSD and NetBSD security can be found at
Copyright 2019, The NetBSD Foundation, Inc. All Rights Reserved.
Redistribution permitted only in full, unmodified form.
$NetBSD: NetBSD-SA2019-006.txt.asc,v 1.1 2019/12/17 00:55:08 christos Exp $
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