CVE-2020-28935

NameCVE-2020-28935
DescriptionNLnet Labs Unbound, up to and including version 1.12.0, and NLnet Labs NSD, up to and including version 4.3.3, contain a local vulnerability that would allow for a local symlink attack. When writing the PID file, Unbound and NSD create the file if it is not there, or open an existing file for writing. In case the file was already present, they would follow symlinks if the file happened to be a symlink instead of a regular file. An additional chown of the file would then take place after it was written, making the user Unbound/NSD is supposed to run as the new owner of the file. If an attacker has local access to the user Unbound/NSD runs as, she could create a symlink in place of the PID file pointing to a file that she would like to erase. If then Unbound/NSD is killed and the PID file is not cleared, upon restarting with root privileges, Unbound/NSD will rewrite any file pointed at by the symlink. This is a local vulnerability that could create a Denial of Service of the system Unbound/NSD is running on. It requires an attacker having access to the limited permission user Unbound/NSD runs as and point through the symlink to a critical file on the system.
SourceCVE (at NVD; CERT, LWN, oss-sec, fulldisc, bugtraq, EDB, Metasploit, Red Hat, Ubuntu, Gentoo, SUSE bugzilla/CVE, Mageia, GitHub code/issues, web search, more)
ReferencesDLA-2556-1
NVD severitylow
Debian Bugs977165

Vulnerable and fixed packages

The table below lists information on source packages.

Source PackageReleaseVersionStatus
nsd (PTS)stretch4.1.14-1vulnerable
buster4.1.26-1vulnerable
bullseye4.3.5-1fixed
bookworm, sid4.3.8-1fixed
unbound (PTS)stretch1.6.0-3+deb9u2vulnerable
buster, buster (security)1.9.0-2+deb10u2vulnerable
bookworm, sid, bullseye1.13.1-1fixed
unbound1.9 (PTS)stretch (security)1.9.0-2+deb10u2~deb9u2fixed

The information below is based on the following data on fixed versions.

PackageTypeReleaseFixed VersionUrgencyOriginDebian Bugs
nsdsource(unstable)4.3.4-1
unboundsourcestretch(unfixed)end-of-life
unboundsource(unstable)1.13.0-1977165
unbound1.9sourcestretch1.9.0-2+deb10u2~deb9u1DLA-2556-1

Notes

[buster] - nsd <no-dsa> (Minor issue)
[stretch] - nsd <no-dsa> (Minor issue)
[buster] - unbound <no-dsa> (Minor issue)
[stretch] - unbound <end-of-life> (DSA 4694-1)
https://www.nlnetlabs.nl/downloads/nsd/CVE-2020-28935.txt
https://github.com/NLnetLabs/nsd/commit/a4caec3137a1bc9eca05d38d66e2bce572ca9bd3 (NSD_4_3_4_RC1)
https://github.com/NLnetLabs/unbound/issues/303
Fixed by: https://github.com/NLnetLabs/unbound/commit/ad387832979b6ce4c93f64fe706301cd7d034e87 (release-1.13.0rc1)

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