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Vulnerability · Published Dec 7, 2020 · Assigned by NLnet Labs

CVE-2020-28935Local symlink attack in Unbound and NSD

Medium 5.5

In plain English

A medium-severity security flaw in NSD from NLnet Labs. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.5% (higher than 43% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS NVD)5.5Medium
Exploit odds (30 days)0.5%top 57.4% of all CVEs
Exploited in the wildNo reportsnot in CISA KEV
Attack toolingNoneno public tooling known

Official description

NLnet 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.

Affected packages & fixed versions

EcosystemPackageFixed inAdvisory
Alpine:v3.20nsd4.3.4-r0ALPINE-CVE-2020-28935
Alpine:v3.21nsd4.3.4-r0ALPINE-CVE-2020-28935
Alpine:v3.22nsd4.3.4-r0ALPINE-CVE-2020-28935
Alpine:v3.23nsd4.3.4-r0ALPINE-CVE-2020-28935
Alpine:v3.24nsd4.3.4-r0ALPINE-CVE-2020-28935
Debian:12nsd4.3.4-1DEBIAN-CVE-2020-28935
Debian:13nsd4.3.4-1DEBIAN-CVE-2020-28935
Debian:14nsd4.3.4-1DEBIAN-CVE-2020-28935
Debian:12unbound1.13.0-1DEBIAN-CVE-2020-28935
Debian:13unbound1.13.0-1DEBIAN-CVE-2020-28935
Debian:14unbound1.13.0-1DEBIAN-CVE-2020-28935
Rocky Linux:8unbound0:1.7.3-15.el8RLSA-2021:1853

Affected products (as reported)

VendorProductVersions
NLnet Labs Unbound <= 1.12.0
NLnet Labs NSD <= 4.3.3

Weakness: CWE-59

References