Link following in Unbound



Published: 2020-12-07 | Updated: 2020-12-09
Risk Low
Patch available YES
Number of vulnerabilities 1
CVE-ID CVE-2020-28935
CWE-ID CWE-59
Exploitation vector Local
Public exploit N/A
Vulnerable software
Subscribe
Unbound
Server applications / DNS servers

Vendor NLnet Labs

Security Bulletin

This security bulletin contains one low risk vulnerability.

1) Link following

EUVDB-ID: #VU48897

Risk: Low

CVSSv3.1: 4.8 [CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H/E:U/RL:O/RC:C]

CVE-ID: CVE-2020-28935

CWE-ID: CWE-59 - Improper Link Resolution Before File Access ('Link Following')

Exploit availability: No

Description

The vulnerability allows a local authenticated user to perform a denial of service (DoS) attack.

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.

Mitigation

Install update from vendor's website.

Vulnerable software versions

Unbound: 1.0.0 - 1.12.0


CPE2.3 External links

http://www.nlnetlabs.nl/downloads/nsd/CVE-2020-28935.txt
http://www.nlnetlabs.nl/downloads/unbound/CVE-2020-28935.txt

Q & A

Can this vulnerability be exploited remotely?

No. This vulnerability can be exploited locally. The attacker should have authentication credentials and successfully authenticate on the system.

How the attacker can exploit this vulnerability?

The attacker would have to send a specially crafted request to the affected application in order to exploit this vulnerability.

The attacker would have to login to the system and perform certain actions in order to exploit this vulnerability.

Is there known malware, which exploits this vulnerability?

No. We are not aware of malware exploiting this vulnerability.



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