CVE-2020-28935
Local symlink attack in Unbound and NSD
- Published
- Updated
- Reserved
- Assigned by
- NLnet Labs
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.
Timeline
ID reserved
By NLnet Labs
- 19 days later
· 19 days later
Published
- 4 years later
· 4 years later
Record updated
Changes after publication (a new score, a public exploit, EPSS crossing 1, 10 or 50 %, leaving KEV) are kept since November 2026; earlier ones are not known.
Scores
| Scored by | CVSS | Score | Vector |
|---|---|---|---|
| NVDNVD · Modified | 3.1 | Medium5.5 | CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H |
- EPSS (exploitation in 30 days)
- 0.49% · top 57% of all CVEs (7 Nov 2026)
The list uses the assigner's score when there is one, then CISA's, then NVD's: Medium 5.5.
Weakness
Affected products
- affected <= 1.12.0
- affected <= 4.3.3
References (5)
- https://www.nlnetlabs.nl/downloads/unbound/CVE-2020-28935.txt (opens in a new tab)x_transferred
- https://www.nlnetlabs.nl/downloads/nsd/CVE-2020-28935.txt (opens in a new tab)x_transferred
- GLSA-202101-38 (opens in a new tab)vendor-advisoryx_transferred
- [debian-lts-announce] 20210212 [SECURITY] [DLA 2556-1] unbound1.9 security update (opens in a new tab)mailing-listx_transferred
- [debian-lts-announce] 20230329 [SECURITY] [DLA 3371-1] unbound security update (opens in a new tab)mailing-listx_transferred
Sources
The same CVE in the primary databases and aggregators, to check against.
- CVE.org record (opens in a new tab)
- CVE List JSON (GitHub) (opens in a new tab)
- NVD (opens in a new tab)
- FIRST EPSS (opens in a new tab)
- OSV.dev (opens in a new tab)
- GitHub Advisories (opens in a new tab)
- CIRCL Vulnerability-Lookup (opens in a new tab)
Badge for a README or a ticket
[](https://trackcve.app/cve/CVE-2020-28935)Record last updated .