CVE-2020-13776
Description
A flaw was found in systemd, where it mishandles numerical usernames beginning with decimal digits, or "0x" followed by hexadecimal digits. When the usernames are used by systemd, for example in service units, an unexpected user may be used instead. In some particular configurations, this flaw allows local attackers to elevate their privileges.
Statement
The flaw is rated as Moderate because several uncommon conditions have to be met to make it exploitable. Numerical usernames with decimal digits or starting with "0x" followed by hexadecimal digits must exist on the system. Systemd would need to process those particular usernames (e.g. while using the User= directive in a systemd service unit). If the service was supposed to run as a regular user and the binary being executed can be controlled by a local attacker, he could abuse this flaw to unexpectedly execute code as a root when the service is started. If the service was run as a regular user to limit the impact of a possible flaw in the service, this flaw would not provide the intended additional protection.
Mitigation
Do not use `User=` directive in services with numerical usernames composed by decimal digits or starting with "0x" followed by hexadecimal digits (e.g. 0x[0-9A-Fa-f]+).
Common Vulnerability Scoring System (CVSS) Score Details
Info alert:Important note
CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 6.7 | 6.7 | N/A |
| Attack Vector | Local | Local | N/A |
| Attack Complexity | High | High | N/A |
| Privileges Required | Low | Low | N/A |
| User Interaction | Required | Required | N/A |
| Scope | Unchanged | Unchanged | N/A |
| Confidentiality | High | High | N/A |
| Integrity Impact | High | High | N/A |
| Availability Impact | High | High | N/A |
Vector
Red Hat: CVSS:3.1/AV:L/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H
NVD: CVSS:3.1/AV:L/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H
Understanding the Weakness (CWE)
Other
Technical Impact: Quality Degradation; Varies by Context
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