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GitLab auth uses full name instead of username as user ID, allowing impersonation

High severity GitHub Reviewed Published Aug 11, 2020 in concourse/concourse • Updated May 27, 2025

Package

gomod github.com/concourse/concourse (Go)

Affected versions

= 6.4.0
= 6.3.0
< 0.0.0-20200730151558-b00d1c8d8576

Patched versions

6.4.1
6.3.1
0.0.0-20200730151558-b00d1c8d8576
gomod github.com/concourse/dex (Go)
= 6.4.0
= 6.3.0
< 0.0.0-20200730150203-821b48abfd88
6.4.1
6.3.1
0.0.0-20200730150203-821b48abfd88

Description

Impact

Installations which use the GitLab auth connector are vulnerable to identity spoofing by way of configuring a GitLab account with the same full name as another GitLab user who is granted access to a Concourse team by having their full name listed under users in the team configuration or given to the --gitlab-user flag.

See the GitLab auth docs for details.

Concourse installations which do not configure the GitLab auth connector are not affected.

Patches

Concourse v6.3.1 and v6.4.1 were both released with a fix on August 4th, 2020.

Both versions change the GitLab connector to use the username, rather than the full name. This was always the intent, and the previous behavior was originally reported as a bug (concourse/dex#7) prior to being reported as a security issue.

Any Concourse teams which configure GitLab users will have to switch each user from their full name to their username upon upgrading to these versions.

Workarounds

GitLab groups do not have this vulnerability, so GitLab users may be moved into groups which are then configured in the Concourse team.

References

For more information

If you have any questions or comments about this advisory, you may reach us privately at [email protected].

References

@vito vito published to concourse/concourse Aug 11, 2020
Reviewed May 24, 2021
Published to the GitHub Advisory Database Dec 20, 2021
Last updated May 27, 2025

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
None
Availability
None

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(49th percentile)

Weaknesses

CVE ID

CVE-2020-5415

GHSA ID

GHSA-627p-rr78-99rj

Source code

No known source code

Credits

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