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Duo Product Security Advisory

Advisory ID: DUO-PSA-2014-003
Publication Date: 2014-01-27
Status: Confirmed, Fixed
Document Revision: 2

Overview

Duo Security has identified an issue in which it is possible to bypass second factor authentication of Remote Desktop Protocol (RDP) integrations which are GINA-based (e.g. those installed on Windows XP/Server 2003).

Description

The RDP integration works to add two-factor authentication to Windows logins, both Remote Desktop and (optionally) the local console. When the user has provided valid credentials (username and password), the integration presents a second-factor authentication dialog. Under normal operation, when the second-factor authentication is successful, the user is logged on.

However, under certain circumstances (e.g. race conditions involving the user pressing "Ctrl-Alt-Delete" to interrupt a transitory dialog), the GINA framework would return an undocumented error, which was treated as a successful authentication.

Additionally, it was also identified that under these conditions, a thread used by the Duo integration would enter an undefined state. This could potentially lead to a crash in the Winlogon process, resulting in a Denial of Service (“blue screen”) of the host.

Impact

A user with valid primary authentication credentials (username and password) may be able to bypass the second factor of authentication.

Affected Product(s)

Duo RDP Integration (GINA-based) 1.1.3 and below running on Windows XP/Server 2003

Workaround

There is currently no workaround available for this issue.

Solution

Duo Security is currently working on an update to these integrations at the end of March 2014 that will cover these new transport mechanisms, to ensure two-factor authentication is enforced on all logins.

Vulnerability Metrics

Vulnerability Class: Authentication Bypass Issue (CWE-592), Unchecked Error Condition (CWE-391)
Remotely Exploitable: Yes
Authentication Required: Yes/Partial (first factor required; second factor bypassed)
Severity: High
CVSSv2 Overall Score: 7.2
CVSSv2 Group Scores: Base: 4.9, Temporal: 4.7, Environmental: 7.2
CVSSv2 Vector: AV:N/AC:M/Au:S/C:P/I:P/A:N/E:H/RL:W/RC:C/CDP:LM/TD:ND/CR:H/IR:H/AR:ND

References

CWE-592: Authentication Bypass Issues - https://cwe.mitre.org/data/definitions/592.html
CWE-391: Unchecked Error Condition - https://cwe.mitre.org/data/definitions/391.html

Timeline

2014-01-23

Customer reported possible RDS 2FA bypass to Duo Security
Duo Security acknowledges receipt of report, begins investigation
Duo Security confirms issue, continues investigation
Duo Security creates fix for issue in affected product

2014-01-24

Fixed GINA integration is released

2014-01-27

Advisory is created, shared with affected Duo Security Business and Enterprise customers

2014-02-04

Advisory is shared with affected Duo Security Personal customers

Credits/Contact

Feedback regarding this issue should be sent to security@duosecurity.com and reference "DUO-PSA-2014-003" in the subject.