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Authentication bypass due to improper user-provided security token verification

High
DaneEveritt published GHSA-5vfx-8w6m-h3v4 Oct 2, 2021

Package

pterodactyl/panel (php)

Affected versions

>= 1.0.0 < 1.6.2

Patched versions

1.6.2

Description

A malicious user can modify the contents of a confirmation_token input during the two-factor authentication process to reference a cache value not associated with the login attempt. In rare cases this can allow a malicious actor to authenticate as a random user in the Panel. The malicious user must target an account with two-factor authentication enabled, and then must provide a correct two-factor authentication token before being authenticated as that user.

Impact

Due to a validation flaw in the logic handling user authentication during the two-factor authentication process a malicious user can trick the system into loading credentials for an arbitrary user by modifying the token sent to the server. This authentication flaw is present in the LoginCheckpointController@__invoke method which handles two-factor authentication for a user.

This controller looks for a request input parameter called confirmation_token which is expected to be a 64 character random alpha-numeric string that references a value within the Panel's cache containing a user_id value. This value is then used to fetch the user that attempted to login, and lookup their two-factor authentication token. Due to the design of this system, any element in the cache that contains only digits could be referenced by a malicious user, and whatever value is stored at that position would be used as the user_id.

There are a few different areas of the Panel that store values into the cache that are integers, and a user who determines what those cache keys are could pass one of those keys which would cause this code pathway to reference an arbitrary user.

Scope

At its heart this is a high-risk login bypass vulnerability. However, there are a few additional conditions that must be met in order for this to be successfully executed, notably:

1.) The account referenced by the malicious cache key must have two-factor authentication enabled. An account without two-factor authentication would cause an exception to be triggered by the authentication logic, thusly exiting this authentication flow.
2.) Even if the malicious user is able to reference a valid cache key that references a valid user account with two-factor authentication, they must provide a valid two-factor authentication token.

However, due to the design of this endpoint once a valid user account is found with two-factor authentication enabled there is no rate-limiting present, thusly allowing an attacker to brute force combinations until successful. This leads to a third condition that must be met:

3.) For the duration of this attack sequence the cache key being referenced must continue to exist with a valid user_id value. Depending on the specific key being used for this attack, this value may disappear quickly, or be changed by other random user interactions on the Panel, outside the control of the attacker.

About the Severity

As you may have noticed, this is not a trivial authentication bypass bug to exploit, and is likely incredibly difficult for a layperson to pull off. However, the severity of this disclosure has been prepared based on the nature of the bug and the potential for unexpected administrative account access under very rare conditions.

Mitigation

In order to mitigate this vulnerability the underlying authentication logic was changed to use an encrypted session store that the user is therefore unable to control the value of. This completely removed the use of a user-controlled value being used. In addition, the code was audited to ensure this type of vulnerability is not present elsewhere.

If you have any questions or concerns about the content of this disclosure please contact Tactical Fish#8008 on Discord, or email dane ät pterodactyl.io.

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
High
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

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:H/PR:N/UI:N/S:U/C:H/I:H/A:H

CVE ID

CVE-2021-41129