Executive Summary

[{'type': 'paragraph', 'content': "CVE-2026-33409 is an authentication bypass vulnerability in Parse Server that occurs when the server option allowExpiredAuthDataToken is set to true. This vulnerability allows an attacker to log in as any user who has linked a third-party authentication provider without knowing the user's credentials."}, {'type': 'paragraph', 'content': "The attacker only needs to know the user's provider ID and can submit partial authentication data (authData), such as just the provider ID without a valid access token, to bypass proper validation during login."}, {'type': 'paragraph', 'content': 'This happens because the server previously allowed login attempts with incomplete authData when allowExpiredAuthDataToken was enabled, skipping full validation and enabling unauthorized access.'}, {'type': 'paragraph', 'content': 'The vulnerability has been fixed by enforcing full validation of all authData on every login attempt regardless of the allowExpiredAuthDataToken setting, preventing attackers from bypassing authentication with partial data.'}] [1, 2, 3, 4, 5]

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Impact Analysis

[{'type': 'paragraph', 'content': "This vulnerability can allow an attacker to gain full access to any user account linked to a third-party authentication provider without knowing the user's credentials."}, {'type': 'paragraph', 'content': 'By exploiting the bypass, the attacker can obtain a valid session token and fully impersonate the victim user.'}, {'type': 'paragraph', 'content': 'This leads to unauthorized access, potential data theft, unauthorized modifications, and loss of account integrity.'}, {'type': 'paragraph', 'content': 'The impact is significant because it compromises confidentiality and integrity of user accounts on affected Parse Server deployments where allowExpiredAuthDataToken is enabled.'}] [1, 2, 4, 5]

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Compliance Impact

I don't know

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Detection Guidance

This vulnerability involves an authentication bypass in Parse Server when the configuration option allowExpiredAuthDataToken is set to true, allowing login with partial authentication data. Detection involves monitoring login attempts that use partial authData, such as only a provider ID without a valid access token.

To detect exploitation attempts, you can analyze server logs for login requests containing incomplete authData payloads or unusual login patterns where only provider IDs are submitted without full credentials.

Specific commands are not provided in the available resources, but general approaches include:

• Inspect Parse Server logs for POST /login requests with authData fields missing expected tokens.
• Use network monitoring tools to capture and analyze authentication traffic for partial authData submissions.
• Implement custom logging or alerting in Parse Server to flag login attempts where authData is a strict subset of stored data.

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Mitigation Strategies

To mitigate this vulnerability immediately, ensure that the Parse Server configuration option allowExpiredAuthDataToken is set to false or removed entirely, as the default is false and enabling it introduces the risk.

Upgrade Parse Server to version 8.6.52, 9.6.0-alpha.41, or later, where the vulnerability has been patched by enforcing full validation of authentication data on every login attempt.

Avoid using partial authData for login and ensure that all authentication providers validate the entire authData payload during login.

• Set allowExpiredAuthDataToken to false or remove it from your server configuration.
• Update Parse Server to version 8.6.52, 9.6.0-alpha.41, or newer.
• Review and monitor authentication logs for suspicious login attempts.

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