Executive Summary

[{'type': 'paragraph', 'content': 'CVE-2026-27804 is a critical security vulnerability in the Parse Server\'s Google authentication adapter. The issue arises because the server previously trusted the algorithm specified in the JWT (JSON Web Token) header during verification. An attacker can exploit this by forging a Google authentication token with the algorithm set to "none", which bypasses signature verification. This allows the attacker to log in as any user linked to a Google account without knowing their credentials.'}, {'type': 'paragraph', 'content': "The vulnerability is caused by algorithm confusion, where the server accepts tokens with manipulated or missing signatures due to trusting the JWT header's algorithm field. The fix hardcodes the expected JWT algorithm to RS256 during token verification, ignoring the algorithm declared in the token header, and replaces the custom key fetcher with a more secure one that rejects unknown key IDs."}] [1, 3, 4]

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Impact Analysis

This vulnerability can have severe impacts because it allows an unauthenticated attacker to perform a full account takeover by forging authentication tokens. The attacker can log in as any user linked to a Google account without needing their credentials, leading to unauthorized access and potential modification of user data.

The attack can be performed remotely over the network with low complexity and no user interaction required, making it highly exploitable. The impacts include high confidentiality and integrity breaches, as attackers gain unauthorized access to user accounts and can potentially manipulate data.

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Compliance Impact

I don't know

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Detection Guidance

[{'type': 'paragraph', 'content': 'This vulnerability involves the acceptance of forged JWT tokens with the algorithm field set to "none" in the Google authentication adapter of Parse Server. Detection involves identifying attempts to authenticate using JWT tokens that specify an insecure or no algorithm (e.g., "alg:none").'}, {'type': 'paragraph', 'content': 'To detect exploitation attempts on your system or network, you can monitor authentication logs for JWT tokens with the "alg:none" header or tokens that fail signature verification but are still accepted.'}, {'type': 'paragraph', 'content': 'Since the vulnerability is related to JWT token verification, you can use commands or scripts to decode JWT tokens and inspect their headers for the "alg" field set to "none".'}, {'type': 'list_item', 'content': 'Use a command-line tool like `jq` and `base64` to decode JWT headers from logs or intercepted tokens: `echo \'<jwt_token>\' | cut -d \'.\' -f1 | base64 -d | jq \'.\'` and check the "alg" field.'}, {'type': 'list_item', 'content': 'Search your server logs for tokens containing "alg":"none" using grep: `grep -r \'"alg":"none"\' /path/to/logs`.'}, {'type': 'list_item', 'content': 'Implement or review unit tests similar to those added in the Parse Server codebase that reject tokens with "alg:none" to verify your system\'s JWT verification behavior.'}] [1, 3]

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Mitigation Strategies

The primary mitigation is to upgrade Parse Server to version 8.6.3 or later (including 9.1.1-alpha.4 and above), where the vulnerability is fixed by hardcoding the JWT verification algorithm to RS256 and replacing the custom key fetcher with a secure implementation.

If upgrading immediately is not possible, the recommended workaround is to disable Google authentication in your Parse Server deployment until the patch can be applied.

Ensure that your JWT verification process does not trust the algorithm specified in the JWT header and enforces the use of RS256 algorithm explicitly.

• Upgrade Parse Server to version 8.6.3 or 9.1.1-alpha.4 or later.
• Temporarily disable Google authentication if upgrading is not feasible immediately.
• Review and apply patches that hardcode the JWT algorithm to RS256 and use the jwks-rsa library for key fetching.

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