CVE-2026-75866 in Punk::OAuth2::Server
Summary
by MITRE • 08/22/2026
Punk::OAuth2::Server versions through 0.03 for Perl issue access tokens outside a client's registered scopes and grant types because no authorization path reads them.
Punk::OAuth2::Server::Store registers scopes and grant_types per client and documents both as client registration. token dispatches on the grant_type in the request body, so a client registered for authorization_code alone can ask for client_credentials, and that arm passes the requested scope straight to the minter, which signs it into the at+jwt access token. authorize copies the query scope into the authorization code record without comparing it against the registration, leaving the optional consent hook as the only check between an arbitrary scope and the issued code. redirect_uris on the same client row is read and enforced.
A registered client can obtain a correctly signed token carrying any scope it names, and a resource server running Punk::OAuth2::Checker accepts that token and honours the scope. A client registered without a secret authenticates on its client_id alone, so anyone who knows that identifier can request one.
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Analysis
by VulDB Data Team • 08/22/2026
The vulnerability in Punk::OAuth2::Server versions through 0.03 represents a critical failure in access control logic within an OAuth 2.0 authorization framework implemented for Perl applications. The core issue stems from the server's inability to enforce scope and grant type restrictions during token issuance, allowing clients to obtain access tokens with privileges far exceeding their registered permissions. This flaw fundamentally undermines the principle of least privilege that is central to secure identity management systems. By failing to validate requested scopes against client registration data or restricting grant types to those explicitly permitted for a specific client, the system permits unauthorized escalation of user and application privileges.
The technical mechanism of this vulnerability lies in how token dispatching handles incoming requests. When a client initiates a token request, the server parses the grant_type from the request body but does not verify whether that grant type is actually registered or allowed for the requesting client identifier. For instance, if a client is strictly configured to use only the authorization_code flow, it can still submit a request specifying client_credentials as the grant type. The system proceeds with this mismatched operation without raising an error or rejecting the request. Furthermore, when processing scopes, the server copies the scope parameter from the query string directly into the authorization code record and subsequently into the access token minting process. There is no intermediate step where these requested scopes are compared against the client's registered scope list. The only potential safeguard mentioned is an optional consent hook, which may not be enabled or properly configured in many deployments, leaving a significant gap in security enforcement.
This architectural oversight has severe operational implications for resource servers that rely on Punk::OAuth2::Checker to validate incoming tokens. Because the access token is correctly signed and structurally valid according to JWT standards, downstream services accept it as authentic. The checker validates the signature and expiration but does not re-evaluate whether the scopes contained within the token were legitimately issued based on client registration policies. Consequently, an attacker or a misconfigured application can present a token with arbitrary administrative or sensitive data access rights that were never intended for that specific client identity. This leads to potential unauthorized access to protected resources, data exfiltration, and complete compromise of system integrity depending on the sensitivity of the exposed scopes.
The risk is further exacerbated by the authentication mechanism used for certain clients. The framework allows clients registered without a secret to authenticate using only their client_id. Since client identifiers are often public or easily discoverable information, any entity that knows a valid client identifier can initiate token requests. Combined with the lack of scope and grant type validation, this means an attacker does not need to compromise secrets or intercept authorization codes to exploit this flaw. They simply need to know a legitimate client ID and then request tokens with elevated scopes using unauthorized grant types. This effectively turns public clients into vectors for privilege escalation without any additional credentials required.
To mitigate these risks, immediate updates to the Punk::OAuth2::Server software are necessary if patches have been released by the vendor. In the interim, administrators should enforce strict validation logic at the application layer before token issuance occurs. Developers must ensure that every request checks both the grant_type and scope parameters against a whitelist defined in the client registration database. It is crucial to reject any request where the requested grant type does not match the allowed types for that specific client_id. Additionally, scopes should be intersected with the registered scopes of the client, ensuring that only permitted permissions are included in issued tokens. Enabling and properly configuring consent hooks can provide an additional layer of defense by requiring explicit user approval for scope changes, though this alone is insufficient without server-side validation.
From a classification perspective, this vulnerability aligns closely with CWE-269 Improper Privilege Management, as the system fails to restrict users or programs from elevating privileges beyond their assigned roles. It also relates to CWE-862 Missing Authorization, since the application does not properly enforce access control policies on protected resources and functions during token generation. In terms of attack patterns, this behavior mirrors techniques found in MITRE ATT&CK under privilege escalation vectors where attackers exploit misconfigured identity providers or API gateways to gain unauthorized access. The specific failure to validate grant types against client configurations is a common pitfall in OAuth implementations that can lead to significant security breaches if not addressed through rigorous input validation and policy enforcement at the authorization server level.