Bug 2517237 (CVE-2026-72887)

Summary: CVE-2026-72887 Net-OAuth: Net::OAuth::Client: Session fixation via OAuth 1.0a downgrade
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
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Priority: high    
Version: unspecifiedKeywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in Net::OAuth::Client. A malicious service provider can silently downgrade an OAuth 1.0a connection to OAuth 1.0 by omitting the `oauth_callback_confirmed` parameter in the request token response. This downgrade vulnerability removes the `oauth_verifier` binding, making the application vulnerable to session fixation. An attacker can then obtain a request token, have the victim authorize it, and complete the exchange, linking the victim's provider account to a session the attacker controls.
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Bug Depends On: 2545812, 2545811    
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Description OSIDB Bzimport 2026-08-16 14:01:21 UTC 
Net::OAuth::Client versions before 0.32 for Perl allow the service provider to silently downgrade OAuth 1.0a to OAuth 1.0 in get_request_token.

Passing a callback to the constructor selects OAuth 1.0a. get_request_token then revokes that choice when the request token response omits oauth_callback_confirmed, with no exception, no warning and no option to require 1.0a. The access token request is built from the OAuth 1.0 message class, which has no verifier parameter, so oauth_verifier is dropped from the request even when get_access_token was passed one.

oauth_verifier is the binding that OAuth 1.0a added between the authorization step and the token exchange. An application that asked for 1.0a and gets 1.0 is open to OAuth 1.0 session fixation, where an attacker obtains a request token, has the victim authorize it, and then completes the exchange themselves, linking the victim's provider account to a session the attacker controls. No attacker action sets up the downgrade: a provider that does not confirm the callback is enough.