Why does Google call Thunderbird “less secure”

emailgmailimapSecuritythunderbird

I've never had problems using Gmail with Thunderbird yet, but while trying to use a free software client for Google Talk/Chat/Hangout I've discovered that, according to Google's document on "less secure apps":

Some examples of apps that do not support the latest security
standards include […] Desktop mail clients like Microsoft Outlook
and Mozilla Thunderbird.

Google then offers an all-or-nothing secure vs. non secure account switch ("Allow less secure apps").

Why does Google say Thunderbird "does not support the latest security standards"? Is Google trying to say that standard protocols like IMAP, SMTP and POP3 are "less secure" ways to access a mailbox? Are they trying to say that the use users make of that software puts their accounts at risk? Or what?

Secunia's Vulnerability Report: Mozilla Thunderbird 24.x (where is 31?) says «Unpatched 11% (1 of 9 Secunia advisories) […] The most severe unpatched Secunia advisory affecting Mozilla Thunderbird 24.x, with all vendor patches applied, is rated Highly critical», apparently SA59803.

Update 2: as of 2018, Google double downs by sending messages to invite disabling "less secure" access:

Google notification

Update: OAuth2 is available in Thunderbird 38, with further fixes in later releases, and bug 849540 has been closed. I'm still not clear about the goals of all this circus.
Italian Thunderbird 38.1.0 SMTP server screenshot

Best Answer

It's because those clients (currently) don't support OAuth 2.0.

...beginning in the second half of 2014, we'll start gradually increasing the security checks performed when users log in to Google. These additional checks will ensure that only the intended user has access to their account, whether through a browser, device or application. These changes will affect any application that sends a username and/or password to Google.

To better protect your users, we recommend you upgrade all of your applications to OAuth 2.0. If you choose not to do so, your users will be required to take extra steps in order to keep accessing your applications.

...

In summary, if your application currently uses plain passwords to authenticate to Google, we strongly encourage you to minimize user disruption by switching to OAuth 2.0.

Source: "New Security Measures Will Affect Older (non-OAuth 2.0) Applications" - Google Online Security Blog

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