XOAUTH2 submission - "500 5.5.2 Line too long"

[Rosario Esposito]

Hello,

I setup a dovecot submission server, using xoauth2 authentication.
My roundcube webmail points to dovecot submission.
In roundcube smtp logs, I see:


[08-Mar-2022 15:47:16 +0100]: <qjonb57c> Recv: 250-PIPELINING
[08-Mar-2022 15:47:16 +0100]: <qjonb57c> Recv: 250 XCLIENT ADDR PORT 
PROTO HELO LOGIN SESSION TTL TIMEOUT FORWARD
[08-Mar-2022 15:47:16 +0100]: <qjonb57c> Send: AUTH XOAUTH2 
dXNlcj1yZXNwb3NpdAFhdXRoPWJlYXJlci<<<...*very long base64 string*....>>>
[08-Mar-2022 15:47:16 +0100]: <qjonb57c> Recv: 500 5.5.2 Line too long
[08-Mar-2022 15:47:16 +0100]: <qjonb57c> Send: ****** [4]
[08-Mar-2022 15:47:16 +0100]: <qjonb57c> Recv: 221 2.0.0 Bye


And, in dovecot logs I found:

Mar  8 15:47:17 ### dovecot[97376]: submission-login: Debug: 
smtp-server: conn 172.18.6.100:38658 [1]: command EHLO: 250 reply: Sent: 
250-###.<#####> 8BITMIME AUTH PLAIN OAUTHBEARER XOAUTH2 BURL imap 
CHUNKING ENHANCEDSTATUSCODES SIZE PIPELINING XCLIENT ADDR PORT PROTO 
HELO LOGIN SESSION TTL TIMEOUT FORWARD
Mar  8 15:47:17 ### dovecot[97376]: submission-login: Debug: 
smtp-server: conn 172.18.6.100:38658 [1]: command EHLO: Finished
Mar  8 15:47:17 ### dovecot[97376]: submission-login: Debug: 
smtp-server: conn 172.18.6.100:38658 [1]: command EHLO: Destroy
Mar  8 15:47:17 ### dovecot[97376]: submission-login: Debug: 
smtp-server: conn 172.18.6.100:38658 [1]: command EHLO: 250 reply: Destroy
Mar  8 15:47:17 ### dovecot[97376]: submission-login: Debug: 
smtp-server: conn 172.18.6.100:38658 [1]: Trigger output
Mar  8 15:47:17 ### dovecot[97376]: submission-login: Debug: 
smtp-server: conn 172.18.6.100:38658 [1]: No more commands pending
Mar  8 15:47:17 ### dovecot[97376]: submission-login: Debug: 
smtp-server: conn 172.18.6.100:38658 [1]: Sending replies
Mar  8 15:47:17 ### dovecot[97376]: submission-login: Debug: 
smtp-server: conn 172.18.6.100:38658 [1]: No more commands pending
Mar  8 15:47:17 ### dovecot[97376]: submission-login: Debug: 
smtp-server: conn 172.18.6.100:38658 [1]: Client sent invalid command: 
Command line is too long

Apparently, the problem came out after upgrading from dovecot 2.3.17 to 
2.3.18

Any ideas ?