Comment 0 for bug 1088136

smtp_cmd_buffer_size is currently 2048 bytes. 2048 bytes is not sufficient for
clients that send an AUTH with an initial-response for GSSAPI when Windows
Kerberos tickets are used that contain a PAC -- as of Windows 2003, the maximum
ticket size is 12000 bytes.

MUAs that use AUTH GSSAPI without an initial-response are not impacted by the
2048 limit, since the remainder of the SASL session is handled by auth_get_data
in Exim, which uses big_buffer and has sufficient space to process large
Kerberos tickets.

Thunderbird will always send an AUTH GSSAPI with an initial-response, which
makes it subject to the 2048 byte limit. A large Kerberos ticket will easily
surpass 2048 bytes when base64-encoded, causing the AUTH to fail.

RFC 4954 recommends 12288 bytes as a line limit to handle AUTH. For a base64
encoded max-size Windows Kerberos ticket, at least 16000 bytes are needed.

This bug is fixed upstream (4.77).

[Impact]
smtp_cmd_buffer_size is currently 2048 bytes. 2048 bytes is not sufficient for
clients that send an AUTH with an initial-response for GSSAPI when Windows
Kerberos tickets are used that contain a PAC. For a base64
encoded max-size Windows Kerberos ticket, at least 16000 bytes are needed.

[Test Case]
1. Configure exim4 to use GSSAPI auth.
2. Configure thunderbird to use GSSAPI smtp auth on windows xp/vista/7/2003/2008.
3. Auth will always fail.

[Regression Potential]
The fix for this bug is one-line-patch applied to upstream (4.77) more than year ago, so it already has got sufficient testing.