Mercurial > dropbear
comparison cli-session.c @ 1788:1fc0012b9c38
Fix handling of replies to global requests (#112)
The current code assumes that all global requests want / need a reply.
This isn't always true and the request itself indicates if it wants a
reply or not.
It causes a specific problem with [email protected] messages.
These are sent by OpenSSH after authentication to inform the client of
potential other host keys for the host. This can be used to add a new
type of host key or to rotate host keys.
The initial information message from the server is sent as a global
request, but with want_reply set to false. This means that the server
doesn't expect an answer to this message. Instead the client needs to
send a prove request as a reply if it wants to receive proof of
ownership for the host keys.
The bug doesn't cause any current problems with due to how OpenSSH
treats receiving the failure message. It instead treats it as a
keepalive message and further ignores it.
Arguably this is a protocol violation though of Dropbear and it is only
accidental that it doesn't cause a problem with OpenSSH.
The bug was found when adding host keys support to libssh, which is more
strict protocol wise and treats the unexpected failure message an error,
also see https://gitlab.com/libssh/libssh-mirror/-/merge_requests/145
for more information.
The fix here is to honor the want_reply flag in the global request and
to only send a reply if the other side expects a reply.
| author | Dirkjan Bussink <d.bussink@gmail.com> |
|---|---|
| date | Thu, 10 Dec 2020 16:13:13 +0100 |
| parents | 6e71440b1e47 |
| children | 249681d9ecda |
comparison
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| 1787:ce3ce75a6e04 | 1788:1fc0012b9c38 |
|---|---|
| 406 /* Null terminate */ | 406 /* Null terminate */ |
| 407 dirtytext[j] = '\0'; | 407 dirtytext[j] = '\0'; |
| 408 } | 408 } |
| 409 | 409 |
| 410 static void recv_msg_global_request_cli(void) { | 410 static void recv_msg_global_request_cli(void) { |
| 411 TRACE(("recv_msg_global_request_cli")) | 411 unsigned int len = 0; |
| 412 /* Send a proper rejection */ | 412 unsigned int wantreply = 0; |
| 413 send_msg_request_failure(); | 413 |
| 414 len = buf_getint(ses.payload); | |
| 415 buf_incrpos(ses.payload, len); | |
| 416 wantreply = buf_getbool(ses.payload); | |
| 417 | |
| 418 TRACE(("recv_msg_global_request_cli: want_reply: %u", wantreply)); | |
| 419 | |
| 420 if (wantreply) { | |
| 421 /* Send a proper rejection */ | |
| 422 send_msg_request_failure(); | |
| 423 } | |
| 414 } | 424 } |
| 415 | 425 |
| 416 void cli_dropbear_exit(int exitcode, const char* format, va_list param) { | 426 void cli_dropbear_exit(int exitcode, const char* format, va_list param) { |
| 417 char exitmsg[150]; | 427 char exitmsg[150]; |
| 418 char fullmsg[300]; | 428 char fullmsg[300]; |