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| 2015-04-10 | 2015-04-10 | ||
| log record associated with Requires padding on SSH-2 RSA signatures (martin) | limit acronym recognition (martin) | ||
| Line 9: | Line 9: | ||
| Not all SSH servers work properly. Various existing servers have bugs in them, which can make it impossible for a client to talk to them unless it knows about the bug and works around it. | Not all SSH servers work properly. Various existing servers have bugs in them, which can make it impossible for a client to talk to them unless it knows about the bug and works around it. | ||
| - | Since most servers announce their software version number at the beginning of the SSH connection, WinSCP will attempt to detect which bugs it can expect to see in the server and automatically enable workarounds. However, sometimes it will make mistakes; if the server has been deliberately configured to conceal its version number, or if the server is a version which WinSCP's bug database does not know | + | Since most servers announce their software version number at the beginning of the %%SSH%% connection, WinSCP will attempt to detect which bugs it can expect to see in the server and automatically enable workarounds. However, sometimes it will make mistakes; if the server has been deliberately configured to conceal its version number, or if the server is a version which WinSCP's bug database does not know |
| about, then WinSCP will not know what bugs to expect. | about, then WinSCP will not know what bugs to expect. | ||
| Line 21: | Line 21: | ||
| If this bug is detected, WinSCP will stop using ignore messages. This means that keepalives will stop working, and WinSCP will have to fall back to a secondary defense against [[#refuses_all_ssh-1_password_camouflage|SSH-1 password-length eavesdropping]]. If this bug is enabled when talking to a correct server, the session will succeed, but keepalives will not work and the session might be more vulnerable to eavesdroppers than it could be. | If this bug is detected, WinSCP will stop using ignore messages. This means that keepalives will stop working, and WinSCP will have to fall back to a secondary defense against [[#refuses_all_ssh-1_password_camouflage|SSH-1 password-length eavesdropping]]. If this bug is enabled when talking to a correct server, the session will succeed, but keepalives will not work and the session might be more vulnerable to eavesdroppers than it could be. | ||
| - | This is an SSH-1-specific bug. No known SSH-2 server fails to deal with SSH-2 ignore messages. | + | This is an %%SSH-1%%-specific bug. No known %%SSH-2%% server fails to deal with %%SSH-2%% ignore messages. |
| ===== Refuses all SSH-1 password camouflage ===== | ===== Refuses all SSH-1 password camouflage ===== | ||
| - | When talking to an SSH-1 server which cannot deal with [[#chokes_on_ssh-1_ignore_messages|ignore messages]], WinSCP will attempt to disguise the length of the user's password by sending additional padding within the password packet. This is technically a violation of the SSH-1 specification, and so WinSCP will only do it when it cannot use standards-compliant ignore messages as camouflage. In this sense, for a server to refuse to accept a padded password packet is not really a bug, but it does make life inconvenient if the server can also not handle ignore messages. | + | When talking to an SSH-1 server which cannot deal with [[#chokes_on_ssh-1_ignore_messages|ignore messages]], WinSCP will attempt to disguise the length of the user's password by sending additional padding within the password packet. This is technically a violation of the %%SSH-1%% specification, and so WinSCP will only do it when it cannot use standards-compliant ignore messages as camouflage. In this sense, for a server to refuse to accept a padded password packet is not really a bug, but it does make life inconvenient if the server can also not handle ignore messages. |
| If this bug is detected, WinSCP will assume that neither ignore messages nor padding are acceptable, and that it thus has no choice but to send the user's password with no form of camouflage, so that an eavesdropping user will be easily able to find out the exact length of the password. If this bug is enabled when talking to a correct server, the session will succeed, but will be more vulnerable to eavesdroppers than it could be. | If this bug is detected, WinSCP will assume that neither ignore messages nor padding are acceptable, and that it thus has no choice but to send the user's password with no form of camouflage, so that an eavesdropping user will be easily able to find out the exact length of the password. If this bug is enabled when talking to a correct server, the session will succeed, but will be more vulnerable to eavesdroppers than it could be. | ||
| - | This is an SSH-1-specific bug. SSH-2 is secure against this type of attack. | + | This is an %%SSH-1%%-specific bug. SSH-2 is secure against this type of attack. |
| ===== Chokes on SSH-1 RSA authentication ===== | ===== Chokes on SSH-1 RSA authentication ===== | ||
| - | Some SSH-1 servers cannot deal with RSA authentication messages at all. If [[ui_pageant|Pageant]] is running and contains any SSH-1 keys, WinSCP will normally automatically try RSA authentication before falling back to passwords, so these servers will crash when they see the RSA attempt. | + | Some SSH-1 servers cannot deal with RSA authentication messages at all. If [[ui_pageant|Pageant]] is running and contains any %%SSH-1%% keys, WinSCP will normally automatically try %%RSA%% authentication before falling back to passwords, so these servers will crash when they see the %%RSA%% attempt. |
| - | If this bug is detected, WinSCP will go straight to password authentication. If this bug is enabled when talking to a correct server, the session will succeed, but of course RSA authentication will be impossible. | + | If this bug is detected, WinSCP will go straight to password authentication. If this bug is enabled when talking to a correct server, the session will succeed, but of course %%RSA%% authentication will be impossible. |
| - | This is an SSH-1-specific bug. | + | This is an %%SSH-1%%-specific bug. |
| ===== Chokes on SSH-2 ignore messages ===== | ===== Chokes on SSH-2 ignore messages ===== | ||
| Line 47: | Line 47: | ||
| WinSCP sometimes sends a special request to SSH servers in the middle of channel data, with the name ''winadj@putty.projects.tartarus.org''. The purpose of this request is to measure the round-trip time to the server, which WinSCP uses to tune its flow control. The server does not actually have to understand the message; it is expected to send back a ''SSH_MSG_CHANNEL_FAILURE'' message indicating that it didn't understand it. (All WinSCP needs for its timing calculations is some kind of response.) | WinSCP sometimes sends a special request to SSH servers in the middle of channel data, with the name ''winadj@putty.projects.tartarus.org''. The purpose of this request is to measure the round-trip time to the server, which WinSCP uses to tune its flow control. The server does not actually have to understand the message; it is expected to send back a ''SSH_MSG_CHANNEL_FAILURE'' message indicating that it didn't understand it. (All WinSCP needs for its timing calculations is some kind of response.) | ||
| - | It has been known for some SSH servers to get confused by this message in one way or another – because it has a long name, or because they can't cope with unrecognized request names even to the extent of sending back the correct failure response, or because they handle it sensibly but fill up the server's log file with pointless spam, or whatever. WinSCP therefore supports this bug-compatibility flag: if it believes the server has this bug, it will never send its ''winadj@putty.projects.tartarus.org'' request, and will make do without its timing data. | + | It has been known for some %%SSH%% servers to get confused by this message in one way or another – because it has a long name, or because they can't cope with unrecognized request names even to the extent of sending back the correct failure response, or because they handle it sensibly but fill up the server's log file with pointless spam, or whatever. WinSCP therefore supports this bug-compatibility flag: if it believes the server has this bug, it will never send its ''winadj@putty.projects.tartarus.org'' request, and will make do without its timing data. |
| ===== Miscomputes SSH-2 HMAC keys ===== | ===== Miscomputes SSH-2 HMAC keys ===== | ||
| Line 67: | Line 67: | ||
| ===== [[rsa_padding]] Requires padding on SSH-2 RSA signatures ===== | ===== [[rsa_padding]] Requires padding on SSH-2 RSA signatures ===== | ||
| - | Versions below 3.3 of [[&openssh|OpenSSH]] and versions below 1.3.4d/1.3.5rc4 of ProFTPD/mod_sftp require SSH-2 RSA signatures to be padded with zero bytes to the same length as the RSA key modulus. The SSH-2 draft specification says that an unpadded signature MUST be accepted, so this is a bug. A typical symptom of this problem is that WinSCP mysteriously fails RSA authentication once in every few hundred attempts, and falls back to passwords. In session log file you will typically see this record: | + | Versions below 3.3 of [[&openssh|OpenSSH]] and versions below 1.3.4d/1.3.5rc4 of ProFTPD/mod_sftp require SSH-2 RSA signatures to be padded with zero bytes to the same length as the %%RSA%% key modulus. The SSH-2 draft specification says that an unpadded signature MUST be accepted, so this is a bug. A typical symptom of this problem is that WinSCP mysteriously fails %%RSA%% authentication once in every few hundred attempts, and falls back to passwords. In session log file you will typically see this record: |
| <code> | <code> | ||
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| If this bug is detected, WinSCP will pad its signatures in the way the buggy servers expect. If this bug is enabled when talking to a correct server, it is likely that no damage will be done, since correct servers usually still accept padded signatures because they're used to talking to OpenSSH. | If this bug is detected, WinSCP will pad its signatures in the way the buggy servers expect. If this bug is enabled when talking to a correct server, it is likely that no damage will be done, since correct servers usually still accept padded signatures because they're used to talking to OpenSSH. | ||
| - | This is an SSH-2-specific bug. | + | This is an %%SSH-2%%-specific bug. |
| ===== Misuses the session ID in SSH-2 PK auth ===== | ===== Misuses the session ID in SSH-2 PK auth ===== | ||
| Line 81: | Line 81: | ||
| Versions below 2.3 of [[&openssh|OpenSSH]] require SSH-2 public-key authentication to be done slightly differently: the data to be signed by the client contains the session ID formatted in a different way. If public-key authentication mysteriously does not work but the session log shows that WinSCP has successfully sent a signature, it might be worth enabling the workaround for this bug to see if it helps. | Versions below 2.3 of [[&openssh|OpenSSH]] require SSH-2 public-key authentication to be done slightly differently: the data to be signed by the client contains the session ID formatted in a different way. If public-key authentication mysteriously does not work but the session log shows that WinSCP has successfully sent a signature, it might be worth enabling the workaround for this bug to see if it helps. | ||
| - | If this bug is detected, WinSCP will sign data in the way OpenSSH expects. If this bug is enabled when talking to a correct server, SSH-2 public-key authentication will fail. | + | If this bug is detected, WinSCP will sign data in the way OpenSSH expects. If this bug is enabled when talking to a correct server, %%SSH-2%% public-key authentication will fail. |
| - | This is an SSH-2-specific bug. | + | This is an %%SSH-2%%-specific bug. |
| ===== Handles SSH-2 key re-exchange badly ===== | ===== Handles SSH-2 key re-exchange badly ===== | ||
| - | Some SSH servers cannot cope with repeat key exchange at all, and will ignore attempts by the client to start one. Since WinSCP pauses the session while performing a repeat key exchange, the effect of this would be to cause the session to hang after an hour (unless you have your [[ui_login_kex|rekey timeout set differently]]). Other, very old, SSH servers handle repeat key exchange even more badly, and disconnect upon receiving a repeat key exchange request. | + | Some SSH servers cannot cope with repeat key exchange at all, and will ignore attempts by the client to start one. Since WinSCP pauses the session while performing a repeat key exchange, the effect of this would be to cause the session to hang after an hour (unless you have your [[ui_login_kex|rekey timeout set differently]]). Other, very old, %%SSH%% servers handle repeat key exchange even more badly, and disconnect upon receiving a repeat key exchange request. |
| If this bug is detected, WinSCP will never initiate a repeat key exchange. If this bug is enabled when talking to a correct server, the session should still function, but may be less secure than you would expect. | If this bug is detected, WinSCP will never initiate a repeat key exchange. If this bug is enabled when talking to a correct server, the session should still function, but may be less secure than you would expect. | ||