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SSH: make banner parsing more robust

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This change revamps SSH banner parsing.  The previous behavior was both
a bit too strict in some regards, and too permissive in other.

Specifically, clients are now required to send a line starting with
"SSH-" as the first line.  This is in line with the RFC, as well with
observed behavior. This also prevents the creation of `ssh.log` for
non-SSH traffic on port 22.

For the server side, we now accept text before the SSH banner. This
previously led to a protocol violation but is allowed by the spec.

New tests are added to cover these cases.
This commit is contained in:
Johanna Amann 2025-03-13 15:14:12 +00:00
parent 629f2bd03a
commit 6023c8b906
14 changed files with 142 additions and 10 deletions
Download patch file Download diff file Expand all files Collapse all files

13
src/analyzer/protocol/ssh/events.bif
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@ -12,7 +12,7 @@
## ssh_capabilities ssh2_server_host_key ssh1_server_host_key
## ssh_server_host_key ssh_encrypted_packet ssh2_dh_server_params
## ssh2_gss_error ssh2_ecc_key ssh2_ecc_init ssh2_dh_gex_init
## ssh2_gss_init ssh2_rsa_secret
## ssh2_gss_init ssh2_rsa_secret ssh_server_pre_banner_data
event ssh_server_version%(c: connection, version: string%);
## An :abbr:`SSH (Secure Shell)` Protocol Version Exchange message
@ -336,3 +336,14 @@ event ssh2_gss_init%(c: connection, is_orig: bool%);
## ssh2_gss_error ssh2_ecc_key ssh2_ecc_init ssh2_dh_gex_init
## ssh2_gss_init ssh2_rsa_secret
event ssh2_rsa_secret%(c: connection, is_orig: bool%);
## SSH servers can send textual data to the client before sending
## a banner. The primary use case of this are error messages of TCP
## wrappers.
##
## As this event happens before the SSH banner is exchanged, it is
## possible that it contains data from different protocols; e.g. if
## an SSH client connects to a non-SSH-server.
##
## .. zeek:see:: ssh_server_version
event ssh_server_pre_banner_data%(c: connection, data: string%);

14
src/analyzer/protocol/ssh/ssh-analyzer.pac
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@ -68,9 +68,17 @@ refine flow SSH_Flow += {
function proc_ssh_version_server(msg: SSH_Version_Server): bool
%{
zeek::BifEvent::enqueue_ssh_server_version(connection()->zeek_analyzer(),
connection()->zeek_analyzer()->Conn(),
to_stringval(${msg.version}));
if ( ${msg.version}.length() > 0 )
{
zeek::BifEvent::enqueue_ssh_server_version(connection()->zeek_analyzer(),
connection()->zeek_analyzer()->Conn(),
to_stringval(${msg.version}));
}
else if ( ssh_server_pre_banner_data )
{
zeek::BifEvent::enqueue_ssh_server_pre_banner_data(connection()->zeek_analyzer(),
connection()->zeek_analyzer()->Conn(), to_stringval(${msg.nonversiondata}));
}
return true;
%}

14
src/analyzer/protocol/ssh/ssh-protocol.pac
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@ -30,15 +30,19 @@ type SSH_Version_Switch(is_orig: bool) = case is_orig of {
false -> server_version: SSH_Version_Server;
};
# SSH servers can have banners before their SSH version. Which... fun.
type SSH_Version_Server = record {
version : bytestring &oneline;
version: RE/(SSH-.*)?/;
# only UTF-8 data. This doesn't catch all bad cases, but some
nonversiondata: RE/([^[\xC0-\xC1]|[\xF5-\xFF]])*/;
} &let {
update_state : bool = $context.connection.update_state(KEX_INIT, false);
update_version : bool = $context.connection.update_version(version, false);
};
update_state : bool = $context.connection.update_state(KEX_INIT, false) &if(sizeof(version) > 0);
update_version : bool = $context.connection.update_version(version, false) &if(sizeof(version) > 0);
} &oneline;
# SSH clients _always_ have to send a line starting with SSH- first.
type SSH_Version_Client = record {
version : bytestring &oneline;
version : RE/SSH-.*/ &oneline;
} &let {
update_state : bool = $context.connection.update_state(KEX_INIT, true);
update_version : bool = $context.connection.update_version(version, true);

11
testing/btest/Baseline/scripts.base.protocols.ssh.http-port-22/analyzer.log Normal file
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@ -0,0 +1,11 @@
### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
#separator \x09
#set_separator ,
#empty_field (empty)
#unset_field -
#path analyzer
#open XXXX-XX-XX-XX-XX-XX
#fields ts cause analyzer_kind analyzer_name uid fuid id.orig_h id.orig_p id.resp_h id.resp_p failure_reason failure_data
#types time string string string string string addr port addr port string string
XXXXXXXXXX.XXXXXX violation protocol SSH CHhAvVGS1DHFjwGM9 - 10.0.0.1 51889 192.168.0.1 22 Binpac exception: binpac exception: string mismatch at <...>/ssh-protocol.pac:45: \x0aexpected pattern: "SSH-.*"\x0aactual data: "GET / HTTP/1.1" -
#close XXXX-XX-XX-XX-XX-XX

11
testing/btest/Baseline/scripts.base.protocols.ssh.http-port-22/conn.log Normal file
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@ -0,0 +1,11 @@
### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
#separator \x09
#set_separator ,
#empty_field (empty)
#unset_field -
#path conn
#open XXXX-XX-XX-XX-XX-XX
#fields ts uid id.orig_h id.orig_p id.resp_h id.resp_p proto service duration orig_bytes resp_bytes conn_state local_orig local_resp missed_bytes history orig_pkts orig_ip_bytes resp_pkts resp_ip_bytes tunnel_parents ip_proto
#types time string addr port addr port enum string interval count count string bool bool count string count count count count set[string] count
XXXXXXXXXX.XXXXXX CHhAvVGS1DHFjwGM9 10.0.0.1 51889 192.168.0.1 22 tcp http 0.000260 18 12649 SF T T 0 ShADadFf 15 618 13 13169 - 6
#close XXXX-XX-XX-XX-XX-XX

11
testing/btest/Baseline/scripts.base.protocols.ssh.http-port-22/http.log Normal file
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@ -0,0 +1,11 @@
### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
#separator \x09
#set_separator ,
#empty_field (empty)
#unset_field -
#path http
#open XXXX-XX-XX-XX-XX-XX
#fields ts uid id.orig_h id.orig_p id.resp_h id.resp_p trans_depth method host uri referrer version user_agent origin request_body_len response_body_len status_code status_msg info_code info_msg tags username password proxied orig_fuids orig_filenames orig_mime_types resp_fuids resp_filenames resp_mime_types
#types time string addr port addr port count string string string string string string string count count count string count string set[enum] string string set[string] vector[string] vector[string] vector[string] vector[string] vector[string] vector[string]
XXXXXXXXXX.XXXXXX CHhAvVGS1DHFjwGM9 10.0.0.1 51889 192.168.0.1 22 1 GET - / - 1.1 - - 0 12632 200 OK - - (empty) - - - - - - FsaSIr11Ze8VUH5yPj - text/plain
#close XXXX-XX-XX-XX-XX-XX

36
testing/btest/Baseline/scripts.base.protocols.ssh.pre-banner/.stdout Normal file
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@ -0,0 +1,36 @@
### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
_____________
< Hi stranger >
-------------
\ \
\ \_
\ \\
\ \\/\
\ _\\/
\ / -\
\ / oo -\
\ / \
| ---\ -\
\--/ \ \
| -\
\ -\ -------------\ /-\
\ \-------/ ---/ \
\ |\ \
| / | |
\ | \ |
| / \ |
| / \ |
\ \ \|
- /--------\ | o
\+ +--------- \ |
| | | \
| | \ |
| | | \
| | \ |
\ | | |
| | \ \
| | | |
+--+ ---+
Habit is habit, and not to be flung out of the window by any man, but coaxed
down-stairs a step at a time.
\x09\x09-- Mark Twain, "Pudd'nhead Wilson's Calendar

11
testing/btest/Baseline/scripts.base.protocols.ssh.pre-banner/conn.log Normal file
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@ -0,0 +1,11 @@
### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
#separator \x09
#set_separator ,
#empty_field (empty)
#unset_field -
#path conn
#open XXXX-XX-XX-XX-XX-XX
#fields ts uid id.orig_h id.orig_p id.resp_h id.resp_p proto service duration orig_bytes resp_bytes conn_state local_orig local_resp missed_bytes history orig_pkts orig_ip_bytes resp_pkts resp_ip_bytes tunnel_parents ip_proto
#types time string addr port addr port enum string interval count count string bool bool count string count count count count set[string] count
XXXXXXXXXX.XXXXXX CHhAvVGS1DHFjwGM9 10.0.2.1 55343 10.0.2.10 22 tcp ssh 0.201784 2869 4728 S1 T T 0 ShADad 21 3973 15 5516 - 6
#close XXXX-XX-XX-XX-XX-XX

11
testing/btest/Baseline/scripts.base.protocols.ssh.pre-banner/ssh.log Normal file
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@ -0,0 +1,11 @@
### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
#separator \x09
#set_separator ,
#empty_field (empty)
#unset_field -
#path ssh
#open XXXX-XX-XX-XX-XX-XX
#fields ts uid id.orig_h id.orig_p id.resp_h id.resp_p version auth_success auth_attempts direction client server cipher_alg mac_alg compression_alg kex_alg host_key_alg host_key
#types time string addr port addr port count bool count enum string string string string string string string string
XXXXXXXXXX.XXXXXX CHhAvVGS1DHFjwGM9 10.0.2.1 55343 10.0.2.10 22 2 - 0 - SSH-2.0-OpenSSH_9.7 SSH-2.0-OpenSSH_9.2p1 Debian-2+deb12u5 aes192-ctr hmac-sha2-256 zlib@openssh.com sntrup761x25519-sha512@openssh.com ssh-ed25519 27:27:33:7a:1a:4f:46:b2:58:1c:04:c2:ad:6d:8a:86
#close XXXX-XX-XX-XX-XX-XX

BIN
testing/btest/Traces/http/http-single-conn-22.pcap Normal file
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BIN
testing/btest/Traces/ssh/server-pre-banner-data.pcap Normal file
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Binary file not shown.

7
testing/btest/scripts/base/protocols/ssh/http-port-22.test Normal file
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@ -0,0 +1,7 @@
# Validate that a text-based protocol pn port 22 does not generate a ssh logfile.
# @TEST-EXEC: zeek -r $TRACES/http/http-single-conn-22.pcap %INPUT
# @TEST-EXEC: test ! -f ssh.log
# @TEST-EXEC: btest-diff http.log
# @TEST-EXEC: btest-diff conn.log
# @TEST-EXEC: TEST_DIFF_CANONIFIER="$SCRIPTS/diff-remove-abspath | $SCRIPTS/diff-remove-timestamps" btest-diff analyzer.log

11
testing/btest/scripts/base/protocols/ssh/pre-banner.test Normal file
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@ -0,0 +1,11 @@
# This tests a trace that has data before the banner.
# @TEST-EXEC: zeek -r $TRACES/ssh/server-pre-banner-data.pcap %INPUT
# @TEST-EXEC: btest-diff ssh.log
# @TEST-EXEC: btest-diff conn.log
# @TEST-EXEC: btest-diff .stdout
event ssh_server_pre_banner_data(c: connection, data: string)
{
print data;
}

2
testing/external/commit-hash.zeek-testing-private vendored
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@ -1 +1 @@
296a3b2bfd36a74c8aa22f175cea4c00a9f4d079
2fa4bd6a18c376c64629a6d5679c230423f60913