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Add note regarding OpenSSH's 2048-bit GEX fallback, and suppress the related recommendation since the user cannot control it (partly related to #168).

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This commit is contained in:
Joe Testa 2023-03-21 11:44:45 -04:00
parent c02ab8f170
commit 71feaa191e
4 changed files with 35 additions and 8 deletions
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34
src/ssh_audit/ssh_audit.py
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@ -308,7 +308,7 @@ def output_fingerprints(out: OutputBuffer, algs: Algorithms, is_json_output: boo
# Returns True if no warnings or failures encountered in configuration.
def output_recommendations(out: OutputBuffer, algs: Algorithms, software: Optional[Software], is_json_output: bool, padlen: int = 0) -> bool:
def output_recommendations(out: OutputBuffer, algs: Algorithms, algorithm_recommendation_suppress_list: List[str], software: Optional[Software], is_json_output: bool, padlen: int = 0) -> bool:
ret = True
# PuTTY's algorithms cannot be modified, so there's no point in issuing recommendations.
@ -352,6 +352,11 @@ def output_recommendations(out: OutputBuffer, algs: Algorithms, software: Option
if action not in alg_rec[sshv][alg_type]:
continue
for name in alg_rec[sshv][alg_type][action]:
# If this algorithm should be suppressed, skip it.
if name in algorithm_recommendation_suppress_list:
continue
p = '' if out.batch else ' ' * (padlen - len(name))
chg_additional_info = ''
if action == 'del':
@ -396,6 +401,27 @@ def output_info(out: OutputBuffer, software: Optional['Software'], client_audit:
out.sep()
def post_process_findings(banner: Optional[Banner], algs: Algorithms) -> List[str]:
'''Perform post-processing on scan results before reporting them to the user. Returns a list of algorithms that should not be recommended'''
algorithm_recommendation_suppress_list = []
# If the server is OpenSSH, and the diffie-hellman-group-exchange-sha256 key exchange was found with modulus size 2048, add a note regarding the bug that causes the server to support 2048-bit moduli no matter the configuration.
if (algs.ssh2kex is not None and 'diffie-hellman-group-exchange-sha256' in algs.ssh2kex.kex_algorithms and 'diffie-hellman-group-exchange-sha256' in algs.ssh2kex.dh_modulus_sizes() and algs.ssh2kex.dh_modulus_sizes()['diffie-hellman-group-exchange-sha256'][0] == 2048) and (banner is not None and banner.software is not None and banner.software.find('OpenSSH') != -1):
# Ensure a list for notes exists.
while len(SSH2_KexDB.ALGORITHMS['kex']['diffie-hellman-group-exchange-sha256']) < 4:
SSH2_KexDB.ALGORITHMS['kex']['diffie-hellman-group-exchange-sha256'].append([])
SSH2_KexDB.ALGORITHMS['kex']['diffie-hellman-group-exchange-sha256'][3].append("A bug in OpenSSH causes it to fall back to a 2048-bit modulus regardless of server configuration (https://bugzilla.mindrot.org/show_bug.cgi?id=2793)")
# Ensure that this algorithm doesn't appear in the recommendations section since the user cannot control this OpenSSH bug.
algorithm_recommendation_suppress_list.append('diffie-hellman-group-exchange-sha256')
return algorithm_recommendation_suppress_list
# Returns a exitcodes.* flag to denote if any failures or warnings were encountered.
def output(out: OutputBuffer, aconf: AuditConf, banner: Optional[Banner], header: List[str], client_host: Optional[str] = None, kex: Optional[SSH2_Kex] = None, pkm: Optional[SSH1_PublicKeyMessage] = None, print_target: bool = False) -> int:
@ -403,6 +429,10 @@ def output(out: OutputBuffer, aconf: AuditConf, banner: Optional[Banner], header
client_audit = client_host is not None # If set, this is a client audit.
sshv = 1 if pkm is not None else 2
algs = Algorithms(pkm, kex)
# Perform post-processing on the findings to make final adjustments before outputting the results.
algorithm_recommendation_suppress_list = post_process_findings(banner, algs)
with out:
if print_target:
host = aconf.host
@ -475,7 +505,7 @@ def output(out: OutputBuffer, aconf: AuditConf, banner: Optional[Banner], header
title, atype = 'message authentication code algorithms', 'mac'
program_retval = output_algorithms(out, title, adb, atype, kex.server.mac, unknown_algorithms, aconf.json, program_retval, maxlen)
output_fingerprints(out, algs, aconf.json)
perfect_config = output_recommendations(out, algs, software, aconf.json, maxlen)
perfect_config = output_recommendations(out, algs, algorithm_recommendation_suppress_list, software, aconf.json, maxlen)
output_info(out, software, client_audit, not perfect_config, aconf.json)
if aconf.json:

2
test/docker/expected_results/openssh_8.0p1_test1.txt
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@ -21,6 +21,7 @@
`- [info] available since OpenSSH 5.7, Dropbear SSH 2013.62
(kex) diffie-hellman-group-exchange-sha256 (2048-bit) -- [warn] 2048-bit modulus only provides 112-bits of symmetric strength
`- [info] available since OpenSSH 4.4
`- [info] A bug in OpenSSH causes it to fall back to a 2048-bit modulus regardless of server configuration (https://bugzilla.mindrot.org/show_bug.cgi?id=2793)
(kex) diffie-hellman-group16-sha512 -- [info] available since OpenSSH 7.3, Dropbear SSH 2016.73
(kex) diffie-hellman-group18-sha512 -- [info] available since OpenSSH 7.3
(kex) diffie-hellman-group14-sha256 -- [warn] 2048-bit modulus only provides 112-bits of symmetric strength
@ -75,7 +76,6 @@
(fin) ssh-rsa: SHA256:nsWtdJ9Z67Vrf7OsUzQov7esXhsWAfVppArGh25u244
# algorithm recommendations (for OpenSSH 8.0)
(rec) !diffie-hellman-group-exchange-sha256 -- kex algorithm to change (increase modulus size to 3072 bits or larger) 
(rec) -ecdh-sha2-nistp256 -- kex algorithm to remove 
(rec) -ecdh-sha2-nistp384 -- kex algorithm to remove 
(rec) -ecdh-sha2-nistp521 -- kex algorithm to remove 

2
test/docker/expected_results/openssh_8.0p1_test2.txt
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@ -21,6 +21,7 @@
`- [info] available since OpenSSH 5.7, Dropbear SSH 2013.62
(kex) diffie-hellman-group-exchange-sha256 (2048-bit) -- [warn] 2048-bit modulus only provides 112-bits of symmetric strength
`- [info] available since OpenSSH 4.4
`- [info] A bug in OpenSSH causes it to fall back to a 2048-bit modulus regardless of server configuration (https://bugzilla.mindrot.org/show_bug.cgi?id=2793)
(kex) diffie-hellman-group16-sha512 -- [info] available since OpenSSH 7.3, Dropbear SSH 2016.73
(kex) diffie-hellman-group18-sha512 -- [info] available since OpenSSH 7.3
(kex) diffie-hellman-group14-sha256 -- [warn] 2048-bit modulus only provides 112-bits of symmetric strength
@ -67,7 +68,6 @@
(fin) ssh-ed25519: SHA256:UrnXIVH+7dlw8UqYocl48yUEcKrthGDQG2CPCgp7MxU
# algorithm recommendations (for OpenSSH 8.0)
(rec) !diffie-hellman-group-exchange-sha256 -- kex algorithm to change (increase modulus size to 3072 bits or larger) 
(rec) -ecdh-sha2-nistp256 -- kex algorithm to remove 
(rec) -ecdh-sha2-nistp384 -- kex algorithm to remove 
(rec) -ecdh-sha2-nistp521 -- kex algorithm to remove 

5
test/docker/expected_results/openssh_8.0p1_test3.txt
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@ -15,6 +15,7 @@
(kex) curve25519-sha256@libssh.org -- [info] available since OpenSSH 6.5, Dropbear SSH 2013.62
(kex) diffie-hellman-group-exchange-sha256 (2048-bit) -- [warn] 2048-bit modulus only provides 112-bits of symmetric strength
`- [info] available since OpenSSH 4.4
`- [info] A bug in OpenSSH causes it to fall back to a 2048-bit modulus regardless of server configuration (https://bugzilla.mindrot.org/show_bug.cgi?id=2793)
# host-key algorithms
(key) ssh-ed25519 -- [info] available since OpenSSH 6.5
@ -37,12 +38,8 @@
(fin) ssh-ed25519: SHA256:UrnXIVH+7dlw8UqYocl48yUEcKrthGDQG2CPCgp7MxU
# algorithm recommendations (for OpenSSH 8.0)
(rec) !diffie-hellman-group-exchange-sha256 -- kex algorithm to change (increase modulus size to 3072 bits or larger) 
(rec) +diffie-hellman-group16-sha512 -- kex algorithm to append 
(rec) +diffie-hellman-group18-sha512 -- kex algorithm to append 
(rec) +rsa-sha2-256 -- key algorithm to append 
(rec) +rsa-sha2-512 -- key algorithm to append 
# additional info
(nfo) For hardening guides on common OSes, please see: <https://www.ssh-audit.com/hardening_guides.html>