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ssh: tighten DH bounds to 1<e<p-1 and add K check #11303

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Merged
u3s merged 1 commit into from
Jul 13, 2026
+84 −36
Merged

ssh: tighten DH bounds to 1<e<p-1 and add K check #11303

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82 changes: 47 additions & 35 deletions lib/ssh/src/ssh_transport.erl
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Original file line number Diff line number Diff line change
Expand Up @@ -617,28 +617,34 @@ handle_kexdh_init(#ssh_msg_kexdh_init{e = E},
%% server
{G, P} = dh_group(Kex),
if
1=<E, E=<(P-1) ->
1<E, E<(P-1) ->
Sz = dh_bits(Algs),
{Public, Private} = generate_key(dh, [P,G,2*Sz]),
K = compute_key(dh, E, Private, [P,G]),
MyPrivHostKey = get_host_key(SignAlg, Opts),
MyPubHostKey = ssh_file:extract_public_key(MyPrivHostKey),
H = kex_hash(Ssh0, MyPubHostKey, sha(Kex), {E,Public,K}),
case sign(H, SignAlg, MyPrivHostKey, Ssh0) of
{ok,H_SIG} ->
{SshPacket, Ssh1} =
ssh_packet(#ssh_msg_kexdh_reply{public_host_key = {MyPubHostKey,SignAlg},
f = Public,
h_sig = H_SIG
}, Ssh0),
{ok, SshPacket, Ssh1#ssh{keyex_key = {{Private, Public}, {G, P}},
shared_secret = ssh_bits:mpint(K),
exchanged_hash = H,
session_id = sid(Ssh1, H)}};
{error,unsupported_sign_alg} ->
if
1<K, K<(P-1) ->
MyPrivHostKey = get_host_key(SignAlg, Opts),
MyPubHostKey = ssh_file:extract_public_key(MyPrivHostKey),
H = kex_hash(Ssh0, MyPubHostKey, sha(Kex), {E,Public,K}),
case sign(H, SignAlg, MyPrivHostKey, Ssh0) of
{ok,H_SIG} ->
{SshPacket, Ssh1} =
ssh_packet(#ssh_msg_kexdh_reply{public_host_key = {MyPubHostKey,SignAlg},
f = Public,
h_sig = H_SIG
}, Ssh0),
{ok, SshPacket, Ssh1#ssh{keyex_key = {{Private, Public}, {G, P}},
shared_secret = ssh_bits:mpint(K),
exchanged_hash = H,
session_id = sid(Ssh1, H)}};
{error,unsupported_sign_alg} ->
?DISCONNECT(?SSH_DISCONNECT_KEY_EXCHANGE_FAILED,
io_lib:format("Unsupported algorithm ~p", [SignAlg],
[{chars_limit, ssh_lib:max_log_len(Opts)}]))
end;
true ->
?DISCONNECT(?SSH_DISCONNECT_KEY_EXCHANGE_FAILED,
io_lib:format("Unsupported algorithm ~p", [SignAlg],
[{chars_limit, ssh_lib:max_log_len(Opts)}]))
"Key exchange failed, 'K' out of bounds")
end;
true ->
MsgFun =
Expand All @@ -658,22 +664,28 @@ handle_kexdh_reply(#ssh_msg_kexdh_reply{public_host_key = PeerPubHostKey,
#ssh{keyex_key = {{Private, Public}, {G, P}},
algorithms = #alg{kex=Kex}} = Ssh0) ->
%% client
if
1=<F, F=<(P-1)->
if
1<F, F<(P-1) ->
K = compute_key(dh, F, Private, [P,G]),
H = kex_hash(Ssh0, PeerPubHostKey, sha(Kex), {Public,F,K}),
case verify_host_key(Ssh0, PeerPubHostKey, H, H_SIG) of
ok ->
{SshPacket, Ssh} = ssh_packet(#ssh_msg_newkeys{}, Ssh0),
{ok, SshPacket, install_alg(snd, Ssh#ssh{shared_secret = ssh_bits:mpint(K),
exchanged_hash = H,
session_id = sid(Ssh, H)})};
Error ->
if
1<K, K<(P-1) ->
H = kex_hash(Ssh0, PeerPubHostKey, sha(Kex), {Public,F,K}),
case verify_host_key(Ssh0, PeerPubHostKey, H, H_SIG) of
ok ->
{SshPacket, Ssh} = ssh_packet(#ssh_msg_newkeys{}, Ssh0),
{ok, SshPacket, install_alg(snd, Ssh#ssh{shared_secret = ssh_bits:mpint(K),
exchanged_hash = H,
session_id = sid(Ssh, H)})};
Error ->
?DISCONNECT(?SSH_DISCONNECT_KEY_EXCHANGE_FAILED,
io_lib:format("Kexdh init failed. Verify host key: ~p",[Error],
[{chars_limit, ssh_lib:max_log_len(Ssh0)}])
)
end;
true ->
?DISCONNECT(?SSH_DISCONNECT_KEY_EXCHANGE_FAILED,
io_lib:format("Kexdh init failed. Verify host key: ~p",[Error],
[{chars_limit, ssh_lib:max_log_len(Ssh0)}])
)
end;
"Key exchange failed, 'K' out of bounds")
end;

true ->
?DISCONNECT(?SSH_DISCONNECT_KEY_EXCHANGE_FAILED,
Expand Down Expand Up @@ -775,7 +787,7 @@ handle_kex_dh_gex_init(#ssh_msg_kex_dh_gex_init{e = E},
opts = Opts} = Ssh0) ->
%% server
if
1=<E, E=<(P-1) ->
1<E, E<(P-1) ->
K = compute_key(dh, E, Private, [P,G]),
if
1<K, K<(P-1) ->
Expand Down Expand Up @@ -820,8 +832,8 @@ handle_kex_dh_gex_reply(#ssh_msg_kex_dh_gex_reply{public_host_key = PeerPubHostK
algorithms = #alg{kex=Kex}} =
Ssh0) ->
%% client
if
1=<F, F=<(P-1)->
if
1<F, F<(P-1) ->
K = compute_key(dh, F, Private, [P,G]),
if
1<K, K<(P-1) ->
Expand Down
38 changes: 37 additions & 1 deletion lib/ssh/test/ssh_protocol_SUITE.erl
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Original file line number Diff line number Diff line change
Expand Up @@ -68,6 +68,7 @@
kex_strict_violation/1,
kex_strict_violation_2/1,
kex_strict_msg_unknown/1,
dh_kexdh_init_e_out_of_bounds/1,
gex_client_init_option_groups/1,
gex_client_init_option_groups_file/1,
gex_client_init_option_groups_moduli_file/1,
Expand Down Expand Up @@ -200,7 +201,8 @@ groups() ->
kex_strict_violation_new_keys,
kex_strict_violation,
kex_strict_violation_2,
kex_strict_msg_unknown]},
kex_strict_msg_unknown,
dh_kexdh_init_e_out_of_bounds]},
{service_requests, [], [bad_service_name,
bad_long_service_name,
bad_very_long_service_name,
Expand Down Expand Up @@ -1339,6 +1341,40 @@ kex_strict_msg_unknown(Config) ->
{match, disconnect(?SSH_DISCONNECT_KEY_EXCHANGE_FAILED), receive_msg}],
kex_strict_helper(Config, TestMessages, ExpectedReason).

%% RFC 4253 §8 / RFC 4419 §3: a peer's DH value must satisfy 1 < e < p-1.
%% A peer driving e (or, on the client side, f) to one of {0, 1, p-1, p}
%% must be rejected with SSH_DISCONNECT_KEY_EXCHANGE_FAILED. Verify this
%% for the server, which receives e from the client.
dh_kexdh_init_e_out_of_bounds(Config) ->
{_G, P} = ?dh_group14,
[verify_kexdh_init_rejected(Config, E) || E <- [0, 1, P-1, P]],
ok.

verify_kexdh_init_rejected(Config, E) ->
ct:log("Trying kexdh_init with e=~p", [E]),
{ok, InitialState} = ssh_trpt_test_lib:exec(
[{set_options, [print_ops, print_seqnums, print_messages]}]),
{ok, _} =
ssh_trpt_test_lib:exec(
[{connect,
ssh_test_lib:server_host(Config), ssh_test_lib:server_port(Config),
[{preferred_algorithms,
[{kex, ['diffie-hellman-group14-sha256']},
{cipher, ?DEFAULT_CIPHERS}]},
{silently_accept_hosts, true},
{recv_ext_info, false},
{user_dir, ssh_test_lib:user_dir(Config)},
{user_interaction, false}
| proplists:get_value(extra_options, Config, [])
]},
receive_hello,
{send, hello},
{send, ssh_msg_kexinit},
{match, #ssh_msg_kexinit{_='_'}, receive_msg},
{send, #ssh_msg_kexdh_init{e = E}},
{match, disconnect(?SSH_DISCONNECT_KEY_EXCHANGE_FAILED), receive_msg}],
InitialState).

kex_strict_helper(Config0, TestMessages, ExpectedReason) ->
Config = ssh_test_lib:add_log_handler(?FUNCTION_NAME, Config0),
Level = ssh_test_lib:get_log_level(),
Expand Down
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