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kex: skip signaling-type kex entries when negotiating by vszakats · Pull Request #2199 · libssh2/libssh2 · GitHub
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kex: skip signaling-type kex entries when negotiating #2199

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kex: skip signaling-type kex entries when negotiating #2199

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d8757de
kex: skip signaling kex entries when negotiating
vszakats Jul 3, 2026
9447115
add extra check
vszakats Jul 3, 2026
64c4ead
fixup
vszakats Jul 22, 2026
8a22cfd
Revert "add extra check"
vszakats Jul 22, 2026
abaf8d3
dbg1
vszakats Jul 23, 2026
e76e46f
dbg5
vszakats Jul 23, 2026
c87ac11
Revert "dbg5"
vszakats Jul 23, 2026
e723fa2
Revert "dbg1"
vszakats Jul 29, 2026
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38 changes: 22 additions & 16 deletions src/kex.c
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Original file line number Diff line number Diff line change
Expand Up @@ -3325,7 +3325,8 @@ static int kex_agree_kex_hostkey(LIBSSH2_SESSION *session,
/* We have agreed on a key exchange method,
* Can we agree on a hostkey that works with this kex?
*/
if(kex_agree_hostkey(session, method->flags, hostkey,
if(method->exchange_keys &&
kex_agree_hostkey(session, method->flags, hostkey,
hostkey_len) == 0) {
session->kex = method;
if(session->burn_optimistic_kexinit && kex == q)
Expand All @@ -3344,22 +3345,27 @@ static int kex_agree_kex_hostkey(LIBSSH2_SESSION *session,
}

while(*kexp && (*kexp)->name) {
s = ssh2_kex_agree_instr(kex, kex_len,
(*kexp)->name, strlen((*kexp)->name));
if(s) {
/* We have agreed on a key exchange method,
* Can we agree on a hostkey that works with this kex?
*/
if(kex_agree_hostkey(session, (*kexp)->flags, hostkey,
hostkey_len) == 0) {
session->kex = *kexp;
if(session->burn_optimistic_kexinit && kex == s)
/* Server sent an optimistic packet, and client agrees
* with preference cancel burning the first KEX_INIT
* packet that comes in */
session->burn_optimistic_kexinit = 0;
/* Skip signalling-only entries (ext-info-c,
* kex-strict-c-v00@openssh.com) that carry no real KEX implementation
* and must never be selected as the negotiated method. */
if((*kexp)->exchange_keys) {
s = ssh2_kex_agree_instr(kex, kex_len,
(*kexp)->name, strlen((*kexp)->name));
if(s) {
/* We have agreed on a key exchange method,
* Can we agree on a hostkey that works with this kex?
*/
if(kex_agree_hostkey(session, (*kexp)->flags, hostkey,
hostkey_len) == 0) {
session->kex = *kexp;
if(session->burn_optimistic_kexinit && kex == s)
/* Server sent an optimistic packet, and client agrees
* with preference cancel burning the first KEX_INIT
* packet that comes in */
session->burn_optimistic_kexinit = 0;

return 0;
return 0;
}
}
}
kexp++;
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