fix: send SSH_MSG_DISCONNECT on KEX failure#1091
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Ensures the library sends a best-effort SSH_MSG_DISCONNECT when KEX negotiation fails due to no mutually supported algorithms, aligning behavior with RFC 4253 guidance and existing disconnect-on-error patterns in the codebase.
Changes:
- Adds a best-effort
(void)SendDisconnect(...)call inDoKexInit()when KEX/hostkey/cipher/MAC matching fails. - Uses
WOLFSSH_DISCONNECT_KEY_EXCHANGE_FAILEDas the disconnect reason for these negotiation failures.
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| /* RFC 4253 11.1: best-effort SSH_MSG_DISCONNECT on negotiation failure. */ | ||
| if (ret == WS_MATCH_KEX_ALGO_E || ret == WS_MATCH_KEY_ALGO_E || | ||
| ret == WS_MATCH_ENC_ALGO_E || ret == WS_MATCH_MAC_ALGO_E) { | ||
| (void)SendDisconnect(ssh, WOLFSSH_DISCONNECT_KEY_EXCHANGE_FAILED); |
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Bug
DoKexInit()insrc/internal.csetsretto one of the negotiation-failure codesWS_MATCH_KEX_ALGO_E/WS_MATCH_KEY_ALGO_E/WS_MATCH_ENC_ALGO_E/WS_MATCH_MAC_ALGO_Ewhen no common KEX / host-key / cipher / MAC algorithm can be negotiated, then returns. That code propagates unchanged throughDoPacket()andDoReceive(), which setssh->errorand returnWS_FATAL_ERROR. NoSSH_MSG_DISCONNECTis ever sent on these paths, so the peer is dropped without notification.RFC 4253 s11.1 says an implementation SHOULD send
SSH_MSG_DISCONNECTon a protocol error. The library already follows this best-effort pattern with(void)SendDisconnect(...)for service-request/accept errors (internal.c:7691, 7723); the KEX negotiation paths were inconsistent with that pattern.Fix
At the single
DoKexInit()exit point, send a best-effortSSH_MSG_DISCONNECTwith reasonWOLFSSH_DISCONNECT_KEY_EXCHANGE_FAILED(3) whenretis any of the four match-failure codes. Uses the same(void)SendDisconnectbest-effort form as existing call sites. Safe:retis not overwritten for match errors (theSendKexInit/ssh->errorpropagation block is gated onret == WS_SUCCESS).Build verification
Compiled the flagged translation unit against a full wolfSSL build (
make src/libwolfssh_la-internal.lo, exit 0) on wolfssh master 974f4fd.Reported by static analysis (Fenrir finding 608). Severity Low (RFC uses SHOULD, not MUST).