From 6bc89ac2f4510f5a526731d84321416f1c0891f6 Mon Sep 17 00:00:00 2001
From: Yosuke Shimizu <yosuke@wolfssl.com>
Date: Mon, 29 Jun 2026 13:55:07 +0900
Subject: [PATCH] keyboard-interactive: send USERAUTH_FAILURE on response-count
 mismatch

---
 src/internal.c  |  10 ++---
 tests/regress.c | 101 ++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 105 insertions(+), 6 deletions(-)

diff --git a/src/internal.c b/src/internal.c
index 35b5e73df..7b1a15718 100644
--- a/src/internal.c
+++ b/src/internal.c
@@ -8223,12 +8223,10 @@ static int DoUserAuthInfoResponse(WOLFSSH* ssh,
     if ((ret == WS_SUCCESS) &&
         (ssh->kbAuth.promptCount != kb->responseCount)) {
         WLOG(WS_LOG_DEBUG, "DUARKB: Invalid number of responses received");
-        ret = WS_USER_AUTH_E;
-    }
-
-    if (ret == WS_SUCCESS && kb->responseCount > WOLFSSH_MAX_PROMPTS) {
-        WLOG(WS_LOG_DEBUG, "DUARKB: Received too many responses (%d), max: %d",
-             kb->responseCount, WOLFSSH_MAX_PROMPTS);
+        /* Answer with USERAUTH_FAILURE instead of tearing down the transport.
+         * Keeping ret non-success skips the allocation, parse, and callback
+         * below; past here responseCount equals the capped promptCount. */
+        authFailure = 1;
         ret = WS_USER_AUTH_E;
     }
 
diff --git a/tests/regress.c b/tests/regress.c
index 27a89f7d8..5f02e01c0 100644
--- a/tests/regress.c
+++ b/tests/regress.c
@@ -340,6 +340,103 @@ static void FreeChannelOpenHarness(ChannelOpenHarness* harness)
         wolfSSH_CTX_free(harness->ctx);
 }
 
+#ifdef WOLFSSH_KEYBOARD_INTERACTIVE
+/* Build a plaintext SSH_MSG_USERAUTH_INFO_RESPONSE. The wire response count is
+ * "responseCount"; "stringCount" "x" response strings are actually appended.
+ * The count-mismatch guard rejects before parsing the body, so a short body is
+ * sufficient to drive it. */
+static word32 BuildInfoResponsePacket(word32 responseCount, word32 stringCount,
+        byte* out, word32 outSz)
+{
+    byte payload[128];
+    word32 idx = 0;
+    word32 i;
+
+    idx = AppendUint32(payload, sizeof(payload), idx, responseCount);
+    for (i = 0; i < stringCount; i++) {
+        idx = AppendString(payload, sizeof(payload), idx, "x");
+    }
+
+    return WrapPacket(MSGID_USERAUTH_INFO_RESPONSE, payload, idx, out, outSz);
+}
+
+/* Place the server mid keyboard-interactive auth, expecting an INFO_RESPONSE
+ * for "promptCount" prompts. */
+static void InitKbInfoResponseHarness(ChannelOpenHarness* harness,
+        byte* in, word32 inSz, word32 promptCount)
+{
+    InitChannelOpenHarness(harness, in, inSz);
+    harness->ssh->acceptState = ACCEPT_CLIENT_USERAUTH_REQUEST_DONE;
+    harness->ssh->authId = ID_USERAUTH_KEYBOARD;
+    harness->ssh->kbAuth.promptCount = promptCount;
+}
+
+/* Post-condition for a rejected INFO_RESPONSE: the transport survives and the
+ * server's reply is a USERAUTH_FAILURE rather than a teardown. */
+static void AssertKbInfoResponseSentFailure(const ChannelOpenHarness* harness,
+        int ret)
+{
+    AssertIntEQ(ret, WS_SUCCESS);
+    AssertIntEQ(harness->io.inOff, harness->io.inSz);
+    AssertTrue(harness->io.outSz > 0);
+    AssertIntEQ(ParseMsgId(harness->io.out, harness->io.outSz),
+            MSGID_USERAUTH_FAILURE);
+}
+
+/* A response count that disagrees with the server's prompt count must yield a
+ * USERAUTH_FAILURE, not a transport teardown (RFC 4256). */
+static void TestKbInfoResponseCountMismatchSendsFailure(void)
+{
+    ChannelOpenHarness harness;
+    byte in[128];
+    word32 inSz;
+    int ret;
+
+    /* Server expects 1 response; client sends 2. */
+    inSz = BuildInfoResponsePacket(2, 2, in, sizeof(in));
+    InitKbInfoResponseHarness(&harness, in, inSz, 1);
+
+    ret = DoReceive(harness.ssh);
+
+    AssertKbInfoResponseSentFailure(&harness, ret);
+
+    FreeChannelOpenHarness(&harness);
+}
+
+/* A rejected INFO_RESPONSE must leave packet framing intact: the next packet on
+ * the surviving connection still parses and draws its own failure. */
+static void TestKbInfoResponseMismatchKeepsFraming(void)
+{
+    ChannelOpenHarness harness;
+    byte in[256];
+    word32 inSz, nextSz, firstOutSz;
+    int ret;
+
+    /* Two back-to-back responses, both disagreeing with a prompt count of 1. */
+    inSz = BuildInfoResponsePacket(2, 2, in, sizeof(in));
+    nextSz = BuildInfoResponsePacket(3, 3, in + inSz,
+            (word32)sizeof(in) - inSz);
+    InitKbInfoResponseHarness(&harness, in, inSz + nextSz, 1);
+
+    ret = DoReceive(harness.ssh);
+    AssertIntEQ(ret, WS_SUCCESS);
+    AssertTrue(harness.io.outSz > 0);
+    AssertIntEQ(ParseMsgId(harness.io.out, harness.io.outSz),
+            MSGID_USERAUTH_FAILURE);
+    firstOutSz = harness.io.outSz;
+
+    ret = DoReceive(harness.ssh);
+    AssertIntEQ(ret, WS_SUCCESS);
+    AssertIntEQ(harness.io.inOff, harness.io.inSz);
+    AssertTrue(harness.io.outSz > firstOutSz);
+    AssertIntEQ(ParseMsgId(harness.io.out + firstOutSz,
+            harness.io.outSz - firstOutSz), MSGID_USERAUTH_FAILURE);
+
+    FreeChannelOpenHarness(&harness);
+}
+
+#endif /* WOLFSSH_KEYBOARD_INTERACTIVE */
+
 /* Needs server, client, key files, and one covered host-key algorithm. */
 #if !defined(NO_WOLFSSH_SERVER) && !defined(NO_WOLFSSH_CLIENT) && \
     !defined(NO_FILESYSTEM) && \
@@ -5097,6 +5194,10 @@ int main(int argc, char** argv)
     TestSecondSessionChannelRejected();
     TestUsernameChangeDisconnects();
     TestSameUserRetryAllowed();
+#ifdef WOLFSSH_KEYBOARD_INTERACTIVE
+    TestKbInfoResponseCountMismatchSendsFailure();
+    TestKbInfoResponseMismatchKeepsFraming();
+#endif
 #ifdef WOLFSSH_FWD
     TestDirectTcpipRejectSendsOpenFail();
     TestDirectTcpipNoFwdCbSendsOpenFail();