Reject packets with too-little padding

- enforce RFC 4253 section 6 minimum of 4 padding bytes on receive
- return WS_BUFFER_E when padding_length is below MIN_PAD_LENGTH
- add negative unit test driving DoReceive with a short-padded packet

Issue: #1047 (5)

Author: ejohnstown

src/internal.c

@@ -10273,6 +10273,13 @@ static int DoPacket(WOLFSSH* ssh, byte* bufferConsumed)
     idx += UINT32_SZ;
     padSz = buf[idx++];
 
+    /* RFC 4253 section 6 requires at least four bytes of padding. */
+    if (padSz < MIN_PAD_LENGTH) {
+        WLOG(WS_LOG_DEBUG, "Packet padding length %u below minimum %u",
+             (word32)padSz, (word32)MIN_PAD_LENGTH);
+        return WS_BUFFER_E;
+    }
+
     /* check for underflow */
     if ((word32)(PAD_LENGTH_SZ + padSz + MSG_ID_SZ) > ssh->curSz) {
         return WS_OVERFLOW_E;

tests/unit.c

@@ -950,6 +950,78 @@ static int test_DoReceive_VerifyMacFailure(void)
 #endif /* WOLFSSH_TEST_INTERNAL && any HMAC SHA variant enabled */
 
 
+#ifdef WOLFSSH_TEST_INTERNAL
+/* Verify DoReceive rejects a binary packet whose padding_length is below the
+ * RFC 4253 section 6 minimum of four bytes, returning WS_BUFFER_E. The packet
+ * is delivered in the clear (no cipher, no MAC), matching the pre-key-exchange
+ * transport, so DoPacket's padding check is what rejects it. */
+static int test_DoReceive_RejectsShortPadding(void)
+{
+    WOLFSSH_CTX* ctx = NULL;
+    WOLFSSH* ssh = NULL;
+    int ret;
+    int result = 0;
+    /* A well-formed MSGID_IGNORE packet carrying an empty string, but with
+     * padding_length = 1 (below MIN_PAD_LENGTH). Aside from the short padding
+     * the packet parses cleanly, so the padding check is the only thing that
+     * can reject it. Layout: uint32 packet_length=7, padding_length=1,
+     * msgId, uint32 string_len=0, 1 pad byte => 11 bytes total. */
+    byte pkt[11];
+    word32 totalLen = (word32)sizeof(pkt);
+
+    ctx = wolfSSH_CTX_new(WOLFSSH_ENDPOINT_CLIENT, NULL);
+    if (ctx == NULL)
+        return -760;
+    ssh = wolfSSH_new(ctx);
+    if (ssh == NULL) {
+        wolfSSH_CTX_free(ctx);
+        return -761;
+    }
+
+    pkt[0] = 0; pkt[1] = 0; pkt[2] = 0; pkt[3] = 7; /* packet_length */
+    pkt[4] = 1;             /* padding_length, below MIN_PAD_LENGTH (4) */
+    pkt[5] = MSGID_IGNORE;
+    pkt[6] = 0; pkt[7] = 0; pkt[8] = 0; pkt[9] = 0; /* string_len = 0 */
+    pkt[10] = 0;            /* padding */
+
+    ssh->peerEncryptId = ID_NONE;
+    ssh->peerAeadMode = 0;
+    ssh->peerBlockSz = MIN_BLOCK_SZ;
+    ssh->peerMacId = ID_NONE;
+    ssh->peerMacSz = 0;
+    ssh->peerSeq = 0;
+    ssh->curSz = 0;
+    ssh->processReplyState = PROCESS_INIT;
+    ssh->error = 0;
+
+    ShrinkBuffer(&ssh->inputBuffer, 1);
+    ret = GrowBuffer(&ssh->inputBuffer, totalLen);
+    if (ret != WS_SUCCESS) {
+        result = -762;
+        goto done2;
+    }
+    WMEMCPY(ssh->inputBuffer.buffer, pkt, totalLen);
+    ssh->inputBuffer.length = totalLen;
+    ssh->inputBuffer.idx = 0;
+
+    ret = wolfSSH_TestDoReceive(ssh);
+    if (ret != WS_FATAL_ERROR) {
+        result = -763;
+        goto done2;
+    }
+    if (ssh->error != WS_BUFFER_E) {
+        result = -764;
+        goto done2;
+    }
+
+done2:
+    wolfSSH_free(ssh);
+    wolfSSH_CTX_free(ctx);
+    return result;
+}
+#endif /* WOLFSSH_TEST_INTERNAL */
+
+
 #if defined(WOLFSSH_TEST_INTERNAL) && !defined(WOLFSSH_NO_DH_GEX_SHA256)
 
 typedef struct {
@@ -5332,6 +5404,13 @@ int wolfSSH_UnitTest(int argc, char** argv)
     testResult = testResult || unitResult;
 #endif
 
+#ifdef WOLFSSH_TEST_INTERNAL
+    unitResult = test_DoReceive_RejectsShortPadding();
+    printf("DoReceiveRejectsShortPadding: %s\n",
+            (unitResult == 0 ? "SUCCESS" : "FAILED"));
+    testResult = testResult || unitResult;
+#endif
+
 #if defined(WOLFSSH_TEST_INTERNAL) && !defined(WOLFSSH_NO_DH_GEX_SHA256)
     unitResult = test_DhGexGroupValidate();
     printf("DhGexGroupValidate: %s\n",