diff --git a/internal/core/browserreceiver/receiver.go b/internal/core/browserreceiver/receiver.go
index 9d744c0..5d25874 100644
--- a/internal/core/browserreceiver/receiver.go
+++ b/internal/core/browserreceiver/receiver.go
@@ -4,8 +4,11 @@ package browserreceiver
 import (
 	"context"
 	"crypto/subtle"
+	"encoding/base64"
 	"encoding/json"
+	"errors"
 	"fmt"
+	"io"
 	"log"
 	"net"
 	"net/http"
@@ -20,6 +23,24 @@ const capturePath = "/api/browser-inbox/v1/captures"
 const DefaultAddr = "127.0.0.1:47731"
 const receiverTokenHeader = "X-Verstak-Receiver-Token"
 
+const (
+	maxCaptureBodyBytes    = 12 * 1024 * 1024
+	maxCaptureIDBytes      = 256
+	maxCapturedAtBytes     = 64
+	maxCaptureSourceBytes  = 128
+	maxPageURLBytes        = 4096
+	maxPageTitleBytes      = 512
+	maxPageDomainBytes     = 255
+	maxSelectionTextBytes  = 20 * 1024
+	maxLinkTextBytes       = 512
+	maxBrowserNameBytes    = 64
+	maxFileNameBytes       = 255
+	maxFileMimeBytes       = 128
+	maxFileTextBytes       = 2 * 1024 * 1024
+	maxFileBytes           = 8 * 1024 * 1024
+	maxFileDataBase64Bytes = 4 * ((maxFileBytes + 2) / 3)
+)
+
 type Receiver struct {
 	bus               *events.Bus
 	workspaceProvider WorkspaceProvider
@@ -145,8 +166,24 @@ func (r *Receiver) ServeHTTP(w http.ResponseWriter, req *http.Request) {
 		return
 	}
 
+	defer req.Body.Close()
+	decoder := json.NewDecoder(http.MaxBytesReader(w, req.Body, maxCaptureBodyBytes))
 	var payload CapturePayload
-	if err := json.NewDecoder(req.Body).Decode(&payload); err != nil {
+	if err := decoder.Decode(&payload); err != nil {
+		var maxBytesErr *http.MaxBytesError
+		if errors.As(err, &maxBytesErr) {
+			writeError(w, http.StatusRequestEntityTooLarge, "capture payload exceeds limit")
+			return
+		}
+		writeError(w, http.StatusBadRequest, "invalid JSON")
+		return
+	}
+	if err := decoder.Decode(&struct{}{}); err != io.EOF {
+		var maxBytesErr *http.MaxBytesError
+		if errors.As(err, &maxBytesErr) {
+			writeError(w, http.StatusRequestEntityTooLarge, "capture payload exceeds limit")
+			return
+		}
 		writeError(w, http.StatusBadRequest, "invalid JSON")
 		return
 	}
@@ -215,27 +252,105 @@ func (p CapturePayload) Validate() error {
 	if strings.TrimSpace(p.CaptureID) == "" {
 		return fmt.Errorf("captureId is required")
 	}
+	if err := validateCaptureText(p.CaptureID, "captureId", maxCaptureIDBytes); err != nil {
+		return err
+	}
 	if strings.TrimSpace(p.CapturedAt) == "" {
 		return fmt.Errorf("capturedAt is required")
 	}
+	if err := validateCaptureText(p.CapturedAt, "capturedAt", maxCapturedAtBytes); err != nil {
+		return err
+	}
 	if p.Kind != "page" && p.Kind != "selection" && p.Kind != "link" && p.Kind != "file" {
 		return fmt.Errorf("unsupported kind")
 	}
 	if strings.TrimSpace(p.Page.URL) == "" {
 		return fmt.Errorf("page.url is required")
 	}
+	if err := validateCaptureText(p.Source, "source", maxCaptureSourceBytes); err != nil {
+		return err
+	}
+	if err := validateCaptureText(p.Page.URL, "page.url", maxPageURLBytes); err != nil {
+		return err
+	}
+	if err := validateCaptureText(p.Page.Title, "page.title", maxPageTitleBytes); err != nil {
+		return err
+	}
+	if err := validateCaptureText(p.Page.Domain, "page.domain", maxPageDomainBytes); err != nil {
+		return err
+	}
+	if p.Browser != nil {
+		if err := validateCaptureText(p.Browser.Name, "browser.name", maxBrowserNameBytes); err != nil {
+			return err
+		}
+	}
 	if p.Kind == "selection" && (p.Selection == nil || strings.TrimSpace(p.Selection.Text) == "") {
 		return fmt.Errorf("selection.text is required")
 	}
-	if p.Kind == "link" && (p.Link == nil || strings.TrimSpace(p.Link.URL) == "") {
-		return fmt.Errorf("link.url is required")
+	if p.Kind == "selection" {
+		if err := validateCaptureText(p.Selection.Text, "selection.text", maxSelectionTextBytes); err != nil {
+			return err
+		}
 	}
-	if p.Kind == "file" && (p.File == nil || strings.TrimSpace(p.File.Name) == "") {
+	if p.Kind == "link" {
+		if p.Link == nil || strings.TrimSpace(p.Link.URL) == "" {
+			return fmt.Errorf("link.url is required")
+		}
+		if err := validateCaptureText(p.Link.URL, "link.url", maxPageURLBytes); err != nil {
+			return err
+		}
+		if err := validateCaptureText(p.Link.Text, "link.text", maxLinkTextBytes); err != nil {
+			return err
+		}
+	}
+	if p.Kind == "file" {
+		if err := p.validateFile(); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func (p CapturePayload) validateFile() error {
+	if p.File == nil || strings.TrimSpace(p.File.Name) == "" {
 		return fmt.Errorf("file.name is required")
 	}
-	if p.Kind == "file" && (p.File == nil || (p.File.Text == "" && strings.TrimSpace(p.File.DataBase64) == "")) {
+	if p.File.Text == "" && strings.TrimSpace(p.File.DataBase64) == "" {
 		return fmt.Errorf("file.text or file.dataBase64 is required")
 	}
+	if p.File.Size < 0 || p.File.Size > maxFileBytes {
+		return fmt.Errorf("file.size exceeds limit")
+	}
+	if err := validateCaptureText(p.File.Name, "file.name", maxFileNameBytes); err != nil {
+		return err
+	}
+	if err := validateCaptureText(p.File.Mime, "file.mime", maxFileMimeBytes); err != nil {
+		return err
+	}
+	if err := validateCaptureText(p.File.Text, "file.text", maxFileTextBytes); err != nil {
+		return err
+	}
+	dataBase64 := strings.TrimSpace(p.File.DataBase64)
+	if dataBase64 == "" {
+		return nil
+	}
+	if len(dataBase64) > maxFileDataBase64Bytes {
+		return fmt.Errorf("file.dataBase64 exceeds limit")
+	}
+	data, err := base64.StdEncoding.DecodeString(dataBase64)
+	if err != nil {
+		return fmt.Errorf("file.dataBase64 is invalid")
+	}
+	if len(data) > maxFileBytes {
+		return fmt.Errorf("file.dataBase64 exceeds limit")
+	}
+	return nil
+}
+
+func validateCaptureText(value, field string, maxBytes int) error {
+	if len(value) > maxBytes {
+		return fmt.Errorf("%s exceeds limit", field)
+	}
 	return nil
 }
 
diff --git a/internal/core/browserreceiver/receiver_test.go b/internal/core/browserreceiver/receiver_test.go
index 4beb920..1e90701 100644
--- a/internal/core/browserreceiver/receiver_test.go
+++ b/internal/core/browserreceiver/receiver_test.go
@@ -2,10 +2,13 @@ package browserreceiver
 
 import (
 	"bytes"
+	"encoding/base64"
 	"encoding/json"
+	"fmt"
 	"io"
 	"net/http"
 	"net/http/httptest"
+	"strings"
 	"testing"
 
 	"github.com/verstak/verstak-desktop/internal/core/events"
@@ -363,3 +366,96 @@ func TestReceiverRejectsInvalidCapturePayload(t *testing.T) {
 		t.Fatalf("response body = %q, want validation error", rec.Body.String())
 	}
 }
+
+func TestReceiverRejectsOversizedCaptureBody(t *testing.T) {
+	bus := events.NewBus()
+	received := make(chan events.Event, 1)
+	bus.Subscribe("browser.capture.page", func(event events.Event) {
+		received <- event
+	})
+	receiver := New(bus)
+
+	const maxCaptureBodyBytes = 12 * 1024 * 1024
+	body := fmt.Sprintf(`{
+		"schemaVersion": 1,
+		"captureId": "capture-oversized",
+		"capturedAt": "2026-06-27T00:00:00.000Z",
+		"kind": "page",
+		"page": {"url": "https://example.com", "title": %q}
+	}`, strings.Repeat("x", maxCaptureBodyBytes))
+	req := httptest.NewRequest(http.MethodPost, "/api/browser-inbox/v1/captures", strings.NewReader(body))
+	rec := httptest.NewRecorder()
+
+	receiver.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusRequestEntityTooLarge {
+		t.Fatalf("status = %d, want %d; body=%s", rec.Code, http.StatusRequestEntityTooLarge, rec.Body.String())
+	}
+	select {
+	case event := <-received:
+		t.Fatalf("unexpected event published for oversized capture: %#v", event)
+	default:
+	}
+}
+
+func TestCapturePayloadRejectsUnsafeFileContent(t *testing.T) {
+	const maxFileBytes = 8 * 1024 * 1024
+	const maxFileTextBytes = 2 * 1024 * 1024
+
+	newFilePayload := func() CapturePayload {
+		return CapturePayload{
+			SchemaVersion: 1,
+			CaptureID:     "capture-file-validation",
+			CapturedAt:    "2026-06-27T00:00:00.000Z",
+			Kind:          "file",
+			Page:          CapturePage{URL: "https://example.com"},
+			File:          &CaptureFile{Name: "attachment.bin", Size: 1, DataBase64: "AQ=="},
+		}
+	}
+
+	tests := []struct {
+		name string
+		edit func(*CapturePayload)
+		want string
+	}{
+		{
+			name: "oversized declared size",
+			edit: func(payload *CapturePayload) {
+				payload.File.Size = maxFileBytes + 1
+			},
+			want: "file.size exceeds limit",
+		},
+		{
+			name: "invalid base64",
+			edit: func(payload *CapturePayload) {
+				payload.File.DataBase64 = "not base64"
+			},
+			want: "file.dataBase64 is invalid",
+		},
+		{
+			name: "oversized decoded data",
+			edit: func(payload *CapturePayload) {
+				payload.File.DataBase64 = base64.StdEncoding.EncodeToString(make([]byte, maxFileBytes+1))
+			},
+			want: "file.dataBase64 exceeds limit",
+		},
+		{
+			name: "oversized text",
+			edit: func(payload *CapturePayload) {
+				payload.File.DataBase64 = ""
+				payload.File.Text = strings.Repeat("x", maxFileTextBytes+1)
+			},
+			want: "file.text exceeds limit",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			payload := newFilePayload()
+			tt.edit(&payload)
+			if err := payload.Validate(); err == nil || !strings.Contains(err.Error(), tt.want) {
+				t.Fatalf("Validate() error = %v, want %q", err, tt.want)
+			}
+		})
+	}
+}