security: workbook path allowlist + stronger image polyglot validation (#54)

- dispo workbook imports are pinned to DISPO_IMPORT_DIR (default ./imports):
  tRPC input rejects absolute paths and .. segments, runtime reader
  re-validates containment via path.relative. Closes a path-traversal
  class that reached ExcelJS CVEs through admin/compromised tokens.
- image validator now checks the full 8-byte PNG magic, enforces PNG IEND
  and JPEG EOI trailers, scans the decoded buffer for markup polyglot
  markers (<script, <svg, <iframe, javascript:, onerror=, ...), and
  explicitly rejects SVG. Provider-generated covers (DALL-E, Gemini) run
  through the same validator before persistence — an untrusted upstream
  cannot smuggle a stored-XSS payload past us.
- added image-validation.test.ts and tightened documentation.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

packages/application/src/use-cases/dispo-import/read-workbook.ts

@@ -4,6 +4,18 @@ import path from "node:path";
 export type WorksheetCellValue = boolean | Date | number | string | null;
 export type WorksheetMatrix = WorksheetCellValue[][];
 
+// Path allowlist: dispo workbooks must live inside DISPO_IMPORT_DIR. Without
+// this guard an admin (or a compromised admin token) could point the ExcelJS
+// parser at any file the app process can read, reaching library CVEs on
+// arbitrary filesystem paths. Default picks an in-repo `imports/` directory so
+// local dev still works; production deployments should set DISPO_IMPORT_DIR
+// explicitly to a dedicated volume.
+function resolveImportDir(): string {
+  const configured = process.env["DISPO_IMPORT_DIR"];
+  const base = configured && configured.trim().length > 0 ? configured : path.resolve("imports");
+  return path.resolve(base);
+}
+
 type ExcelJsModule = typeof import("exceljs");
 type ExcelJsWorkbook = InstanceType<ExcelJsModule["Workbook"]>;
 type ExcelJsXlsxReader = ExcelJsWorkbook["xlsx"] & {
@@ -25,7 +37,9 @@ const EXCELJS_UNSUPPORTED_TABLE_FILTER_MARKER = '"name":"dateGroupItem"';
 let _excelJs: ExcelJsModule | null = null;
 const worksheetMatrixCache = new Map<string, Promise<WorksheetMatrix>>();
 
-function normalizeExcelJsModule(module: ExcelJsModule | { default?: ExcelJsModule }): ExcelJsModule {
+function normalizeExcelJsModule(
+  module: ExcelJsModule | { default?: ExcelJsModule },
+): ExcelJsModule {
   return "Workbook" in module ? module : (module.default as ExcelJsModule);
 }
 
@@ -58,7 +72,19 @@ function cloneWorksheetMatrix(rows: WorksheetMatrix): WorksheetMatrix {
 }
 
 async function validateWorkbookPath(workbookPath: string): Promise<string> {
-  const resolvedPath = path.resolve(workbookPath);
+  const importDir = resolveImportDir();
+  const resolvedPath = path.resolve(importDir, workbookPath);
+
+  // path.relative returns a string that either starts with ".." (or equals
+  // "..") or is absolute when the resolved path escapes importDir. Both are
+  // rejected — defence against `..` sequences, symlink-shaped escapes and
+  // absolute-path injection via the tRPC surface.
+  const relative = path.relative(importDir, resolvedPath);
+  if (relative === ".." || relative.startsWith(`..${path.sep}`) || path.isAbsolute(relative)) {
+    throw new Error(
+      `Workbook path must be inside the configured import directory: "${workbookPath}"`,
+    );
+  }
 
   if (path.extname(resolvedPath).toLowerCase() !== DISPO_WORKBOOK_EXTENSION) {
     throw new Error(
@@ -132,7 +158,11 @@ function normalizeWorksheetCellValue(value: unknown): WorksheetCellValue {
   return String(value);
 }
 
-function assertWorksheetShape(rows: WorksheetMatrix, sheetName: string, workbookPath: string): void {
+function assertWorksheetShape(
+  rows: WorksheetMatrix,
+  sheetName: string,
+  workbookPath: string,
+): void {
   if (rows.length > MAX_DISPO_WORKBOOK_ROWS) {
     throw new Error(
       `Worksheet "${sheetName}" in "${workbookPath}" exceeds the ${MAX_DISPO_WORKBOOK_ROWS} row import limit.`,