diff --git a/CHANGELOG.md b/CHANGELOG.md
index f82cf606f75..7ecf12602fb 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -36,6 +36,7 @@ Docs: https://docs.openclaw.ai
 - Gateway HTTP: match models, session kill, and session history route paths without trusting malformed Host headers, avoiding pre-auth 500s on those endpoints.
 - Google Meet/Codex: report malformed node proxy `payloadJSON` responses with plugin-owned errors instead of leaking raw JSON parser failures.
 - Debug proxy: reject malformed relative-form proxy targets with a controlled 400 response instead of letting URL parsing escape the request handler.
+- File transfer: reject malformed inline `file_write` base64 before computing hashes or invoking paired nodes, avoiding Node's lenient base64 decoder.
 - Models config/auth: stop inferring provider env-var markers from broad `^[A-Z_][A-Z0-9_]*$` strings, and resolve config-backed provider `apiKey` values only through structured env SecretRefs (`secrets.providers[id]` / `secrets.defaults`), so unrelated env vars cannot accidentally become provider credentials. Thanks @sallyom.
 - Media fetch: skip allocating and buffering the response body for bodyless media responses (HEAD probes and 204-style empty bodies), avoiding wasted heap on streams that carry no payload. Thanks @shakkernerd.
 - CLI/onboarding: forward provider-specific auth flags (e.g. `--openai-api-key`) through the onboarding wizard so they reach provider auth methods via `ctx.opts`, letting `--openai-api-key "$OPENAI_API_KEY"` skip the redundant "use existing env var?" prompt in non-interactive harnesses. (#81669) Thanks @sjf.
diff --git a/extensions/file-transfer/src/tools/file-write-tool.test.ts b/extensions/file-transfer/src/tools/file-write-tool.test.ts
new file mode 100644
index 00000000000..e6b47bb01b5
--- /dev/null
+++ b/extensions/file-transfer/src/tools/file-write-tool.test.ts
@@ -0,0 +1,29 @@
+import { callGatewayTool } from "openclaw/plugin-sdk/agent-harness-runtime";
+import { describe, expect, it, vi } from "vitest";
+import { createFileWriteTool } from "./file-write-tool.js";
+
+vi.mock("openclaw/plugin-sdk/agent-harness-runtime", () => ({
+  callGatewayTool: vi.fn(),
+  listNodes: vi.fn(),
+  resolveNodeIdFromList: vi.fn(),
+}));
+
+vi.mock("openclaw/plugin-sdk/media-store", () => ({
+  readMediaBuffer: vi.fn(),
+}));
+
+describe("file_write tool", () => {
+  it("rejects malformed inline base64 before invoking the node", async () => {
+    const tool = createFileWriteTool();
+
+    await expect(
+      tool.execute("tool-call-1", {
+        node: "node-1",
+        path: "/tmp/out.txt",
+        contentBase64: "AAA@@@",
+      }),
+    ).rejects.toThrow("contentBase64 is not valid base64");
+
+    expect(callGatewayTool).not.toHaveBeenCalled();
+  });
+});
diff --git a/extensions/file-transfer/src/tools/file-write-tool.ts b/extensions/file-transfer/src/tools/file-write-tool.ts
index e900cdb430c..2ae3260bdd6 100644
--- a/extensions/file-transfer/src/tools/file-write-tool.ts
+++ b/extensions/file-transfer/src/tools/file-write-tool.ts
@@ -21,6 +21,18 @@ import {
   FILE_WRITE_TOOL_DESCRIPTOR,
 } from "./descriptors.js";
 
+function normalizeBase64ForCompare(value: string): string {
+  return value.replace(/=+$/u, "").replace(/-/gu, "+").replace(/_/gu, "/");
+}
+
+function decodeStrictBase64(value: string): Buffer {
+  const buffer = Buffer.from(value, "base64");
+  if (normalizeBase64ForCompare(buffer.toString("base64")) !== normalizeBase64ForCompare(value)) {
+    throw new Error("contentBase64 is not valid base64");
+  }
+  return buffer;
+}
+
 async function readSourceBytes(input: {
   contentBase64?: string;
   sourceMediaId?: string;
@@ -37,7 +49,7 @@ async function readSourceBytes(input: {
   if (input.contentBase64 === undefined) {
     throw new Error("contentBase64 or sourceMediaId required");
   }
-  const buffer = Buffer.from(input.contentBase64, "base64");
+  const buffer = decodeStrictBase64(input.contentBase64);
   return { buffer, contentBase64: input.contentBase64, source: "inline" };
 }