diff --git a/src/line/monitor.ts b/src/line/monitor.ts
index 9b40e4460..c6241d97d 100644
--- a/src/line/monitor.ts
+++ b/src/line/monitor.ts
@@ -1,10 +1,10 @@
 import type { WebhookRequestBody } from "@line/bot-sdk";
 import type { IncomingMessage, ServerResponse } from "node:http";
-import crypto from "node:crypto";
 import type { ClawdbotConfig } from "../config/config.js";
 import { danger, logVerbose } from "../globals.js";
 import type { RuntimeEnv } from "../runtime.js";
 import { createLineBot } from "./bot.js";
+import { validateLineSignature } from "./signature.js";
 import { normalizePluginHttpPath } from "../plugins/http-path.js";
 import { registerPluginHttpRoute } from "../plugins/http-registry.js";
 import {
@@ -85,11 +85,6 @@ export function getLineRuntimeState(accountId: string) {
   return runtimeState.get(`line:${accountId}`);
 }
 
-function validateLineSignature(body: string, signature: string, channelSecret: string): boolean {
-  const hash = crypto.createHmac("SHA256", channelSecret).update(body).digest("base64");
-  return hash === signature;
-}
-
 async function readRequestBody(req: IncomingMessage): Promise<string> {
   return new Promise((resolve, reject) => {
     const chunks: Buffer[] = [];
diff --git a/src/line/signature.test.ts b/src/line/signature.test.ts
new file mode 100644
index 000000000..8bd9b1f3f
--- /dev/null
+++ b/src/line/signature.test.ts
@@ -0,0 +1,27 @@
+import crypto from "node:crypto";
+import { describe, expect, it } from "vitest";
+import { validateLineSignature } from "./signature.js";
+
+const sign = (body: string, secret: string) =>
+  crypto.createHmac("SHA256", secret).update(body).digest("base64");
+
+describe("validateLineSignature", () => {
+  it("accepts valid signatures", () => {
+    const secret = "secret";
+    const rawBody = JSON.stringify({ events: [{ type: "message" }] });
+
+    expect(validateLineSignature(rawBody, sign(rawBody, secret), secret)).toBe(true);
+  });
+
+  it("rejects signatures computed with the wrong secret", () => {
+    const rawBody = JSON.stringify({ events: [{ type: "message" }] });
+
+    expect(validateLineSignature(rawBody, sign(rawBody, "wrong-secret"), "secret")).toBe(false);
+  });
+
+  it("rejects signatures with a different length", () => {
+    const rawBody = JSON.stringify({ events: [{ type: "message" }] });
+
+    expect(validateLineSignature(rawBody, "short", "secret")).toBe(false);
+  });
+});
diff --git a/src/line/signature.ts b/src/line/signature.ts
new file mode 100644
index 000000000..771a950ff
--- /dev/null
+++ b/src/line/signature.ts
@@ -0,0 +1,18 @@
+import crypto from "node:crypto";
+
+export function validateLineSignature(
+  body: string,
+  signature: string,
+  channelSecret: string,
+): boolean {
+  const hash = crypto.createHmac("SHA256", channelSecret).update(body).digest("base64");
+  const hashBuffer = Buffer.from(hash);
+  const signatureBuffer = Buffer.from(signature);
+
+  // Use constant-time comparison to prevent timing attacks.
+  if (hashBuffer.length !== signatureBuffer.length) {
+    return false;
+  }
+
+  return crypto.timingSafeEqual(hashBuffer, signatureBuffer);
+}
diff --git a/src/line/webhook.ts b/src/line/webhook.ts
index 846d8d796..9986617f9 100644
--- a/src/line/webhook.ts
+++ b/src/line/webhook.ts
@@ -1,8 +1,8 @@
 import type { Request, Response, NextFunction } from "express";
-import crypto from "node:crypto";
 import type { WebhookRequestBody } from "@line/bot-sdk";
 import { logVerbose, danger } from "../globals.js";
 import type { RuntimeEnv } from "../runtime.js";
+import { validateLineSignature } from "./signature.js";
 
 export interface LineWebhookOptions {
   channelSecret: string;
@@ -10,20 +10,6 @@ export interface LineWebhookOptions {
   runtime?: RuntimeEnv;
 }
 
-function validateSignature(body: string, signature: string, channelSecret: string): boolean {
-  const hash = crypto.createHmac("SHA256", channelSecret).update(body).digest("base64");
-  const hashBuffer = Buffer.from(hash);
-  const signatureBuffer = Buffer.from(signature);
-
-  // Use constant-time comparison to prevent timing attacks
-  // Ensure buffers are same length before comparison to prevent timing leak
-  if (hashBuffer.length !== signatureBuffer.length) {
-    return false;
-  }
-
-  return crypto.timingSafeEqual(hashBuffer, signatureBuffer);
-}
-
 function readRawBody(req: Request): string | null {
   const rawBody =
     (req as { rawBody?: string | Buffer }).rawBody ??
@@ -61,7 +47,7 @@ export function createLineWebhookMiddleware(options: LineWebhookOptions) {
         return;
       }
 
-      if (!validateSignature(rawBody, signature, channelSecret)) {
+      if (!validateLineSignature(rawBody, signature, channelSecret)) {
         logVerbose("line: webhook signature validation failed");
         res.status(401).json({ error: "Invalid signature" });
         return;