diff --git a/__tests__/main.test.js b/__tests__/main.test.js
index ef41967..c749741 100644
--- a/__tests__/main.test.js
+++ b/__tests__/main.test.js
@@ -179,3 +179,54 @@ test('test patch level with latest patches only and unstable throws error', () =
     'The options "unstable" and "latest-patches-only" cannot be used together'
   )
 })
+
+import path from 'path'
+import os from 'os'
+import {validateWorkingDirectory} from '../src/main.js'
+
+describe('validateWorkingDirectory', () => {
+  test('rejects path traversal via ../..', () => {
+    expect(() => {
+      validateWorkingDirectory('../..', '/tmp/workspace')
+    }).toThrow(
+      'working-directory must resolve to a path inside the workspace'
+    )
+  })
+
+  test('rejects absolute path outside workspace', () => {
+    expect(() => {
+      validateWorkingDirectory('/etc', '/tmp/workspace')
+    }).toThrow(
+      'working-directory must resolve to a path inside the workspace'
+    )
+  })
+
+  test('accepts empty working-directory (workspace root)', () => {
+    expect(() => {
+      validateWorkingDirectory('', '/tmp/workspace')
+    }).not.toThrow()
+  })
+
+  test('accepts valid subdirectory inside workspace', () => {
+    expect(() => {
+      validateWorkingDirectory('subdir', '/tmp/workspace')
+    }).not.toThrow()
+  })
+
+  test('rejects symlink inside workspace pointing outside', () => {
+    const workspace = fs.mkdtempSync(path.join(os.tmpdir(), 'ws-'))
+    const outside = fs.mkdtempSync(path.join(os.tmpdir(), 'outside-'))
+    const symlinkInWorkspace = path.join(workspace, 'escape')
+    fs.symlinkSync(outside, symlinkInWorkspace)
+    try {
+      expect(() => {
+        validateWorkingDirectory('escape', workspace)
+      }).toThrow(
+        'working-directory must resolve to a path inside the workspace'
+      )
+    } finally {
+      fs.rmSync(workspace, {recursive: true, force: true})
+      fs.rmSync(outside, {recursive: true, force: true})
+    }
+  })
+})
diff --git a/src/main.js b/src/main.js
index b3c5735..1cd96ca 100644
--- a/src/main.js
+++ b/src/main.js
@@ -1,3 +1,5 @@
+import path from 'path'
+import fs from 'fs'
 import * as core from '@actions/core'
 import {
   getGoModVersion,
@@ -9,6 +11,37 @@ import {
   modulename
 } from './go-versions.js'
 
+export function validateWorkingDirectory(workingDirectory, workspace) {
+  const resolvedDirectory = path.resolve(workspace, workingDirectory)
+  if (
+    resolvedDirectory !== workspace &&
+    !resolvedDirectory.startsWith(workspace + path.sep)
+  ) {
+    throw new Error(
+      'working-directory must resolve to a path inside the workspace'
+    )
+  }
+  // Resolve symlinks to prevent symlink-based workspace escapes
+  try {
+    const realDirectory = fs.realpathSync(resolvedDirectory)
+    const realWorkspace = fs.realpathSync(workspace)
+    if (
+      realDirectory !== realWorkspace &&
+      !realDirectory.startsWith(realWorkspace + path.sep)
+    ) {
+      throw new Error(
+        'working-directory must resolve to a path inside the workspace'
+      )
+    }
+  } catch (e) {
+    if (e.code !== 'ENOENT') {
+      throw e
+    }
+    // Directory does not exist yet; the downstream file read will handle it
+  }
+  return resolvedDirectory
+}
+
 async function run() {
   try {
     if (process.env.GITHUB_TOKEN !== undefined) {
@@ -23,7 +56,9 @@ async function run() {
     const withPatchLevel = core.getBooleanInput('patch-level')
     const withLatestPatches = core.getBooleanInput('latest-patches-only')
     const withStrictSemver = core.getBooleanInput('strict-semver')
-    const content = gomod(`${workingDirectory}/go.mod`)
+    const workspace = path.resolve(process.env.GITHUB_WORKSPACE || process.cwd())
+    const resolvedDirectory = validateWorkingDirectory(workingDirectory, workspace)
+    const content = gomod(path.join(resolvedDirectory, 'go.mod'))
     const name = modulename(content)
     const goModVersion = getGoModVersion(content)
     const versions = await getVersions(withUnsupported)
@@ -74,4 +109,7 @@ async function run() {
   }
 }
 
-run()
+/* istanbul ignore next */
+if (process.env.NODE_ENV !== 'test') {
+  run()
+}