diff --git a/.changeset/tighten-credentials-permissions.md b/.changeset/tighten-credentials-permissions.md
new file mode 100644
index 00000000..0aa781bf
--- /dev/null
+++ b/.changeset/tighten-credentials-permissions.md
@@ -0,0 +1,5 @@
+---
+"nansen-cli": patch
+---
+
+Tighten POSIX permissions when rewriting the fallback wallet credentials file and refuse non-regular credential paths.
diff --git a/src/__tests__/keychain.test.js b/src/__tests__/keychain.test.js
index 7b1cb858..cf63a689 100644
--- a/src/__tests__/keychain.test.js
+++ b/src/__tests__/keychain.test.js
@@ -72,6 +72,42 @@ describe('keychain', () => {
       expect(content).toContain('NANSEN_WALLET_PASSWORD_B64=');
       const b64 = content.match(/NANSEN_WALLET_PASSWORD_B64=(.+)/)[1].trim();
       expect(Buffer.from(b64, 'base64').toString('utf8')).toBe('mypassword');
+      if (originalPlatform !== 'win32') {
+        expect(fs.statSync(credPath).mode & 0o777).toBe(0o600);
+      }
+    });
+
+    it.skipIf(process.platform === 'win32')('should tighten permissions when rewriting an existing credentials file on POSIX', () => {
+      setPlatform('linux');
+      execFileSync.mockImplementation(() => { throw new Error('secret-tool unavailable'); });
+
+      const credDir = path.join(tempDir, '.nansen', 'wallets');
+      const credPath = path.join(credDir, '.credentials');
+      fs.mkdirSync(credDir, { recursive: true });
+      fs.writeFileSync(credPath, 'old credentials\n');
+      // writeFileSync's mode is filtered by the process umask, so set and
+      // verify the insecure starting state explicitly.
+      fs.chmodSync(credPath, 0o644);
+      expect(fs.statSync(credPath).mode & 0o777).toBe(0o644);
+
+      expect(storePassword('replacement-password')).toEqual({ stored: true, method: 'file' });
+      expect(fs.statSync(credPath).mode & 0o777).toBe(0o600);
+      expect(retrievePassword()).toEqual({ password: 'replacement-password', source: 'file' });
+    });
+
+    it.skipIf(process.platform === 'win32')('should refuse to overwrite a credentials symlink on POSIX', () => {
+      setPlatform('linux');
+      execFileSync.mockImplementation(() => { throw new Error('secret-tool unavailable'); });
+
+      const credDir = path.join(tempDir, '.nansen', 'wallets');
+      const credPath = path.join(credDir, '.credentials');
+      const targetPath = path.join(tempDir, 'symlink-target');
+      fs.mkdirSync(credDir, { recursive: true });
+      fs.writeFileSync(targetPath, 'leave unchanged\n');
+      fs.symlinkSync(targetPath, credPath);
+
+      expect(storePassword('replacement-password')).toEqual({ stored: false, method: 'none' });
+      expect(fs.readFileSync(targetPath, 'utf8')).toBe('leave unchanged\n');
     });
 
     it('should fall back to .credentials on unsupported platform', () => {
diff --git a/src/keychain.js b/src/keychain.js
index a7743edd..e37b7db3 100644
--- a/src/keychain.js
+++ b/src/keychain.js
@@ -151,8 +151,33 @@ function credentialsFileWrite(password) {
       fs.mkdirSync(dir, { mode: 0o700, recursive: true });
     }
     const encoded = Buffer.from(password, 'utf8').toString('base64');
-    fs.writeFileSync(filePath, `NANSEN_WALLET_PASSWORD_B64=${encoded}\n`, { mode: 0o600 });
-    return true;
+    const content = `NANSEN_WALLET_PASSWORD_B64=${encoded}\n`;
+
+    // Windows does not implement POSIX modes or O_NOFOLLOW consistently.
+    if (process.platform === 'win32') {
+      fs.writeFileSync(filePath, content, { mode: 0o600 });
+      return true;
+    }
+
+    let fd;
+    try {
+      const flags = fs.constants.O_WRONLY
+        | fs.constants.O_CREAT
+        | fs.constants.O_NOFOLLOW
+        | fs.constants.O_NONBLOCK;
+      fd = fs.openSync(filePath, flags, 0o600);
+      if (!fs.fstatSync(fd).isFile()) throw new Error('Credentials path is not a regular file');
+
+      // Opening an existing file does not apply the requested mode. Tighten it
+      // before replacing the secret, then enforce the final mode after writing.
+      fs.fchmodSync(fd, 0o600);
+      fs.ftruncateSync(fd, 0);
+      fs.writeFileSync(fd, content, 'utf8');
+      fs.fchmodSync(fd, 0o600);
+      return true;
+    } finally {
+      if (fd !== undefined) fs.closeSync(fd);
+    }
   } catch {
     return false;
   }