@ZaphodB fix(security): S/MIME verify reported altered messages as validly signed

OpenSSL::verify() called openssl_pkcs7_verify with PKCS7_NOSIGS, which skips
the signature check, and returned success whenever that call did. Any
well-formed signed message therefore came back as success: true, including one
whose content had been changed after signing, and the client shows that as a
good signature (Message.js smimeSigned). Applies to the message view, the
explicit verify action and signed content inside decrypted mail.

The call is also how the content of an opaque signed message is extracted for
display, so it stays as it is for that, and a second openssl_pkcs7_verify
without PKCS7_NOSIGS now decides success and does not throw on failure. A
message with a broken signature remains readable but is no longer reported as
signed. PKCS7_NOVERIFY is kept on purpose: the signer certificate is not checked
against a CA, so success means the content is what the certificate holder
signed, not that the certificate is trusted; the comment says so.

test/smime-verify.php signs detached and opaque messages with a throwaway key,
checks the intact ones verify, and that altering one byte of signed content
flips success to false while the content stays readable. Before this change
the altered detached message was reported as validly signed.

.gitignore

@@ -20,7 +20,6 @@
 /issues
 .idea/
 .env
-/test
 /public_html
 /vendors/knockout/spec
 /vendors/openpgp-5

snappymail/v/0.0.0/app/libraries/snappymail/smime/openssl.php

@@ -324,10 +324,11 @@ class OpenSSL
 			$opaque |= \str_contains($input, 'application/pkcs7-mime') || \str_contains($input, 'application/x-pkcs7-mime');
 			$input = $tmp;
 		}
+		// First pass extracts the content and nothing else: PKCS7_NOSIGS skips the
+		// signature check, so a message with a broken signature can still be read.
 		$output = $opaque ? new Temporary('smimeout-') : null;
 		if (true !== \openssl_pkcs7_verify(
 			$input->filename(),
-//			$flags = 0, // \PKCS7_NOVERIFY | \PKCS7_NOCHAIN | \PKCS7_NOSIGS
 			\PKCS7_NOVERIFY | \PKCS7_NOCHAIN | \PKCS7_NOSIGS,
 			$signers_certificates_filename ?: null,
 			$ca_info = [],
@@ -337,9 +338,23 @@ class OpenSSL
 		)) {
 			throw new \RuntimeException('OpenSSL verify: ' . \openssl_error_string());
 		}
+		// Second pass is the verdict. It used to be the first pass returning true,
+		// which with PKCS7_NOSIGS it does for any well-formed message, so altered
+		// or forged content was reported as validly signed. This checks the
+		// signature against the signer certificate in the message. PKCS7_NOVERIFY
+		// stays: the signer's chain is not checked against any CA, so 'success'
+		// means the content is what the certificate holder signed, not that the
+		// certificate itself is trusted.
+		$verified = \openssl_pkcs7_verify(
+			$input->filename(),
+			\PKCS7_NOVERIFY,
+			null,
+			[],
+			$this->untrusted_certificates_filename
+		);
 		return [
 			'body' => $output ? $output->getContents() : null,
-			'success' => true
+			'success' => true === $verified
 		];
 	}
 }

test/smime-verify.php

@@ -0,0 +1,69 @@
+<?php
+
+// Run with: php test/smime-verify.php
+spl_autoload_register(static function (string $class): void {
+	$base = dirname(__DIR__).'/tachyon/v/0.0.0/app/libraries/';
+	$file = str_starts_with($class, 'Tachyon\\Util\\')
+		? $base.'tachyon_util/'.strtolower(str_replace('\\', '/', substr($class, 13))).'.php'
+		: $base.str_replace('\\', '/', $class).'.php';
+	if (is_file($file)) {
+		require_once $file;
+	}
+});
+
+function check(bool $condition, string $message): void
+{
+	if (!$condition) {
+		throw new RuntimeException($message);
+	}
+}
+
+$dir = sys_get_temp_dir().'/smime-verify-test-'.getmypid();
+mkdir($dir);
+$key = openssl_pkey_new(['private_key_bits' => 2048, 'private_key_type' => OPENSSL_KEYTYPE_RSA]);
+$cert = openssl_csr_sign(openssl_csr_new(['commonName' => 'signer@example.com'], $key), null, $key, 1);
+openssl_x509_export_to_file($cert, "{$dir}/cert.pem");
+openssl_pkey_export_to_file($key, "{$dir}/key.pem");
+
+$content = "Content-Type: text/plain\r\n\r\nPay 100 EUR to account A.\r\n";
+file_put_contents("{$dir}/in.txt", $content);
+
+$smime = new \Tachyon\Util\SMime\OpenSSL($dir);
+
+// Detached (multipart/signed) and opaque (application/pkcs7-mime) signatures.
+foreach (['detached' => PKCS7_DETACHED, 'opaque' => 0] as $kind => $flags) {
+	check(openssl_pkcs7_sign("{$dir}/in.txt", "{$dir}/signed.eml", "file://{$dir}/cert.pem", "file://{$dir}/key.pem", [], $flags),
+		"{$kind}: signing failed: ".openssl_error_string());
+	$signed = file_get_contents("{$dir}/signed.eml");
+	$opaque = 'opaque' === $kind;
+
+	$result = $smime->verify($signed, null, $opaque);
+	check(true === $result['success'], "{$kind}: intact message not reported as validly signed");
+	if ($opaque) {
+		check(str_contains($result['body'], 'Pay 100 EUR to account A.'), "{$kind}: content not extracted");
+	}
+
+	// Alter the signed content. Detached: the clear text part. Opaque: a byte of
+	// the base64 payload, chosen inside the encapsulated content.
+	if ($opaque) {
+		[$head, $b64] = explode("\n\n", str_replace("\r\n", "\n", $signed), 2);
+		$der = base64_decode($b64);
+		$pos = strpos($der, 'account A');
+		check(false !== $pos, "{$kind}: test could not find the content to alter");
+		$der[$pos + 8] = 'B';
+		$tampered = $head."\n\n".chunk_split(base64_encode($der), 64, "\n");
+	} else {
+		$tampered = str_replace('account A', 'account B', $signed);
+	}
+	check($tampered !== $signed, "{$kind}: test failed to alter the message");
+
+	$result = $smime->verify($tampered, null, $opaque);
+	check(false === $result['success'], "{$kind}: ALTERED message reported as validly signed");
+	if ($opaque) {
+		check(str_contains($result['body'], 'account B'), "{$kind}: altered content no longer readable");
+	}
+}
+
+array_map('unlink', glob("{$dir}/*"));
+rmdir($dir);
+echo "smime-verify: ok\n";