From 96dfd842b1413173623925072864a326d629577d Mon Sep 17 00:00:00 2001
From: Sarah Krakowiak Criel <skrakowi@chromium.org>
Date: Thu, 30 Jul 2026 12:14:33 -0700
Subject: [PATCH] unrar: skip key derivation when no password is supplied

When opening header-encrypted RAR archives without a caller-supplied
password, UnRAR attempts decryption using a dummy placeholder password.
This causes UnRAR to execute PBKDF2-HMAC-SHA256 for every encrypted
header and entry, even though decryption will never succeed. On archives
specifying the maximum KDF iteration exponent (2^24) and distinct salts,
this wastes significant CPU time on the sequenced task runner.

Set UnRAR's SkipEncrypted option when no password is provided so that
encrypted archive headers are reported as undecryptable immediately
without running KDF. Also short-circuit ExtractNextEntry for encrypted
file headers when password is empty to take the metadata reporting path
directly without attempting key derivation.

Add a unit test (AnalyzeEncryptedRarMaxIterationCount) and test archive
to verify entries are enumerated quickly without executing key
derivation.

The test archive
(chrome/test/data/safe_browsing/rar/encrypted_high_kdf.rar) was
generated by constructing a RAR5 archive with 8 dummy encrypted file
entries (e0.dat .. e7.dat) and 1 trailing unencrypted executable payload
(payload.exe). Each encrypted entry's FHEXTRA_CRYPT record was set to
the maximum KDF iteration exponent (Lg2Count = 24, corresponding to 2^24
= 16,777,216 PBKDF2 iterations) with distinct 16-byte salts.

TAG=agy

Bug: 502611474
Change-Id: Idc77ca1d39970cf17c2f8f5565e66c12bf496b9e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8160612
Reviewed-by: Yaw Frempong <yawfrempong@google.com>
Commit-Queue: Sarah Krakowiak <skrakowi@chromium.org>
SLSA-Policy-Verified: SLSA Policy Verification Service <devtools-gerritcodereview-exitgate@google.com>
Auto-Submit: Sarah Krakowiak <skrakowi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1671306}
---

diff --git a/chrome/services/file_util/public/cpp/sandboxed_rar_analyzer_unittest.cc b/chrome/services/file_util/public/cpp/sandboxed_rar_analyzer_unittest.cc
index 9004060..15c3c5e 100644
--- a/chrome/services/file_util/public/cpp/sandboxed_rar_analyzer_unittest.cc
+++ b/chrome/services/file_util/public/cpp/sandboxed_rar_analyzer_unittest.cc
@@ -281,6 +281,34 @@
             EncryptionInfo::kKnownIncorrect);
 }
 
+TEST_F(SandboxedRarAnalyzerTest, AnalyzeEncryptedRarMaxIterationCount) {
+  // Can quickly enumerate entries in an archive whose encrypted file headers
+  // specify the maximum permitted KDF iteration count, even though no
+  // password is supplied. Key derivation is unnecessary in this case because
+  // there is no candidate password to verify.
+  // encrypted_high_kdf.rar contains: e0.dat .. e7.dat (encrypted), payload.exe
+  base::FilePath path;
+  ASSERT_NO_FATAL_FAILURE(path = GetFilePath("encrypted_high_kdf.rar"));
+
+  safe_browsing::ArchiveAnalyzerResults results;
+  AnalyzeFile(path, &results);
+
+  ASSERT_TRUE(results.success);
+  EXPECT_TRUE(results.has_executable);
+  ASSERT_EQ(results.archived_binary.size(), 9);
+  EXPECT_EQ(results.archived_binary[0].file_path(), "e0.dat");
+  EXPECT_TRUE(results.archived_binary[0].is_encrypted());
+  EXPECT_EQ(results.archived_binary[7].file_path(), "e7.dat");
+  EXPECT_TRUE(results.archived_binary[7].is_encrypted());
+  EXPECT_EQ(results.archived_binary[8].file_path(), "payload.exe");
+  EXPECT_TRUE(results.archived_binary[8].is_executable());
+  EXPECT_FALSE(results.archived_binary[8].is_encrypted());
+
+  EXPECT_TRUE(results.encryption_info.is_encrypted);
+  EXPECT_EQ(results.encryption_info.password_status,
+            EncryptionInfo::kKnownIncorrect);
+}
+
 TEST_F(SandboxedRarAnalyzerTest, AnalyzeRarWithPasswordMultipleFiles) {
   // Can list files inside an archive that has password protected data.
   // passwd1234_two_files.rar contains 2 files: signed.exe and text.txt
diff --git a/chrome/test/data/safe_browsing/rar/encrypted_high_kdf.rar b/chrome/test/data/safe_browsing/rar/encrypted_high_kdf.rar
new file mode 100644
index 0000000..6e215af
--- /dev/null
+++ b/chrome/test/data/safe_browsing/rar/encrypted_high_kdf.rar
Binary files differ
diff --git a/third_party/unrar/google/unrar_wrapper.cc b/third_party/unrar/google/unrar_wrapper.cc
index 7ea986f..6fd9e9b9 100644
--- a/third_party/unrar/google/unrar_wrapper.cc
+++ b/third_party/unrar/google/unrar_wrapper.cc
@@ -65,6 +65,14 @@
   command_->ParseArg(password_flag.data());
   command_->ParseArg(const_cast<wchar_t*>(L"t"));
   command_->ParseDone();
+  if (password_.empty()) {
+    // Without a real password there is nothing useful key derivation can
+    // produce, so tell UnRAR to skip encrypted headers and entries instead of
+    // attempting decryption with the dummy password above. The work avoided
+    // scales with the archive's KDF iteration parameter and the number of
+    // distinct salts.
+    command_->SkipEncrypted = true;
+  }
   // Disables an optimization that can allow specially crafted archives to
   // bypass analysis. See crbug.com/506473226.
   command_->Recurse = RECURSE_ALWAYS;
@@ -100,8 +108,15 @@
     temp_file_.SetLength(0);
     size_t header_size = archive_->ReadHeader();
     repeat = false;
-    success = extractor_->ExtractCurrentFile(
-        *archive_, header_size, repeat);  // |repeat| is passed by reference
+    // When no password was supplied, attempting to decrypt the entry will
+    // never succeed; skip extraction and report the available metadata.
+    bool skip_encrypted_entry = password_.empty() &&
+                                archive_->GetHeaderType() == HEAD_FILE &&
+                                archive_->FileHead.Encrypted;
+    success =
+        !skip_encrypted_entry &&
+        extractor_->ExtractCurrentFile(
+            *archive_, header_size, repeat);  // |repeat| is passed by reference
 
     if (archive_->GetHeaderType() == HEAD_FILE) {
 #if defined(OS_WIN)

diff --git a/chrome/services/file_util/public/cpp/sandboxed_rar_analyzer_unittest.cc b/chrome/services/file_util/public/cpp/sandboxed_rar_analyzer_unittest.cc
index 9004060..15c3c5e 100644
--- a/chrome/services/file_util/public/cpp/sandboxed_rar_analyzer_unittest.cc
+++ b/chrome/services/file_util/public/cpp/sandboxed_rar_analyzer_unittest.cc
@@ -281,6 +281,34 @@
             EncryptionInfo::kKnownIncorrect);
 }
 
+TEST_F(SandboxedRarAnalyzerTest, AnalyzeEncryptedRarMaxIterationCount) {
+  // Can quickly enumerate entries in an archive whose encrypted file headers
+  // specify the maximum permitted KDF iteration count, even though no
+  // password is supplied. Key derivation is unnecessary in this case because
+  // there is no candidate password to verify.
+  // encrypted_high_kdf.rar contains: e0.dat .. e7.dat (encrypted), payload.exe
+  base::FilePath path;
+  ASSERT_NO_FATAL_FAILURE(path = GetFilePath("encrypted_high_kdf.rar"));
+
+  safe_browsing::ArchiveAnalyzerResults results;
+  AnalyzeFile(path, &results);
+
+  ASSERT_TRUE(results.success);
+  EXPECT_TRUE(results.has_executable);
+  ASSERT_EQ(results.archived_binary.size(), 9);
+  EXPECT_EQ(results.archived_binary[0].file_path(), "e0.dat");
+  EXPECT_TRUE(results.archived_binary[0].is_encrypted());
+  EXPECT_EQ(results.archived_binary[7].file_path(), "e7.dat");
+  EXPECT_TRUE(results.archived_binary[7].is_encrypted());
+  EXPECT_EQ(results.archived_binary[8].file_path(), "payload.exe");
+  EXPECT_TRUE(results.archived_binary[8].is_executable());
+  EXPECT_FALSE(results.archived_binary[8].is_encrypted());
+
+  EXPECT_TRUE(results.encryption_info.is_encrypted);
+  EXPECT_EQ(results.encryption_info.password_status,
+            EncryptionInfo::kKnownIncorrect);
+}
+
 TEST_F(SandboxedRarAnalyzerTest, AnalyzeRarWithPasswordMultipleFiles) {
   // Can list files inside an archive that has password protected data.
   // passwd1234_two_files.rar contains 2 files: signed.exe and text.txt
diff --git a/chrome/test/data/safe_browsing/rar/encrypted_high_kdf.rar b/chrome/test/data/safe_browsing/rar/encrypted_high_kdf.rar
new file mode 100644
index 0000000..6e215af
--- /dev/null
+++ b/chrome/test/data/safe_browsing/rar/encrypted_high_kdf.rar
Binary files differ