GH-49614: [C++] Fix silent truncation in base64_decode on invalid input

Author: Reranko05

cpp/src/arrow/util/string_test.cc

@@ -28,6 +28,7 @@
 #include "arrow/testing/gtest_util.h"
 #include "arrow/util/regex.h"
 #include "arrow/util/string.h"
+#include "arrow/util/base64.h"
 
 namespace arrow {
 namespace internal {
@@ -238,6 +239,49 @@ TEST(ToChars, FloatingPoint) {
   }
 }
 
+TEST(Base64DecodeTest, ValidInputs) {
+  EXPECT_EQ(arrow::util::base64_decode("Zg=="), "f");
+  EXPECT_EQ(arrow::util::base64_decode("Zm8="), "fo");
+  EXPECT_EQ(arrow::util::base64_decode("Zm9v"), "foo");
+  EXPECT_EQ(arrow::util::base64_decode("aGVsbG8gd29ybGQ="), "hello world");
+}
+
+TEST(Base64DecodeTest, InvalidLength) {
+  EXPECT_EQ(arrow::util::base64_decode("abc"), "");
+  EXPECT_EQ(arrow::util::base64_decode("abcde"), "");
+}
+
+TEST(Base64DecodeTest, InvalidCharacters) {
+  EXPECT_EQ(arrow::util::base64_decode("ab$="), "");
+  EXPECT_EQ(arrow::util::base64_decode("Zm9v*"), "");
+  EXPECT_EQ(arrow::util::base64_decode("abcd$AAA"), "");
+}
+
+TEST(Base64DecodeTest, InvalidPadding) {
+  EXPECT_EQ(arrow::util::base64_decode("ab=c"), "");
+  EXPECT_EQ(arrow::util::base64_decode("abc==="), "");
+  EXPECT_EQ(arrow::util::base64_decode("abcd=AAA"), "");
+  EXPECT_EQ(arrow::util::base64_decode("Zm=9v"), "");
+}
+
+TEST(Base64DecodeTest, EdgeCases) {
+  EXPECT_EQ(arrow::util::base64_decode("===="), "");
+  EXPECT_EQ(arrow::util::base64_decode("TQ=="), "M");
+}
+
+TEST(Base64DecodeTest, EmptyInput) {
+  EXPECT_EQ(arrow::util::base64_decode(""), "");
+}
+
+TEST(Base64DecodeTest, NonAsciiInput) {
+  std::string input = std::string("abcd") + char(0xFF) + "==";
+  EXPECT_EQ(arrow::util::base64_decode(input), "");
+}
+
+TEST(Base64DecodeTest, PartialCorruption) {
+  EXPECT_EQ(arrow::util::base64_decode("aGVs$G8gd29ybGQ="), "");
+}
+
 #if !defined(_WIN32) || defined(NDEBUG)
 
 TEST(ToChars, LocaleIndependent) {

cpp/src/arrow/vendored/base64.cpp

@@ -30,7 +30,9 @@
 */
 
 #include "arrow/util/base64.h"
+#include "arrow/util/logging.h"
 #include <iostream>
+#include <cctype>
 
 namespace arrow {
 namespace util {
@@ -101,7 +103,46 @@ std::string base64_decode(std::string_view encoded_string) {
   unsigned char char_array_4[4], char_array_3[3];
   std::string ret;
 
-  while (in_len-- && ( encoded_string[in_] != '=') && is_base64(encoded_string[in_])) {
+  static const std::string base64_chars =
+               "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+               "abcdefghijklmnopqrstuvwxyz"
+               "0123456789+/";
+  
+  auto is_base64 = [](unsigned char c) -> bool {
+    return (std::isalnum(c) || (c == '+') || (c == '/'));
+  };
+
+  if (encoded_string.size() % 4 != 0) {
+    ARROW_LOG(ERROR) << "Invalid base64 input: length is not a multiple of 4";
+    return "";
+  }
+
+  size_t padding_start = encoded_string.find('=');
+
+  if (padding_start != std::string::npos) {
+    for (size_t k = padding_start; k < encoded_string.size(); ++k) {
+      if (encoded_string[k] != '=') {
+        ARROW_LOG(ERROR) << "Invalid base64 input: padding character '=' found at invalid position";
+        return "";
+      }
+    }
+
+    size_t padding_count = encoded_string.size() - padding_start;
+
+    if (padding_count > 2) {
+      ARROW_LOG(ERROR) << "Invalid base64 input: too many padding characters";
+      return "";
+    }
+  }
+
+    for (char c : encoded_string) {
+      if (c != '=' && !is_base64(c)) {
+        ARROW_LOG(ERROR) << "Invalid base64 input: contains non-base64 character '" << c << "'";
+        return "";
+      }
+    }
+
+  while (in_len-- && encoded_string[in_] != '=') {
     char_array_4[i++] = encoded_string[in_]; in_++;
     if (i ==4) {
       for (i = 0; i <4; i++)