Fix base64 validation and add unittests (#10515)

Implement proper padding character checks

4 files changed, 124 insertions, 6 deletions

diff --git a/doc/client_lua_api.txt b/doc/client_lua_api.txt
index 1e8015f7b..d239594f7 100644
--- a/doc/client_lua_api.txt
+++ b/doc/client_lua_api.txt
@@ -910,7 +910,9 @@ Call these functions only at load time!
     * Example: `minetest.rgba(10, 20, 30, 40)`, returns `"#0A141E28"`
 * `minetest.encode_base64(string)`: returns string encoded in base64
     * Encodes a string in base64.
-* `minetest.decode_base64(string)`: returns string
+* `minetest.decode_base64(string)`: returns string or nil on failure
+    * Padding characters are only supported starting at version 5.4.0, where
+      5.5.0 and newer perform proper checks.
     * Decodes a string encoded in base64.
 * `minetest.gettext(string)` : returns string
     * look up the translation of a string in the gettext message catalog
diff --git a/doc/lua_api.txt b/doc/lua_api.txt
index 6c7ae0fb5..956919c89 100644
--- a/doc/lua_api.txt
+++ b/doc/lua_api.txt
@@ -5773,7 +5773,9 @@ Misc.
     * Example: `minetest.rgba(10, 20, 30, 40)`, returns `"#0A141E28"`
 * `minetest.encode_base64(string)`: returns string encoded in base64
     * Encodes a string in base64.
-* `minetest.decode_base64(string)`: returns string or nil for invalid base64
+* `minetest.decode_base64(string)`: returns string or nil on failure
+    * Padding characters are only supported starting at version 5.4.0, where
+      5.5.0 and newer perform proper checks.
     * Decodes a string encoded in base64.
 * `minetest.is_protected(pos, name)`: returns boolean
     * Returning `true` restricts the player `name` from modifying (i.e. digging,
diff --git a/src/unittest/test_utilities.cpp b/src/unittest/test_utilities.cpp
index 93ba3f844..039110d54 100644
--- a/src/unittest/test_utilities.cpp
+++ b/src/unittest/test_utilities.cpp
@@ -23,6 +23,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "util/enriched_string.h"
 #include "util/numeric.h"
 #include "util/string.h"
+#include "util/base64.h"
 
 class TestUtilities : public TestBase {
 public:
@@ -56,6 +57,7 @@ public:
 	void testMyround();
 	void testStringJoin();
 	void testEulerConversion();
+	void testBase64();
 };
 
 static TestUtilities g_test_instance;
@@ -87,6 +89,7 @@ void TestUtilities::runTests(IGameDef *gamedef)
 	TEST(testMyround);
 	TEST(testStringJoin);
 	TEST(testEulerConversion);
+	TEST(testBase64);
 }
 
 ////////////////////////////////////////////////////////////////////////////////
@@ -537,3 +540,93 @@ void TestUtilities::testEulerConversion()
 	setPitchYawRoll(m2, v2);
 	UASSERT(within(m1, m2, tolL));
 }
+
+void TestUtilities::testBase64()
+{
+	// Test character set
+	UASSERT(base64_is_valid("ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+		"abcdefghijklmnopqrstuvwxyz"
+		"0123456789+/") == true);
+	UASSERT(base64_is_valid("/+9876543210"
+		"zyxwvutsrqponmlkjihgfedcba"
+		"ZYXWVUTSRQPONMLKJIHGFEDCBA") == true);
+	UASSERT(base64_is_valid("ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+		"abcdefghijklmnopqrstuvwxyz"
+		"0123456789+.") == false);
+	UASSERT(base64_is_valid("ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+		"abcdefghijklmnopqrstuvwxyz"
+		"0123456789 /") == false);
+
+	// Test empty string
+	UASSERT(base64_is_valid("") == true);
+
+	// Test different lengths, with and without padding,
+	// with correct and incorrect padding
+	UASSERT(base64_is_valid("A") == false);
+	UASSERT(base64_is_valid("AA") == true);
+	UASSERT(base64_is_valid("AAA") == true);
+	UASSERT(base64_is_valid("AAAA") == true);
+	UASSERT(base64_is_valid("AAAAA") == false);
+	UASSERT(base64_is_valid("AAAAAA") == true);
+	UASSERT(base64_is_valid("AAAAAAA") == true);
+	UASSERT(base64_is_valid("AAAAAAAA") == true);
+	UASSERT(base64_is_valid("A===") == false);
+	UASSERT(base64_is_valid("AA==") == true);
+	UASSERT(base64_is_valid("AAA=") == true);
+	UASSERT(base64_is_valid("AAAA") == true);
+	UASSERT(base64_is_valid("AAAA====") == false);
+	UASSERT(base64_is_valid("AAAAA===") == false);
+	UASSERT(base64_is_valid("AAAAAA==") == true);
+	UASSERT(base64_is_valid("AAAAAAA=") == true);
+	UASSERT(base64_is_valid("AAAAAAA==") == false);
+	UASSERT(base64_is_valid("AAAAAAA===") == false);
+	UASSERT(base64_is_valid("AAAAAAA====") == false);
+	UASSERT(base64_is_valid("AAAAAAAA") == true);
+	UASSERT(base64_is_valid("AAAAAAAA=") == false);
+	UASSERT(base64_is_valid("AAAAAAAA==") == false);
+	UASSERT(base64_is_valid("AAAAAAAA===") == false);
+	UASSERT(base64_is_valid("AAAAAAAA====") == false);
+
+	// Test if canonical encoding
+	// Last character limitations, length % 4 == 3
+	UASSERT(base64_is_valid("AAB") == false);
+	UASSERT(base64_is_valid("AAE") == true);
+	UASSERT(base64_is_valid("AAQ") == true);
+	UASSERT(base64_is_valid("AAB=") == false);
+	UASSERT(base64_is_valid("AAE=") == true);
+	UASSERT(base64_is_valid("AAQ=") == true);
+	UASSERT(base64_is_valid("AAAAAAB=") == false);
+	UASSERT(base64_is_valid("AAAAAAE=") == true);
+	UASSERT(base64_is_valid("AAAAAAQ=") == true);
+	// Last character limitations, length % 4 == 2
+	UASSERT(base64_is_valid("AB") == false);
+	UASSERT(base64_is_valid("AE") == false);
+	UASSERT(base64_is_valid("AQ") == true);
+	UASSERT(base64_is_valid("AB==") == false);
+	UASSERT(base64_is_valid("AE==") == false);
+	UASSERT(base64_is_valid("AQ==") == true);
+	UASSERT(base64_is_valid("AAAAAB==") == false);
+	UASSERT(base64_is_valid("AAAAAE==") == false);
+	UASSERT(base64_is_valid("AAAAAQ==") == true);
+
+	// Extraneous character present
+	UASSERT(base64_is_valid(".") == false);
+	UASSERT(base64_is_valid("A.") == false);
+	UASSERT(base64_is_valid("AA.") == false);
+	UASSERT(base64_is_valid("AAA.") == false);
+	UASSERT(base64_is_valid("AAAA.") == false);
+	UASSERT(base64_is_valid("AAAAA.") == false);
+	UASSERT(base64_is_valid("A.A") == false);
+	UASSERT(base64_is_valid("AA.A") == false);
+	UASSERT(base64_is_valid("AAA.A") == false);
+	UASSERT(base64_is_valid("AAAA.A") == false);
+	UASSERT(base64_is_valid("AAAAA.A") == false);
+	UASSERT(base64_is_valid("\xE1""AAA") == false);
+
+	// Padding in wrong position
+	UASSERT(base64_is_valid("A=A") == false);
+	UASSERT(base64_is_valid("AA=A") == false);
+	UASSERT(base64_is_valid("AAA=A") == false);
+	UASSERT(base64_is_valid("AAAA=A") == false);
+	UASSERT(base64_is_valid("AAAAA=A") == false);
+}
\ No newline at end of file
diff --git a/src/util/base64.cpp b/src/util/base64.cpp
index 6e1584410..0c2455222 100644
--- a/src/util/base64.cpp
+++ b/src/util/base64.cpp
@@ -33,18 +33,39 @@ static const std::string base64_chars =
 		"abcdefghijklmnopqrstuvwxyz"
 		"0123456789+/";
 
+static const std::string base64_chars_padding_1 = "AEIMQUYcgkosw048";
+static const std::string base64_chars_padding_2 = "AQgw";
 
 static inline bool is_base64(unsigned char c)
 {
-	return isalnum(c) || c == '+' || c == '/' || c == '=';
+	return (c >= '0' && c <= '9')
+			|| (c >= 'A' && c <= 'Z')
+			|| (c >= 'a' && c <= 'z')
+			|| c == '+' || c == '/';
 }
 
 bool base64_is_valid(std::string const& s)
 {
-	for (char i : s)
-		if (!is_base64(i))
+	size_t i = 0;
+	for (; i < s.size(); ++i)
+		if (!is_base64(s[i]))
+			break;
+	unsigned char padding = 3 - ((i + 3) % 4);
+	if ((padding == 1 && base64_chars_padding_1.find(s[i - 1]) == std::string::npos)
+			|| (padding == 2 && base64_chars_padding_2.find(s[i - 1]) == std::string::npos)
+			|| padding == 3)
+		return false;
+	int actual_padding = s.size() - i;
+	// omission of padding characters is allowed
+	if (actual_padding == 0)
+		return true;
+
+	// remaining characters (max. 2) may only be padding
+	for (; i < s.size(); ++i)
+		if (s[i] != '=')
 			return false;
-	return true;
+	// number of padding characters needs to match
+	return padding == actual_padding;
 }
 
 std::string base64_encode(unsigned char const* bytes_to_encode, unsigned int in_len) {