Fix buffer overflow (pull request #81)

CHANGELOG.md

@@ -1,6 +1,13 @@
 Arduino JSON: change log
 ========================
 
+v4.5
+----
+
+* Fixed buffer overflow when input contains a backslash followed by a terminator (issue #81)
+
+**Upgrading is recommended** since previous versions contain a potential security risk.
+
 v4.4
 ----
 

src/Internals/QuotedString.cpp

@@ -58,46 +58,44 @@ static char unescapeChar(char c) {
 static inline bool isQuote(char c) { return c == '\"' || c == '\''; }
 
 char *QuotedString::extractFrom(char *input, char **endPtr) {
-  char firstChar = *input;
-
-  if (!isQuote(firstChar)) {
-    // must start with a quote
-    return NULL;
-  }
-
-  char stopChar = firstChar;  // closing quote is the same as opening quote
-
   char *startPtr = input + 1;  // skip the quote
   char *readPtr = startPtr;
   char *writePtr = startPtr;
   char c;
 
+  char firstChar = *input;
+  char stopChar = firstChar;  // closing quote is the same as opening quote
+
+  if (!isQuote(firstChar)) goto ERROR_OPENING_QUOTE_MISSING;
+
   for (;;) {
     c = *readPtr++;
 
-    if (c == '\0') {
-      // premature ending
-      return NULL;
-    }
+    if (c == '\0') goto ERROR_CLOSING_QUOTE_MISSING;
 
-    if (c == stopChar) {
-      // closing quote
-      break;
-    }
+    if (c == stopChar) goto SUCCESS;
 
     if (c == '\\') {
       // replace char
       c = unescapeChar(*readPtr++);
+      if (c == '\0') goto ERROR_ESCAPE_SEQUENCE_INTERRUPTED;
     }
 
     *writePtr++ = c;
   }
 
+SUCCESS:
   // end the string here
   *writePtr = '\0';
 
   // update end ptr
   *endPtr = readPtr;
 
+  // return pointer to unquoted string
   return startPtr;
+
+ERROR_OPENING_QUOTE_MISSING:
+ERROR_CLOSING_QUOTE_MISSING:
+ERROR_ESCAPE_SEQUENCE_INTERRUPTED:
+  return NULL;
 }

test/QuotedString_ExtractFrom_Tests.cpp

@@ -16,6 +16,11 @@ class QuotedString_ExtractFrom_Tests : public testing::Test {
     _result = QuotedString::extractFrom(_jsonString, &_trailing);
   }
 
+  void whenInputIs(const char *json, size_t len) {
+    memcpy(_jsonString, json, len);
+    _result = QuotedString::extractFrom(_jsonString, &_trailing);
+  }
+
   void resultMustBe(const char *expected) { EXPECT_STREQ(expected, _result); }
 
   void trailingMustBe(const char *expected) {
@@ -134,3 +139,8 @@ TEST_F(QuotedString_ExtractFrom_Tests, AllEscapedCharsTogether) {
   whenInputIs("\"1\\\"2\\\\3\\/4\\b5\\f6\\n7\\r8\\t9\"");
   resultMustBe("1\"2\\3/4\b5\f6\n7\r8\t9");
 }
+
+TEST_F(QuotedString_ExtractFrom_Tests, UnterminatedEscapeSequence) {
+  whenInputIs("\"\\\0\"", 4);
+  resultMustBe(0);
+}