🚑 fix to address #389

src/json.hpp

@@ -10643,7 +10643,22 @@ basic_json_parser_66:
             }
             else if (type == value_t::number_integer)
             {
-                result.m_value.number_integer = -static_cast<number_integer_t>(value);
+                // invariant: if we parsed a '-', the absolute value is between
+                // 0 (we allow -0) and max == -INT64_MIN
+                assert(value >= 0);
+                assert(value <= max);
+
+                if (value == max)
+                {
+                    // we cannot simply negate value (== max == -INT64_MIN),
+                    // see https://github.com/nlohmann/json/issues/389
+                    result.m_value.number_integer = INT64_MIN;
+                }
+                else
+                {
+                    // all other values can be negated safely
+                    result.m_value.number_integer = -static_cast<number_integer_t>(value);
+                }
             }
             else
             {

src/json.hpp.re2c

@@ -9793,7 +9793,22 @@ class basic_json
             }
             else if (type == value_t::number_integer)
             {
-                result.m_value.number_integer = -static_cast<number_integer_t>(value);
+                // invariant: if we parsed a '-', the absolute value is between
+                // 0 (we allow -0) and max == -INT64_MIN
+                assert(value >= 0);
+                assert(value <= max);
+
+                if (value == max)
+                {
+                    // we cannot simply negate value (== max == -INT64_MIN),
+                    // see https://github.com/nlohmann/json/issues/389
+                    result.m_value.number_integer = INT64_MIN;
+                }
+                else
+                {
+                    // all other values can be negated safely
+                    result.m_value.number_integer = -static_cast<number_integer_t>(value);
+                }
             }
             else
             {

test/src/unit-regression.cpp

@@ -516,4 +516,21 @@ TEST_CASE("regression tests")
         CHECK_THROWS_AS(j << ss, std::invalid_argument);
         CHECK_THROWS_WITH(j << ss, "parse error - unexpected end of input");
     }
+
+    SECTION("issue #389 - Integer-overflow (OSS-Fuzz issue 267)")
+    {
+        // original test case
+        json j1 = json::parse("-9223372036854775808");
+        CHECK(j1.is_number_integer());
+        CHECK(j1.get<json::number_integer_t>() == INT64_MIN);
+
+        // edge case (+1; still an integer)
+        json j2 = json::parse("-9223372036854775807");
+        CHECK(j2.is_number_integer());
+        CHECK(j2.get<json::number_integer_t>() == INT64_MIN + 1);
+
+        // edge case (-1; overflow -> floats)
+        json j3 = json::parse("-9223372036854775809");
+        CHECK(j3.is_number_float());
+    }
 }