diff --git a/c++/src/ColumnPrinter.cc b/c++/src/ColumnPrinter.cc
index 6535c612ce..6d969a3dab 100644
--- a/c++/src/ColumnPrinter.cc
+++ b/c++/src/ColumnPrinter.cc
@@ -540,11 +540,17 @@ namespace orc {
     if (hasNulls && !notNull[rowId]) {
       writeString(buffer, "null");
     } else {
+      const size_t tag = static_cast<size_t>(tags_[rowId]);
+      if (tag >= fieldPrinter_.size()) {
+        throw ParseError("Invalid union tag " + to_string(static_cast<int64_t>(tag)) +
+                         " for union with " +
+                         to_string(static_cast<int64_t>(fieldPrinter_.size())) + " children");
+      }
       writeString(buffer, "{\"tag\": ");
-      const auto numBuffer = std::to_string(static_cast<int64_t>(tags_[rowId]));
+      const auto numBuffer = std::to_string(static_cast<int64_t>(tag));
       writeString(buffer, numBuffer.c_str());
       writeString(buffer, ", \"value\": ");
-      fieldPrinter_[tags_[rowId]]->printRow(offsets_[rowId]);
+      fieldPrinter_[tag]->printRow(offsets_[rowId]);
       writeChar(buffer, '}');
     }
   }
diff --git a/c++/src/ColumnReader.cc b/c++/src/ColumnReader.cc
index b4032af734..ab1fe24b1c 100644
--- a/c++/src/ColumnReader.cc
+++ b/c++/src/ColumnReader.cc
@@ -1131,6 +1131,16 @@ namespace orc {
     }
   }
 
+  size_t getCheckedUnionTag(unsigned char tag, uint64_t numChildren) {
+    size_t child = static_cast<size_t>(tag);
+    if (child >= numChildren) {
+      throw ParseError("Invalid union tag " + to_string(static_cast<int64_t>(child)) +
+                       " for union with " + to_string(static_cast<int64_t>(numChildren)) +
+                       " children");
+    }
+    return child;
+  }
+
   class UnionColumnReader : public ColumnReader {
    private:
     std::unique_ptr<ByteRleDecoder> rle_;
@@ -1181,15 +1191,15 @@ namespace orc {
   uint64_t UnionColumnReader::skip(uint64_t numValues) {
     numValues = ColumnReader::skip(numValues);
     const uint64_t BUFFER_SIZE = 1024;
-    char buffer[BUFFER_SIZE];
+    unsigned char buffer[BUFFER_SIZE];
     uint64_t lengthsRead = 0;
     int64_t* counts = childrenCounts_.data();
     memset(counts, 0, sizeof(int64_t) * numChildren_);
     while (lengthsRead < numValues) {
       uint64_t chunk = std::min(numValues - lengthsRead, BUFFER_SIZE);
-      rle_->next(buffer, chunk, nullptr);
+      rle_->next(reinterpret_cast<char*>(buffer), chunk, nullptr);
       for (size_t i = 0; i < chunk; ++i) {
-        counts[static_cast<size_t>(buffer[i])] += 1;
+        counts[getCheckedUnionTag(buffer[i], numChildren_)] += 1;
       }
       lengthsRead += chunk;
     }
@@ -1225,12 +1235,14 @@ namespace orc {
     if (notNull) {
       for (size_t i = 0; i < numValues; ++i) {
         if (notNull[i]) {
-          offsets[i] = static_cast<uint64_t>(counts[static_cast<size_t>(tags[i])]++);
+          size_t tag = getCheckedUnionTag(tags[i], numChildren_);
+          offsets[i] = static_cast<uint64_t>(counts[tag]++);
         }
       }
     } else {
       for (size_t i = 0; i < numValues; ++i) {
-        offsets[i] = static_cast<uint64_t>(counts[static_cast<size_t>(tags[i])]++);
+        size_t tag = getCheckedUnionTag(tags[i], numChildren_);
+        offsets[i] = static_cast<uint64_t>(counts[tag]++);
       }
     }
     // read the right number of each child column
diff --git a/c++/test/TestColumnPrinter.cc b/c++/test/TestColumnPrinter.cc
index 8fa5d01cd4..2553888437 100644
--- a/c++/test/TestColumnPrinter.cc
+++ b/c++/test/TestColumnPrinter.cc
@@ -506,6 +506,25 @@ namespace orc {
     }
   }
 
+  TEST(TestColumnPrinter, UnionColumnPrinterRejectsInvalidTag) {
+    std::string line;
+    std::unique_ptr<Type> type = createUnionType();
+    type->addUnionChild(createPrimitiveType(LONG));
+    type->addUnionChild(createPrimitiveType(INT));
+    std::unique_ptr<ColumnPrinter> printer = createColumnPrinter(line, type.get());
+
+    UnionVectorBatch batch(1, *getDefaultPool());
+    batch.children.push_back(new LongVectorBatch(1, *getDefaultPool()));
+    batch.children.push_back(new LongVectorBatch(1, *getDefaultPool()));
+    batch.numElements = 1;
+    batch.hasNulls = false;
+    batch.tags[0] = 200;
+    batch.offsets[0] = 0;
+
+    printer->reset(batch);
+    EXPECT_THROW(printer->printRow(0), ParseError);
+  }
+
   TEST(TestColumnPrinter, StructColumnPrinter) {
     std::string line;
     std::unique_ptr<Type> type = createStructType();
diff --git a/c++/test/TestColumnReader.cc b/c++/test/TestColumnReader.cc
index fcbf007630..d2aa38cb66 100644
--- a/c++/test/TestColumnReader.cc
+++ b/c++/test/TestColumnReader.cc
@@ -3717,6 +3717,76 @@ namespace orc {
               batch.toString());
   }
 
+  const unsigned char INVALID_UNION_TAG[] = {0xff, 0xc8};
+  const unsigned char ONE_PRESENT_VALUE[] = {0x00, 0x80};
+
+  std::unique_ptr<Type> createTwoChildUnionRowType() {
+    std::unique_ptr<Type> unionType = createUnionType();
+    unionType->addUnionChild(createPrimitiveType(LONG));
+    unionType->addUnionChild(createPrimitiveType(INT));
+    std::unique_ptr<Type> rowType = createStructType();
+    rowType->addStructField("col0", std::move(unionType));
+    return rowType;
+  }
+
+  std::unique_ptr<ColumnReader> buildInvalidUnionTagReader(MockStripeStreams& streams,
+                                                           bool hasNulls = false) {
+    std::vector<bool> selectedColumns(4, false);
+    selectedColumns[0] = true;
+    selectedColumns[1] = true;
+    EXPECT_CALL(streams, getSelectedColumns()).WillRepeatedly(testing::Return(selectedColumns));
+    EXPECT_CALL(streams, getSchemaEvolution()).WillRepeatedly(testing::Return(nullptr));
+
+    proto::ColumnEncoding directEncoding;
+    directEncoding.set_kind(proto::ColumnEncoding_Kind_DIRECT);
+    EXPECT_CALL(streams, getEncoding(testing::_)).WillRepeatedly(testing::Return(directEncoding));
+
+    EXPECT_CALL(streams, getStreamProxy(testing::_, proto::Stream_Kind_PRESENT, true))
+        .WillRepeatedly(testing::Return(nullptr));
+
+    if (hasNulls) {
+      EXPECT_CALL(streams, getStreamProxy(1, proto::Stream_Kind_PRESENT, true))
+          .WillRepeatedly(testing::Return(
+              new SeekableArrayInputStream(ONE_PRESENT_VALUE, ARRAY_SIZE(ONE_PRESENT_VALUE))));
+    }
+
+    EXPECT_CALL(streams, getStreamProxy(1, proto::Stream_Kind_DATA, true))
+        .WillRepeatedly(testing::Return(
+            new SeekableArrayInputStream(INVALID_UNION_TAG, ARRAY_SIZE(INVALID_UNION_TAG))));
+
+    std::unique_ptr<Type> rowType = createTwoChildUnionRowType();
+    return buildReader(*rowType, streams);
+  }
+
+  void addSingleUnionBatch(StructVectorBatch& batch) {
+    batch.fields.push_back(new UnionVectorBatch(1, *getDefaultPool()));
+  }
+
+  TEST(TestColumnReader, testUnionRejectsInvalidTag) {
+    MockStripeStreams streams;
+    std::unique_ptr<ColumnReader> reader = buildInvalidUnionTagReader(streams);
+
+    StructVectorBatch batch(1, *getDefaultPool());
+    addSingleUnionBatch(batch);
+    EXPECT_THROW(reader->next(batch, 1, 0), ParseError);
+  }
+
+  TEST(TestColumnReader, testUnionRejectsInvalidTagWithNulls) {
+    MockStripeStreams streams;
+    std::unique_ptr<ColumnReader> reader = buildInvalidUnionTagReader(streams, true);
+
+    StructVectorBatch batch(1, *getDefaultPool());
+    addSingleUnionBatch(batch);
+    EXPECT_THROW(reader->next(batch, 1, 0), ParseError);
+  }
+
+  TEST(TestColumnReader, testUnionSkipRejectsInvalidTag) {
+    MockStripeStreams streams;
+    std::unique_ptr<ColumnReader> reader = buildInvalidUnionTagReader(streams);
+
+    EXPECT_THROW(reader->skip(1), ParseError);
+  }
+
   TEST(TestColumnReader, testUnionWithNulls) {
     MockStripeStreams streams;