diff --git a/wire-runtime-swift/src/main/swift/ProtoCodable/ProtoReader.swift b/wire-runtime-swift/src/main/swift/ProtoCodable/ProtoReader.swift
index a1935b11ad..0951b98aff 100644
--- a/wire-runtime-swift/src/main/swift/ProtoCodable/ProtoReader.swift
+++ b/wire-runtime-swift/src/main/swift/ProtoCodable/ProtoReader.swift
@@ -183,8 +183,7 @@ public final class ProtoReader {
                 throw ProtoDecoder.Error.unexpectedEndGroupFieldNumber(expected: nil, found: tag)
 
             case .lengthDelimited:
-                let length = try Int32(truncatingIfNeeded: buffer.readVarint())
-                state = .lengthDelimited(length: Int(length))
+                state = .lengthDelimited(length: try readLengthDelimitedLength(tag: tag))
                 currentTag = tag
                 return tag
 
@@ -793,8 +792,7 @@ public final class ProtoReader {
                 return
 
             case .lengthDelimited:
-                let length = try Int32(truncatingIfNeeded: buffer.readVarint())
-                state = .lengthDelimited(length: Int(length))
+                state = .lengthDelimited(length: try readLengthDelimitedLength(tag: tag))
                 let data = try readData()
                 try unknownFieldsWriter.encode(tag: tag, value: data)
 
@@ -856,6 +854,16 @@ public final class ProtoReader {
         return (tag, wireType)
     }
 
+    private func readLengthDelimitedLength(tag: UInt32) throws -> Int {
+        let length = try Int32(truncatingIfNeeded: buffer.readVarint())
+        if length < 0 {
+            throw ProtoDecoder.Error.invalidStructure(
+                message: "Negative length: \(length). Reader position: \(buffer.position). Last read tag: \(tag)."
+            )
+        }
+        return Int(length)
+    }
+
     // MARK: - Private Methods - Decoding - Repeated Field
 
     private func decode<T>(into array: inout [T], decode: () throws -> T?) throws {
diff --git a/wire-runtime-swift/src/main/swift/ProtoCodable/ReadBuffer.swift b/wire-runtime-swift/src/main/swift/ProtoCodable/ReadBuffer.swift
index 44f987db12..605578e6e7 100644
--- a/wire-runtime-swift/src/main/swift/ProtoCodable/ReadBuffer.swift
+++ b/wire-runtime-swift/src/main/swift/ProtoCodable/ReadBuffer.swift
@@ -67,7 +67,7 @@ final class ReadBuffer {
     }
 
     func verifyAdditional(count: Int) throws {
-        guard pointer.advanced(by: count) <= end else {
+        guard count >= 0, pointer.advanced(by: count) <= end else {
             throw ProtoDecoder.Error.unexpectedEndOfData
         }
     }
diff --git a/wire-runtime-swift/src/test/swift/ProtoReaderTests.swift b/wire-runtime-swift/src/test/swift/ProtoReaderTests.swift
index 1b27b8aa5a..7093759b45 100644
--- a/wire-runtime-swift/src/test/swift/ProtoReaderTests.swift
+++ b/wire-runtime-swift/src/test/swift/ProtoReaderTests.swift
@@ -1111,6 +1111,42 @@ final class ProtoReaderTests: XCTestCase {
         }
     }
 
+    func testLengthDelimitedRejectsNegativeLength() throws {
+        let data = Foundation.Data(hexEncoded: """
+            0A         // (Tag 1 | Length Delimited)
+            80FFFFFF0F // Length -128
+        """)!
+
+        XCTAssertThrowsError(
+            try test(data: data) { reader in
+                _ = try reader.forEachTag { _ in
+                    XCTFail("The negative length should have thrown before returning a tag")
+                }
+            }
+        ) { error in
+            assertNegativeLengthError(error, readerPosition: 6)
+        }
+    }
+
+    func testSkipGroupRejectsNegativeLengthDelimited() throws {
+        let data = Foundation.Data(hexEncoded: """
+            9B06       // (Tag 99 | Start Group)
+            0A         // (Tag 1 | Length Delimited)
+            80FFFFFF0F // Length -128
+            9C06       // (Tag 99 | End Group)
+        """)!
+
+        XCTAssertThrowsError(
+            try test(data: data) { reader in
+                _ = try reader.forEachTag { _ in
+                    XCTFail("The group should have been skipped or rejected")
+                }
+            }
+        ) { error in
+            assertNegativeLengthError(error, readerPosition: 8)
+        }
+    }
+
     // MARK: - Tests - Unknown Fields
 
     func testUnknownFields() throws {
@@ -1330,6 +1366,14 @@ final class ProtoReaderTests: XCTestCase {
             try test(reader)
         }
     }
+
+    private func assertNegativeLengthError(_ error: Error, readerPosition: Int) {
+        guard case let ProtoDecoder.Error.invalidStructure(message) = error else {
+            XCTFail("Unexpected error: \(error)")
+            return
+        }
+        XCTAssertEqual(message, "Negative length: -128. Reader position: \(readerPosition). Last read tag: 1.")
+    }
 }
 
 // MARK: -