diff --git a/diff/src/main/scala/proteus/diff/Git.scala b/diff/src/main/scala/proteus/diff/Git.scala
index 8650341..a430572 100644
--- a/diff/src/main/scala/proteus/diff/Git.scala
+++ b/diff/src/main/scala/proteus/diff/Git.scala
@@ -12,7 +12,11 @@ object Git {
     * Extracts all files at the given ref into a temp dir via a `git archive | tar -x` pipeline.
     * Much faster than invoking `git show` per file (one subprocess pair vs N+1).
     */
-  def extractProtos(ref: String, cwd: Option[Path] = None): Either[String, Path] = {
+  def extractProtos(ref: String, cwd: Option[Path] = None): Either[String, Path] =
+    if (ref.startsWith("-")) Left(s"invalid git ref: $ref")
+    else extractProtosUnchecked(ref, cwd)
+
+  private def extractProtosUnchecked(ref: String, cwd: Option[Path]): Either[String, Path] = {
     val tmp = Files.createTempDirectory("proteus-diff-git-")
     sys.addShutdownHook(deleteRecursively(tmp))
 
diff --git a/diff/src/test/scala/proteus/diff/GitSpec.scala b/diff/src/test/scala/proteus/diff/GitSpec.scala
index 5156e53..a8aa20f 100644
--- a/diff/src/test/scala/proteus/diff/GitSpec.scala
+++ b/diff/src/test/scala/proteus/diff/GitSpec.scala
@@ -72,6 +72,16 @@ object GitSpec extends ZIOSpecDefault {
             Git.extractProtos("definitely-not-a-ref-12345", Some(dir)).isLeft
           }
           assertTrue(isLeft)
+        },
+        test("extractProtos rejects a ref that looks like a git option") {
+          val (fileWritten, isLeft) = withRepo { dir =>
+            writeFile(dir.resolve("a.proto"), """syntax = "proto3"; message Foo {}""")
+            commit(dir, "initial")
+            val evil   = dir.resolve("evil.tar")
+            val result = Git.extractProtos(s"--output=${evil.toString}", Some(dir))
+            (Files.exists(evil), result.isLeft)
+          }
+          assertTrue(isLeft, !fileWritten)
         }
       )
 }