diff --git a/lib/concerns/environment_writer.rb b/lib/concerns/environment_writer.rb
index 1bacce3..0d7f0c6 100644
--- a/lib/concerns/environment_writer.rb
+++ b/lib/concerns/environment_writer.rb
@@ -14,6 +14,8 @@ module EnvironmentWriter
   def modify_env(file, name, value)
     return if value.nil?
 
+    raise ArgumentError, "Invalid input: #{name}=#{value}" if value.include?("\n")
+
     File.open(ENV[file], "a") do |f|
       f.puts "#{name}=#{value}"
     end
diff --git a/spec/concerns/environment_writer_spec.rb b/spec/concerns/environment_writer_spec.rb
index 1f0bc5a..dc89f63 100644
--- a/spec/concerns/environment_writer_spec.rb
+++ b/spec/concerns/environment_writer_spec.rb
@@ -23,6 +23,12 @@ RSpec.describe EnvironmentWriter do
 
       it { expect { subject }.not_to change { File.read(ENV["GITHUB_OUTPUT"], chomp: true).last } }
     end
+
+    context "when value contains a newline" do
+      let(:value) { "var_value\n" }
+
+      it { expect { subject }.to raise_error(ArgumentError) }
+    end
   end
 
   describe "#set_environment" do
@@ -39,5 +45,11 @@ RSpec.describe EnvironmentWriter do
 
       it { expect { subject }.not_to change { File.read(ENV["GITHUB_ENV"], chomp: true).last } }
     end
+
+    context "when value contains a newline" do
+      let(:value) { "env_var_value\n" }
+
+      it { expect { subject }.to raise_error(ArgumentError) }
+    end
   end
 end