From 492b45aedc4788337440f936e942e7e5219f9624 Mon Sep 17 00:00:00 2001
From: "Ali Firas (thesmartshadow)" <firaswq12@gmail.com>
Date: Wed, 25 Mar 2026 15:43:39 +0000
Subject: [PATCH] Wrap SystemStackError as ResponseParseError in ResponseParser

---
 lib/net/imap/response_parser.rb       |  2 ++
 test/net/imap/test_response_parser.rb | 20 ++++++++++++++++++++
 2 files changed, 22 insertions(+)

diff --git a/lib/net/imap/response_parser.rb b/lib/net/imap/response_parser.rb
index 4a80fb64..ba6deaf4 100644
--- a/lib/net/imap/response_parser.rb
+++ b/lib/net/imap/response_parser.rb
@@ -691,6 +691,8 @@ def response
         CRLF!
         EOF!
         resp
+      rescue SystemStackError => error
+        raise exception(error.message), cause: error
       end
 
       # RFC3501 & RFC9051:
diff --git a/test/net/imap/test_response_parser.rb b/test/net/imap/test_response_parser.rb
index eaa6ff24..e7309ec7 100644
--- a/test/net/imap/test_response_parser.rb
+++ b/test/net/imap/test_response_parser.rb
@@ -201,6 +201,26 @@ def test_fetch_binary_and_binary_size
     end
   end
 
+  test "deeply nested BODYSTRUCTURE raises ResponseParseError instead of SystemStackError" do
+    parser = Net::IMAP::ResponseParser.new
+
+    def parser.body
+      raise SystemStackError, "stack level too deep"
+    end unless ENV["TEST_RESPONSE_PARSER_STACK_LEVEL"] == "original"
+
+    depth = 10_000
+    leaf = '("TEXT" "PLAIN" NIL NIL NIL "7BIT" 1 1)'
+    bodystructure = depth.times.reduce(leaf) { |part, _| "(#{part} \"MIXED\")" }
+    response = "* 1 FETCH (BODYSTRUCTURE #{bodystructure})\r\n"
+
+    err = assert_raise(Net::IMAP::ResponseParseError) do
+      parser.parse(response)
+    end
+
+    assert_match(/stack level too deep/i, err.message)
+    assert_instance_of(SystemStackError, err.cause)
+  end
+
   def assert_deprecated_appenduid_data_warning
     assert_warn(/#{__FILE__}.*warning.*parser_use_deprecated_uidplus_data is ignored/) do
       result = yield