diff --git a/lib/github_advisory_sync.rb b/lib/github_advisory_sync.rb
index 4aaf290551..59f7550937 100644
--- a/lib/github_advisory_sync.rb
+++ b/lib/github_advisory_sync.rb
@@ -250,6 +250,73 @@ def initialize(advisory)
       @vulnerabilities = []
     end
 
+    def self.formatted_yaml(data)
+      yaml = data.to_yaml
+      formatted_yaml = indent_mapping_sequences(yaml)
+
+      return formatted_yaml if yaml_round_trips?(formatted_yaml, data)
+
+      yaml
+    end
+
+    def self.indent_mapping_sequences(yaml)
+      active_sequence_indents = []
+      pending_sequence_indent = nil
+      block_scalar_indent = nil
+
+      yaml.lines.map do |line|
+        if line.strip.empty? || line.start_with?("---")
+          next line
+        end
+
+        indent = line[/\A */].length
+
+        if block_scalar_indent
+          if indent > block_scalar_indent
+            next "#{'  ' * active_sequence_indents.length}#{line}"
+          end
+
+          block_scalar_indent = nil
+        end
+
+        sequence_item = line.match?(/\A\s*-\s/)
+
+        active_sequence_indents.pop while active_sequence_indents.any? &&
+          indent <= active_sequence_indents.last &&
+          !(sequence_item && indent == active_sequence_indents.last)
+
+        if sequence_item && pending_sequence_indent == indent
+          active_sequence_indents << indent
+        end
+
+        pending_sequence_indent = sequence_indent_after_mapping_key(line, indent)
+        block_scalar_indent = indent if block_scalar_header?(line)
+
+        "#{'  ' * active_sequence_indents.length}#{line}"
+      end.join
+    end
+
+    def self.sequence_indent_after_mapping_key(line, indent)
+      if line.match?(/\A\s*-\s+.*:\s*\z/)
+        indent + 2
+      elsif line.match?(/\A\s*[^#].*:\s*\z/)
+        indent
+      end
+    end
+
+    def self.block_scalar_header?(line)
+      line.match?(/\A\s*(?:-\s+)?[^#]+:\s*[>|](?:[+-]?[1-9]|[1-9][+-]?)?\s*\z/)
+    end
+
+    def self.yaml_round_trips?(yaml, data)
+      YAML.safe_load(yaml, permitted_classes: [Date]) == data
+    rescue Psych::Exception
+      false
+    end
+
+    private_class_method :indent_mapping_sequences, :sequence_indent_after_mapping_key,
+                         :block_scalar_header?, :yaml_round_trips?
+
     def identifier_list
       advisory["identifiers"]
     end
@@ -338,7 +405,7 @@ def update(package)
       return if saved_data == new_data
 
       File.open(package.filename, 'w') do |file|
-        file.write YAML.dump(new_data)
+        file.write self.class.formatted_yaml(new_data)
       end
 
       puts "Updated: #{package.filename}"
@@ -428,7 +495,7 @@ def create(package)
       FileUtils.mkdir_p(File.dirname(filename_to_write))
       File.open(filename_to_write, "w") do |file|
         # create an automatically generated advisory yaml file
-        file.write new_data.to_yaml
+        file.write self.class.formatted_yaml(new_data)
 
         # The data we just wrote is incomplete,
         # and therefore should not be committed as is
@@ -448,7 +515,7 @@ def create(package)
         # Still it should be removed before the data goes into rubysec
         file.write "# GitHub advisory data below - **Remove this data before committing**\n"
         file.write "# Use this data to write patched_versions (and potentially unaffected_versions) above\n"
-        file.write advisory.merge("vulnerabilities" => vulnerabilities).to_yaml
+        file.write self.class.formatted_yaml(advisory.merge("vulnerabilities" => vulnerabilities))
       end
       puts "Wrote: #{filename_to_write}"
       filename_to_write
diff --git a/spec/github_advisory_sync_spec.rb b/spec/github_advisory_sync_spec.rb
new file mode 100644
index 0000000000..019832ffb6
--- /dev/null
+++ b/spec/github_advisory_sync_spec.rb
@@ -0,0 +1,74 @@
+require "spec_helper"
+require "github_advisory_sync"
+
+RSpec.describe GitHub::GitHubAdvisory do
+  describe ".formatted_yaml" do
+    it "indents generated sequence values under their keys" do
+      data = {
+        "patched_versions" => [">= 3.0.1"],
+        "related" => {
+          "url" => [
+            "https://github.com/autolab/Autolab/security/advisories/GHSA-v46j-h43h-rwrm"
+          ]
+        }
+      }
+
+      yaml = described_class.formatted_yaml(data)
+
+      expect(yaml).to include(%(patched_versions:\n  - ">= 3.0.1"\n))
+      expect(yaml).to include(
+        "related:\n" \
+        "  url:\n" \
+        "    - https://github.com/autolab/Autolab/security/advisories/GHSA-v46j-h43h-rwrm\n"
+      )
+      expect(YAML.safe_load(yaml)).to eq(data)
+    end
+
+    it "keeps nested array payloads valid" do
+      data = {
+        "description" => "Impact:\n- user-provided bullet\n",
+        "notes" => "  heading:\n  - keep literal bullet\n",
+        "vulnerabilities" => [
+          {
+            "package" => {
+              "name" => "autolab"
+            },
+            "identifiers" => [
+              {
+                "type" => "CVE",
+                "value" => "CVE-2026-1234"
+              }
+            ]
+          }
+        ]
+      }
+
+      yaml = described_class.formatted_yaml(data)
+
+      expect(yaml).to include(
+        "vulnerabilities:\n" \
+        "  - package:\n" \
+        "      name: autolab\n" \
+        "    identifiers:\n" \
+        "      - type: CVE\n" \
+        "        value: CVE-2026-1234\n"
+      )
+      expect(YAML.safe_load(yaml)).to eq(data)
+    end
+
+    it "does not corrupt multiline quoted scalar payloads" do
+      data = {
+        "vulnerabilities" => [
+          {
+            "desc" => "x\n",
+            "fixed" => true
+          }
+        ]
+      }
+
+      yaml = described_class.formatted_yaml(data)
+
+      expect(YAML.safe_load(yaml)).to eq(data)
+    end
+  end
+end