diff --git a/tests/fixtures/security/proto-bash.jsonl b/tests/fixtures/security/proto-bash.jsonl
new file mode 100644
index 0000000..8e6b5e7
--- /dev/null
+++ b/tests/fixtures/security/proto-bash.jsonl
@@ -0,0 +1 @@
+{"type":"assistant","sessionId":"security-test","timestamp":"2026-04-16T00:00:00Z","message":{"id":"pwn-bash","type":"message","role":"assistant","model":"claude-opus-4-6","content":[{"type":"tool_use","id":"b1","name":"Bash","input":{"command":"/x/__proto__"}}],"usage":{"input_tokens":1,"output_tokens":1}}}
diff --git a/tests/fixtures/security/proto-model.jsonl b/tests/fixtures/security/proto-model.jsonl
new file mode 100644
index 0000000..0aabf71
--- /dev/null
+++ b/tests/fixtures/security/proto-model.jsonl
@@ -0,0 +1 @@
+{"type":"assistant","sessionId":"security-test","timestamp":"2026-04-16T00:00:00Z","message":{"id":"pwn-model","type":"message","role":"assistant","model":"__proto__","content":[{"type":"text","text":"x"}],"usage":{"input_tokens":1,"output_tokens":1}}}
diff --git a/tests/fixtures/security/proto-tool.jsonl b/tests/fixtures/security/proto-tool.jsonl
new file mode 100644
index 0000000..a93a853
--- /dev/null
+++ b/tests/fixtures/security/proto-tool.jsonl
@@ -0,0 +1 @@
+{"type":"assistant","sessionId":"security-test","timestamp":"2026-04-16T00:00:00Z","message":{"id":"pwn-tool","type":"message","role":"assistant","model":"claude-opus-4-6","content":[{"type":"tool_use","id":"t1","name":"__proto__","input":{}}],"usage":{"input_tokens":1,"output_tokens":1}}}
diff --git a/tests/security/prototype-pollution.test.ts b/tests/security/prototype-pollution.test.ts
new file mode 100644
index 0000000..6b6075c
--- /dev/null
+++ b/tests/security/prototype-pollution.test.ts
@@ -0,0 +1,77 @@
+import { describe, it, expect, afterEach, beforeEach } from 'vitest'
+import { mkdtemp, mkdir, cp, rm } from 'fs/promises'
+import { tmpdir } from 'os'
+import { join } from 'path'
+
+import { parseAllSessions } from '../../src/parser.js'
+import type { DateRange } from '../../src/types.js'
+
+// Fixtures carry timestamp 2026-04-16T00:00:00Z. The range below must stay
+// wide enough to include that date; if the fixtures move, move FIXTURE_DAY too.
+const FIXTURE_DAY = Date.UTC(2026, 3, 16) // month index 3 = April (Date.UTC is 0-indexed)
+const RANGE_BEFORE_MS = FIXTURE_DAY - 24 * 60 * 60 * 1000
+const RANGE_AFTER_MS = FIXTURE_DAY + 24 * 60 * 60 * 1000
+const PROJECT_NAME = 'codeburn-poc-testing'
+
+function makeRange(offsetMs: number): DateRange {
+  return {
+    start: new Date(RANGE_BEFORE_MS + offsetMs),
+    end: new Date(RANGE_AFTER_MS + offsetMs),
+  }
+}
+
+// Hermeticity note: the Claude provider also scans a fixed Desktop sessions
+// dir independent of CLAUDE_CONFIG_DIR. The narrow dateRange above excludes
+// any real sessions in practice, but these tests are not strictly isolated
+// on a machine with April 2026 Claude Desktop activity. A stricter fix
+// belongs in a follow-up to discoverSessions itself.
+
+describe('HIGH-1 prototype pollution via unchecked bracket-assign', () => {
+  const tmpDirs: string[] = []
+  let originalConfigDir: string | undefined
+
+  beforeEach(() => {
+    originalConfigDir = process.env['CLAUDE_CONFIG_DIR']
+  })
+
+  afterEach(async () => {
+    delete (Object.prototype as Record<string, unknown>).calls
+    if (originalConfigDir === undefined) {
+      delete process.env['CLAUDE_CONFIG_DIR']
+    } else {
+      process.env['CLAUDE_CONFIG_DIR'] = originalConfigDir
+    }
+    while (tmpDirs.length > 0) {
+      const d = tmpDirs.pop()
+      if (d) await rm(d, { recursive: true, force: true })
+    }
+  })
+
+  async function setupPoc(fixture: string): Promise<string> {
+    const base = await mkdtemp(join(tmpdir(), 'codeburn-sec-'))
+    tmpDirs.push(base)
+    const projectDir = join(base, 'projects', PROJECT_NAME)
+    await mkdir(projectDir, { recursive: true })
+    await cp(join(__dirname, '..', 'fixtures', 'security', fixture), join(projectDir, 'pwn.jsonl'))
+    process.env['CLAUDE_CONFIG_DIR'] = base
+    return base
+  }
+
+  it('does not pollute Object.prototype when session contains tool_use name "__proto__"', async () => {
+    await setupPoc('proto-tool.jsonl')
+    await expect(parseAllSessions(makeRange(0), 'claude')).resolves.not.toThrow()
+    expect(({} as Record<string, unknown>).calls).toBeUndefined()
+  })
+
+  it('does not pollute Object.prototype when bash command basename is "__proto__"', async () => {
+    await setupPoc('proto-bash.jsonl')
+    await expect(parseAllSessions(makeRange(1), 'claude')).resolves.not.toThrow()
+    expect(({} as Record<string, unknown>).calls).toBeUndefined()
+  })
+
+  it('does not pollute Object.prototype when model name is "__proto__"', async () => {
+    await setupPoc('proto-model.jsonl')
+    await expect(parseAllSessions(makeRange(2), 'claude')).resolves.not.toThrow()
+    expect(({} as Record<string, unknown>).calls).toBeUndefined()
+  })
+})