diff --git a/docs/VULNERABILITY_CATALOG.md b/docs/VULNERABILITY_CATALOG.md
index 5763d34..a016faa 100644
--- a/docs/VULNERABILITY_CATALOG.md
+++ b/docs/VULNERABILITY_CATALOG.md
@@ -7,12 +7,12 @@ from each file's header comment, so this page cannot drift from the source.
 
 ## Totals
 
-- **Test cases:** 62
-- **Expected detections:** 62
-- **`VULNERABLE:` markers:** 123 (individual lines a scanner should flag)
-- **`SAFE:` markers:** 73 (lines a scanner must not flag — the false-positive control group)
+- **Test cases:** 63
+- **Expected detections:** 63
+- **`VULNERABLE:` markers:** 124 (individual lines a scanner should flag)
+- **`SAFE:` markers:** 74 (lines a scanner must not flag — the false-positive control group)
 - **Languages:** 8 — dotenv, go, java, javascript, json, python, ruby, text
-- **CWE categories:** 46 — CWE-20, CWE-22, CWE-78, CWE-79, CWE-89, CWE-90, CWE-95, CWE-113, CWE-117, CWE-190, CWE-201, CWE-209, CWE-256, CWE-295, CWE-321, CWE-327, CWE-330, CWE-338, CWE-346, CWE-347, CWE-352, CWE-362, CWE-377, CWE-384, CWE-489, CWE-502, CWE-506, CWE-532, CWE-601, CWE-611, CWE-614, CWE-639, CWE-643, CWE-681, CWE-759, CWE-798, CWE-862, CWE-915, CWE-918, CWE-942, CWE-943, CWE-1236, CWE-1321, CWE-1333, CWE-1336, CWE-1357
+- **CWE categories:** 47 — CWE-20, CWE-22, CWE-78, CWE-79, CWE-89, CWE-90, CWE-95, CWE-113, CWE-117, CWE-170, CWE-190, CWE-201, CWE-209, CWE-256, CWE-295, CWE-321, CWE-327, CWE-330, CWE-338, CWE-346, CWE-347, CWE-352, CWE-362, CWE-377, CWE-384, CWE-489, CWE-502, CWE-506, CWE-532, CWE-601, CWE-611, CWE-614, CWE-639, CWE-643, CWE-681, CWE-759, CWE-798, CWE-862, CWE-915, CWE-918, CWE-942, CWE-943, CWE-1236, CWE-1321, CWE-1333, CWE-1336, CWE-1357
 
 ## How coverage is scored
 
@@ -71,6 +71,7 @@ counts as a detection. See `docs/SCANNER_INTEGRATION.md`.
 | Test case | File | CWE | Severity | Expected | Markers |
 |---|---|---|---|---|---|
 | CSV formula injection from untrusted spreadsheet cells | [`csv-formula-injection.py`](../vulns/python/csv-formula-injection.py) | CWE-1236 | medium | yes | 1 vuln / 1 safe |
+| Format string injection via user-controlled format specifier | [`cwe-170-python.py`](../vulns/python/cwe-170-python.py) | CWE-170 | medium | yes | 1 vuln / 1 safe |
 | Excessive user-record exposure in an API response | [`excessive-data-exposure.py`](../vulns/python/excessive-data-exposure.py) | CWE-201 | high | yes | 1 vuln / 1 safe |
 | Flask debug mode enabled in application configuration | [`flask-debug-enabled.py`](../vulns/python/flask-debug-enabled.py) | CWE-489 | high | yes | 1 vuln / 1 safe |
 | Hardcoded secret used to configure session signing | [`hardcoded-session-secret.py`](../vulns/python/hardcoded-session-secret.py) | CWE-798 | high | yes | 1 vuln / 1 safe |
diff --git a/vulns/VULNERABILITY_CATALOG.json b/vulns/VULNERABILITY_CATALOG.json
index 153fdaf..499cb0d 100644
--- a/vulns/VULNERABILITY_CATALOG.json
+++ b/vulns/VULNERABILITY_CATALOG.json
@@ -2,10 +2,10 @@
   "schema": "threatcrush-testbed-catalog/1",
   "note": "Generated by scripts/generate-catalog.py \u2014 do not edit by hand.",
   "totals": {
-    "test_cases": 62,
-    "expected_detections": 62,
-    "vulnerable_markers": 123,
-    "safe_markers": 73,
+    "test_cases": 63,
+    "expected_detections": 63,
+    "vulnerable_markers": 124,
+    "safe_markers": 74,
     "languages": [
       "dotenv",
       "go",
@@ -35,6 +35,7 @@
       "CWE-95",
       "CWE-113",
       "CWE-117",
+      "CWE-170",
       "CWE-190",
       "CWE-201",
       "CWE-209",
@@ -791,6 +792,29 @@
         40
       ]
     },
+    {
+      "id": "py-cwe-170-format-string-injection",
+      "file": "vulns/python/cwe-170-python.py",
+      "title": "Format string injection via user-controlled format specifier",
+      "category": "python",
+      "language": "python",
+      "cwe": "CWE-170",
+      "cwes": [
+        "CWE-170"
+      ],
+      "severity": "medium",
+      "expected_detection": true,
+      "description": "User-controlled input is used directly as a format string in",
+      "detection_target": "Taint flow from request input into str.format() as the",
+      "safe_guard": "The entire fixture is wrapped in `if False:` and cannot execute. It",
+      "attribution": "line",
+      "vulnerable_lines": [
+        23
+      ],
+      "safe_lines": [
+        32
+      ]
+    },
     {
       "id": "py-excessive-data-exposure",
       "file": "vulns/python/excessive-data-exposure.py",
diff --git a/vulns/python/cwe-170-python.py b/vulns/python/cwe-170-python.py
new file mode 100644
index 0000000..6367e04
--- /dev/null
+++ b/vulns/python/cwe-170-python.py
@@ -0,0 +1,33 @@
+"""
+@id py-cwe-170-format-string-injection
+@test-case Format string injection via user-controlled format specifier
+@cwe CWE-170
+@severity medium
+@language python
+@expected-detection true
+@description User-controlled input is used directly as a format string in
+             str.format(), allowing an attacker to access internal object
+             attributes or cause unexpected behavior through format specifiers.
+@safe-guard The entire fixture is wrapped in `if False:` and cannot execute. It
+            uses only local variables and never performs I/O operations.
+@detection-target Taint flow from request input into str.format() as the
+                  format string argument without sanitization.
+
+NEVER RUN IN PRODUCTION - intentional test case for scanner validation.
+"""
+
+
+if False:
+    def format_message_vulnerable(request):
+        user_input = request.form["user_input"]  # SOURCE: attacker-controlled
+        template = "User message: {}".format(user_input)  # VULNERABLE: CWE-170
+        return template
+
+    def sanitize_format_string(value):
+        # Remove format specifiers that could access object attributes
+        return value.replace("{", "{{").replace("}", "}}")
+
+    def format_message_safe(request):
+        user_input = sanitize_format_string(request.form["user_input"])
+        template = "User message: {}".format(user_input)  # SAFE: format specifiers are escaped
+        return template
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