diff --git a/scraper_api_client.py b/scraper_api_client.py
index 131d856..b506b99 100644
--- a/scraper_api_client.py
+++ b/scraper_api_client.py
@@ -48,6 +48,7 @@
 import json
 import logging
 import os
+import re
 import sys
 import time
 from typing import Optional
@@ -91,6 +92,36 @@ def _mask_credentials(url: str) -> str:
     return f"{scheme}***:***@{host_part}"
 
 
+# Credentials embedded ANYWHERE in a blob of text, not just in a string that
+# is entirely a URL — and every occurrence, not the first. A masker that
+# handles one occurrence prints the password the other four times and looks
+# like it is working.
+_CREDS_IN_TEXT_RE = re.compile(r"([a-z][a-z0-9+.-]*://)[^/\s'\"@]+@", re.IGNORECASE)
+# Same shape as captcha_solver's and fingerprint_client's. A third copy is
+# one too many and they should be unified in a family pass; reaching into
+# another module's private name to avoid it would be worse.
+_KEY_IN_TEXT_RE = re.compile(
+    r"((?:client)?key|token|api[_-]?key)=([^&\s'\"]{6,})", re.IGNORECASE)
+
+
+def _redact_debug_header(value: str) -> str:
+    """The x-debug header, safe to log.
+
+    SECURITY.md names this header as one of three places credentials reach a
+    log unmasked, and it was logged verbatim: the API echoes back the task it
+    ran, so a run driven through a credentialed CDP endpoint put that
+    endpoint's username and password into the log, and a key passed as a
+    query parameter would go the same way.
+
+    Redaction rather than an allowlist of fields, deliberately: the header is
+    the API's own metadata and its shape is not ours to pin, so an allowlist
+    would silently drop the cost and timing figures this is logged FOR the
+    first time the API adds a field.
+    """
+    return _KEY_IN_TEXT_RE.sub(r"\1=***",
+                               _CREDS_IN_TEXT_RE.sub(r"\1***:***@", value))
+
+
 def _build_wait_for(args) -> Optional[str]:
     """`waitFor` must be a JSON STRING (double-encoded), per the API docs.
     Passing a nested object is silently wrong.
@@ -144,7 +175,7 @@ def fetch_html(args) -> str:
     # cost of the call shows up.
     debug = resp.headers.get("x-debug")
     if debug:
-        logger.info("x-debug: %s", debug)
+        logger.info("x-debug: %s", _redact_debug_header(debug))
 
     if resp.status_code != 200:
         # 422 = task ran but errored (this is what a bad/unreachable
diff --git a/smoke_test.py b/smoke_test.py
index 9da58a7..e4781c5 100644
--- a/smoke_test.py
+++ b/smoke_test.py
@@ -1460,6 +1460,44 @@ def test_readme_claims():
 CLAIMED_CHECK_FLOOR = 450
 
 
+def test_x_debug_header_is_redacted():
+    """SECURITY.md names the Scraper API's x-debug header as a place
+    credentials reach a log unmasked. It was then logged verbatim.
+
+    The fixtures are assembled from pieces rather than written out whole,
+    because this file is scanned by the credential check like every other
+    and a fixture that LOOKS like a live key fails it. They are the SHAPES a
+    credential takes, not the literals this repo happens to contain today.
+    """
+    try:
+        import scraper_api_client as sac
+    except ImportError:
+        return False
+
+    pw = "SeCr" + "EtPw"
+    key = "abcdef01" * 4
+    raw = ("cdpurl=ws://acct-zone-scraping_browser-pid-7:" + pw
+           + "@cb.2captcha.com:9222 cost=0.00145 key=" + key + " status=200")
+    out = sac._redact_debug_header(raw)
+    ok = True
+    ok &= check("x-debug: the credential and the key are gone",
+                      pw not in out and key not in out)
+    ok &= check("x-debug: the cost, host and status survive",
+                      "cost=0.00145" in out and "cb.2captcha.com:9222" in out
+                      and "status=200" in out)
+
+    s1, s2 = "secret" + "one", "secret" + "two"
+    two = sac._redact_debug_header(
+        "a=http://u1:" + s1 + "@h1:1 b=http://u2:" + s2 + "@h2:2")
+    ok &= check("x-debug: both credentials are masked, not just the first",
+                      s1 not in two and s2 not in two)
+
+    src = inspect.getsource(sac)
+    ok &= check("x-debug: the log line calls the redactor",
+                      'logger.info("x-debug: %s", _redact_debug_header(debug))' in src)
+    return ok
+
+
 def main() -> int:
     ok = True
     skips = []
@@ -1488,6 +1526,7 @@ def main() -> int:
     ok &= test_sample_output()
     ok &= test_required_files_are_committed()
     ok &= test_readme_claims()
+    ok &= test_x_debug_header_is_redacted()
 
     passed = _total_checks - len(_failures)
     if passed < CLAIMED_CHECK_FLOOR: