diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index a4ec3ec..11de440 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -92,3 +92,47 @@ jobs:
       # the system Python, which is what the LaunchAgent runs the service with
       - name: Unit and end-to-end tests
         run: /usr/bin/python3 -m unittest discover -s tests -v
+
+  fuzz:
+    name: Fuzz
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+    steps:
+      - name: Harden runner
+        uses: step-security/harden-runner@dc2769abcc0fcfbadfc1df54081c1608579ef6b2 # v2.22.2
+        with:
+          egress-policy: block
+          allowed-endpoints: >
+            github.com:443
+            pypi.org:443
+            files.pythonhosted.org:443
+
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          persist-credentials: false
+
+      # Atheris only ships Linux x86-64 wheels, for CPython 3.12-3.14
+      - uses: actions/setup-python@5fda3b95a4ea91299a34e894583c3862153e4b97 # v7.0.0
+        with:
+          python-version: "3.13"
+
+      - name: Install Atheris (hash-checked)
+        run: 'python -m pip install --require-hashes --only-binary :all: -r tests/requirements-fuzz.txt'
+
+      # coverage-guided; -timeout makes any input slower than 10 s (a regex blow-up) a failure
+      - name: Fuzz the lockfile parsers
+        run: |
+          mkdir -p "$RUNNER_TEMP/corpus" crashes
+          cp tests/fuzz-corpus/lockfiles/* "$RUNNER_TEMP/corpus/"
+          python tests/fuzz_lockfiles.py "$RUNNER_TEMP/corpus" -max_total_time=90 -timeout=10 -rss_limit_mb=2048 -artifact_prefix=crashes/
+
+      - name: Fuzz the projects-folder check
+        run: python tests/fuzz_projects_root.py -max_total_time=45 -timeout=10 -artifact_prefix=crashes/
+
+      - name: Keep crashing inputs
+        if: failure()
+        uses: actions/upload-artifact@cf430e030ddbb5b0abf93d22962f4752f3646cd9 # v7.0.2
+        with:
+          name: fuzz-crashes
+          path: crashes/
+          retention-days: 14
diff --git a/README.md b/README.md
index e57dfc3..1ea3775 100644
--- a/README.md
+++ b/README.md
@@ -212,6 +212,11 @@ they never touch an installed pkgdash, and setup is never finished in them, so n
 updated. They cover the Host/Origin/token checks, settings validation, turned-off managers, the Socket
 token check, the settings migration and the demo.
 
+The lockfile parsers read files from other people's projects, so CI also **fuzzes** them with
+[Atheris](https://github.com/google/atheris) (coverage-guided; `tests/fuzz_lockfiles.py`, seeded from
+`tests/fuzz-corpus/`), along with the projects-folder check (`tests/fuzz_projects_root.py`). Atheris
+only runs on Linux x86-64; inputs it has found are kept as regression tests in `test_server.py`.
+
 ## License
 
 [MIT](LICENSE)
diff --git a/ruff.toml b/ruff.toml
index 2cbd965..34330e9 100644
--- a/ruff.toml
+++ b/ruff.toml
@@ -12,6 +12,6 @@ ignore = [
 
 [lint.per-file-ignores]
 # tests only open http://127.0.0.1 URLs they build themselves
-"tests/*" = ["S310"]
+"tests/*" = ["S310", "S101"]  # S101: asserts are how tests check results (never run with -O)
 # the made-up data tables read better one record per line
 "demo.py" = ["E501"]
diff --git a/server.py b/server.py
index 392404c..6ddd5f4 100644
--- a/server.py
+++ b/server.py
@@ -163,7 +163,7 @@ def iter_json(text):
 def loads(text, default):
     try:
         return json.loads(text)
-    except (ValueError, TypeError):
+    except (ValueError, TypeError, RecursionError):  # RecursionError: absurdly nested JSON
         return default
 
 
@@ -1352,18 +1352,24 @@ def lock_deps(fname, text):
     """[(name, version)] pinned by a lockfile, dependencies included (registry packages only)."""
     res = set()
     if fname == "package-lock.json":
+        # lockfiles come from any project, so check every type rather than trust the shape
         data = loads(text, {})
-        for key, info in (data.get("packages") or {}).items():  # lockfile v2/v3
-            if "node_modules/" in key and not info.get("link") and info.get("version"):
-                res.add((info.get("name") or key.rsplit("node_modules/", 1)[1], info["version"]))
-
-        def walk(deps):  # lockfile v1
-            for n, info in (deps or {}).items():
-                if info.get("version"):
-                    res.add((n, info["version"]))
-                walk(info.get("dependencies"))
-        if not data.get("packages"):
-            walk(data.get("dependencies"))
+        data = data if isinstance(data, dict) else {}
+        packages = data.get("packages")
+        for key, info in (packages.items() if isinstance(packages, dict) else ()):  # lockfile v2/v3
+            if (isinstance(info, dict) and "node_modules/" in key and not info.get("link")
+                    and isinstance(info.get("version"), str) and info["version"]):
+                name = info.get("name")
+                res.add((name if isinstance(name, str) and name else key.rsplit("node_modules/", 1)[1], info["version"]))
+        if not packages:  # lockfile v1: nested "dependencies", walked without recursion
+            todo = [data.get("dependencies")]
+            while todo:
+                deps = todo.pop()
+                for n, info in (deps.items() if isinstance(deps, dict) else ()):
+                    if isinstance(info, dict):
+                        if isinstance(info.get("version"), str) and info["version"]:
+                            res.add((n, info["version"]))
+                        todo.append(info.get("dependencies"))
     elif fname == "yarn.lock":  # yarn 1 (`version "1.2.3"`) and yarn 2+ (`version: 1.2.3`)
         name = None
         for line in text.splitlines():
@@ -1558,7 +1564,12 @@ def projects(self):
             except OSError:
                 continue
             rel = os.path.relpath(os.path.dirname(path), root)
-            out[(kind, rel)] = (eco, lock_deps(os.path.basename(path), text), path, f["mtime"])
+            try:
+                deps = lock_deps(os.path.basename(path), text)
+            except Exception as e:  # one unreadable lockfile mustn't stop the other projects' checks
+                print("projects: skipping %s: %s: %s" % (path, type(e).__name__, e), flush=True)
+                continue
+            out[(kind, rel)] = (eco, deps, path, f["mtime"])
         return out
 
     def scan(self, job):
diff --git a/tests/fuzz-corpus/lockfiles/Cargo.lock b/tests/fuzz-corpus/lockfiles/Cargo.lock
new file mode 100644
index 0000000..98ce82e
--- /dev/null
+++ b/tests/fuzz-corpus/lockfiles/Cargo.lock
@@ -0,0 +1,10 @@
+version = 3
+
+[[package]]
+name = "serde"
+version = "1.0.190"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
+name = "myapp"
+version = "0.1.0"
diff --git a/tests/fuzz-corpus/lockfiles/package-lock-v1.json b/tests/fuzz-corpus/lockfiles/package-lock-v1.json
new file mode 100644
index 0000000..21e050b
--- /dev/null
+++ b/tests/fuzz-corpus/lockfiles/package-lock-v1.json
@@ -0,0 +1,2 @@
+{"name": "app", "lockfileVersion": 1, "dependencies": {"express": {"version": "4.18.2",
+ "dependencies": {"debug": {"version": "2.6.9", "dependencies": {"ms": {"version": "2.0.0"}}}}}, "lodash": {"version": "4.17.21"}}}
diff --git a/tests/fuzz-corpus/lockfiles/package-lock-v3.json b/tests/fuzz-corpus/lockfiles/package-lock-v3.json
new file mode 100644
index 0000000..685a10f
--- /dev/null
+++ b/tests/fuzz-corpus/lockfiles/package-lock-v3.json
@@ -0,0 +1,4 @@
+{"name": "app", "lockfileVersion": 3, "packages": {"": {"name": "app", "version": "1.0.0"},
+ "node_modules/left-pad": {"version": "1.3.0", "resolved": "https://registry.npmjs.org/left-pad/-/left-pad-1.3.0.tgz"},
+ "node_modules/@scope/pkg": {"version": "2.1.0"}, "node_modules/a/node_modules/b": {"version": "0.1.0"},
+ "node_modules/linked": {"link": true, "resolved": "../linked"}}}
diff --git a/tests/fuzz-corpus/lockfiles/poetry.lock b/tests/fuzz-corpus/lockfiles/poetry.lock
new file mode 100644
index 0000000..dfa17ae
--- /dev/null
+++ b/tests/fuzz-corpus/lockfiles/poetry.lock
@@ -0,0 +1,11 @@
+[[package]]
+name = "click"
+version = "8.1.7"
+
+[[package]]
+name = "localdep"
+version = "0.1.0"
+
+[package.source]
+type = "directory"
+url = "../localdep"
diff --git a/tests/fuzz-corpus/lockfiles/requirements.txt b/tests/fuzz-corpus/lockfiles/requirements.txt
new file mode 100644
index 0000000..9778989
--- /dev/null
+++ b/tests/fuzz-corpus/lockfiles/requirements.txt
@@ -0,0 +1,4 @@
+requests==2.31.0
+urllib3[socks]===2.0.7 ; python_version >= "3.8"
+flask>=2.0  # not a pin
+-e git+https://example.invalid/x.git#egg=x
diff --git a/tests/fuzz-corpus/lockfiles/uv.lock b/tests/fuzz-corpus/lockfiles/uv.lock
new file mode 100644
index 0000000..2ba1e34
--- /dev/null
+++ b/tests/fuzz-corpus/lockfiles/uv.lock
@@ -0,0 +1,11 @@
+version = 1
+
+[[package]]
+name = "requests"
+version = "2.31.0"
+source = { registry = "https://pypi.org/simple" }
+
+[[package]]
+name = "myproj"
+version = "0.1.0"
+source = { editable = "." }
diff --git a/tests/fuzz-corpus/lockfiles/yarn-berry.lock b/tests/fuzz-corpus/lockfiles/yarn-berry.lock
new file mode 100644
index 0000000..7ff01b2
--- /dev/null
+++ b/tests/fuzz-corpus/lockfiles/yarn-berry.lock
@@ -0,0 +1,6 @@
+__metadata:
+  version: 8
+
+"react@npm:^18.2.0":
+  version: 18.2.0
+  resolution: "react@npm:18.2.0"
diff --git a/tests/fuzz-corpus/lockfiles/yarn-v1.lock b/tests/fuzz-corpus/lockfiles/yarn-v1.lock
new file mode 100644
index 0000000..4edf85b
--- /dev/null
+++ b/tests/fuzz-corpus/lockfiles/yarn-v1.lock
@@ -0,0 +1,8 @@
+# yarn lockfile v1
+
+"@babel/core@^7.0.0", "@babel/core@^7.1.0":
+  version "7.23.2"
+  resolved "https://registry.yarnpkg.com/@babel/core/-/core-7.23.2.tgz"
+
+lodash@^4.17.21:
+  version "4.17.21"
diff --git a/tests/fuzz_lockfiles.py b/tests/fuzz_lockfiles.py
new file mode 100644
index 0000000..38f8638
--- /dev/null
+++ b/tests/fuzz_lockfiles.py
@@ -0,0 +1,35 @@
+"""Coverage-guided fuzzing of the lockfile parsers with Atheris (Linux x86-64; CI runs it).
+
+Lockfiles come from any project in the projects folder, other people's code included, so whatever
+they contain, every parser must return clean (name, version) pairs and never raise: one bad file
+would otherwise stop the malware and CVE checks for every project.
+
+    pip install --require-hashes -r tests/requirements-fuzz.txt
+    python tests/fuzz_lockfiles.py tests/fuzz-corpus/lockfiles -max_total_time=60
+"""
+import os
+import sys
+import tempfile
+
+os.environ["HOME"] = tempfile.mkdtemp(prefix="pkgdash-fuzz-")  # server.py keeps its state under HOME
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), ".."))
+
+import atheris  # noqa: E402
+
+with atheris.instrument_imports():
+    import server  # noqa: E402
+
+NAMES = sorted(server.LOCKFILES)
+
+
+def TestOneInput(data):
+    text = data.decode("utf-8", "replace")
+    for fname in NAMES:  # every input goes through every parser
+        for name, version in server.lock_deps(fname, text):
+            assert isinstance(name, str) and name, (fname, name)
+            assert isinstance(version, str) and version and ":" not in version and "/" not in version, (fname, version)
+
+
+if __name__ == "__main__":
+    atheris.Setup(sys.argv, TestOneInput)
+    atheris.Fuzz()
diff --git a/tests/fuzz_projects_root.py b/tests/fuzz_projects_root.py
new file mode 100644
index 0000000..3a68aff
--- /dev/null
+++ b/tests/fuzz_projects_root.py
@@ -0,0 +1,35 @@
+"""Coverage-guided fuzzing, with Atheris, of the check that keeps the projects folder inside the home
+folder: for any string, the answer is either "no" (None) or "~/…" inside home, not ~/Library.
+
+    python tests/fuzz_projects_root.py -max_total_time=60
+"""
+import os
+import sys
+import tempfile
+
+os.environ["HOME"] = tempfile.mkdtemp(prefix="pkgdash-fuzz-")
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), ".."))
+
+import atheris  # noqa: E402
+
+with atheris.instrument_imports():
+    import server  # noqa: E402
+
+HOME = os.path.realpath(os.environ["HOME"])
+
+
+def TestOneInput(data):
+    value = atheris.FuzzedDataProvider(data).ConsumeUnicodeNoSurrogates(256)
+    root = server.clean_projects_root(value)
+    if root is None:
+        return
+    assert root.startswith("~/"), (value, root)
+    parts = root[2:].split("/")
+    assert parts[0] not in ("", ".", "..", "Library") and ".." not in parts, (value, root)
+    full = os.path.realpath(os.path.expanduser(root))
+    assert full.startswith(HOME + "/"), (value, root, full)
+
+
+if __name__ == "__main__":
+    atheris.Setup(sys.argv, TestOneInput)
+    atheris.Fuzz()
diff --git a/tests/requirements-fuzz.txt b/tests/requirements-fuzz.txt
new file mode 100644
index 0000000..d84ffb4
--- /dev/null
+++ b/tests/requirements-fuzz.txt
@@ -0,0 +1,6 @@
+# Fuzzing dependency (Linux x86-64 wheels only), pinned with hashes; Renovate keeps it current.
+atheris==3.1.0 \
+    --hash=sha256:315a0b5c819852b1ffe1ca72efc389c7724881f2c33e4aacb8c6bcec49bd5011 \
+    --hash=sha256:ec5e11f21a4c197fe91f7aea2b2de88e623c73a21fc07b105ac6329a1588457b \
+    --hash=sha256:f8a9f51ce8369026e8eb7b7174835e8c4c85a1a6db5d9add36c15100779d2a39
+    # via fuzzing
diff --git a/tests/test_server.py b/tests/test_server.py
index d4f9de1..1789b36 100644
--- a/tests/test_server.py
+++ b/tests/test_server.py
@@ -54,6 +54,31 @@ def test_outside_home_or_library_is_refused(self):
             self.assertIsNone(server.clean_projects_root(bad), repr(bad))
 
 
+class LockfileParsing(unittest.TestCase):
+    """Lockfiles come from any project, so a hostile one must be skipped, never crash the scan."""
+
+    def test_hostile_package_locks_parse_to_nothing(self):
+        deep = '{"dependencies": ' + '{"x": {"version": "1", "dependencies": ' * 5000 + "{}" + "}}" * 5000 + "}"
+        for text in ("[]", '{"packages": {"node_modules/x": "1.0.0"}}', '{"packages": ["x"]}',
+                     '{"dependencies": {"x": 5}}', '{"packages": {"node_modules/x": {"version": 3}}}', deep, "not json"):
+            self.assertEqual(server.lock_deps("package-lock.json", text), [], text[:40])
+
+    def test_real_lockfiles_still_parse(self):
+        corpus = os.path.join(HERE, "fuzz-corpus", "lockfiles")
+        def deps(seed, fname):
+            with open(os.path.join(corpus, seed)) as f:
+                return server.lock_deps(fname, f.read())
+        self.assertEqual(deps("package-lock-v3.json", "package-lock.json"),
+                         [("@scope/pkg", "2.1.0"), ("b", "0.1.0"), ("left-pad", "1.3.0")])
+        self.assertEqual(deps("package-lock-v1.json", "package-lock.json"),
+                         [("debug", "2.6.9"), ("express", "4.18.2"), ("lodash", "4.17.21"), ("ms", "2.0.0")])
+        self.assertEqual(deps("yarn-v1.lock", "yarn.lock"), [("@babel/core", "7.23.2"), ("lodash", "4.17.21")])
+        self.assertEqual(deps("requirements.txt", "requirements.txt"), [("requests", "2.31.0"), ("urllib3", "2.0.7")])
+        self.assertEqual(deps("Cargo.lock", "Cargo.lock"), [("serde", "1.0.190")])
+        self.assertEqual(deps("uv.lock", "uv.lock"), [("requests", "2.31.0")])
+        self.assertEqual(deps("poetry.lock", "poetry.lock"), [("click", "8.1.7")])
+
+
 class Migration(unittest.TestCase):
     def setUp(self):
         self.state = server.STATE_DIR / "state.json"