Don't permit pull_request_target for GitHub publishers

tests/unit/oidc/models/test_github.py

@@ -433,6 +433,18 @@ def test_check_repository(self, truth, claim, valid):
         check = github.GitHubPublisher.__required_verifiable_claims__["repository"]
         assert check(truth, claim, pretend.stub()) == valid
 
+    def test_check_event_name_invalid(self):
+        check = github.GitHubPublisher.__required_verifiable_claims__["event_name"]
+
+        with pytest.raises(
+            errors.InvalidPublisherError,
+            match=(
+                "Publishing from a workflow invoked via 'pull_request_target' "
+                "is not supported."
+            ),
+        ):
+            check("throwaway", "pull_request_target", pretend.stub())
+
     @pytest.mark.parametrize(
         ("claim", "ref", "sha", "valid", "expected"),
         [

warehouse/oidc/models/github.py

@@ -152,6 +152,18 @@ def _check_sub(
     return f"{org}:{repo}".lower() == ground_truth.lower()
 
 
+def _check_event_name(
+    ground_truth: str, signed_claim: str, _all_signed_claims: SignedClaims, **_kwargs,
+) -> bool:
+    if signed_claim == "pull_request_target":
+        raise InvalidPublisherError(
+            "Publishing from a workflow invoked via 'pull_request_target' is "
+            "not supported."
+        )
+    else:
+        return True
+
+
 class GitHubPublisherMixin:
     """
     Common functionality for both pending and concrete GitHub OIDC publishers.
@@ -170,6 +182,7 @@ class GitHubPublisherMixin:
         "repository_owner_id": check_claim_binary(str.__eq__),
         "job_workflow_ref": _check_job_workflow_ref,
         "jti": check_existing_jti,
+        "event_name": _check_event_name,
     }
 
     __required_unverifiable_claims__: set[str] = {"ref", "sha"}
@@ -186,7 +199,6 @@ class GitHubPublisherMixin:
         "run_attempt",
         "head_ref",
         "base_ref",
-        "event_name",
         "ref_type",
         "repository_id",
         "workflow",
@@ -275,6 +287,11 @@ def jti(self) -> str:
         """Placeholder value for JTI."""
         return "placeholder"
 
+    @property
+    def event_name(self) -> str:
+        """Placeholder value for event_name (not used)"""
+        return "placeholder"
+
     def publisher_url(self, claims: SignedClaims | None = None) -> str:
         base = self.publisher_base_url
         sha = claims.get("sha") if claims else None