diff --git a/scripts/ci/strix_quick_gate.sh b/scripts/ci/strix_quick_gate.sh
index c25ad0ee2..f08ebe0d7 100644
--- a/scripts/ci/strix_quick_gate.sh
+++ b/scripts/ci/strix_quick_gate.sh
@@ -269,6 +269,17 @@ pull_request_head_blob_required() {
 	[ "${GITHUB_EVENT_NAME:-}" = "pull_request_target" ]
 }
 
+is_valid_git_commit_sha() {
+	local sha="$1"
+	[[ "$sha" =~ ^[0-9a-fA-F]{40}$ || "$sha" =~ ^[0-9a-fA-F]{64}$ ]]
+}
+
+invalid_pull_request_sha() {
+	local label="$1"
+	echo "ERROR: pull request $label commit SHA is invalid; failing closed." >&2
+	return 2
+}
+
 pr_head_regular_file_mode() {
 	local relative_path="$1"
 	local head_sha tree_output line_count metadata tree_path mode object_type _object_hash
@@ -276,7 +287,10 @@ pr_head_regular_file_mode() {
 	if [ -z "$head_sha" ]; then
 		return 2
 	fi
-	if ! git cat-file -e "$head_sha^{commit}" 2>/dev/null; then
+	if ! is_valid_git_commit_sha "$head_sha"; then
+		return 2
+	fi
+	if ! git rev-parse --verify --quiet "$head_sha^{commit}" >/dev/null; then
 		return 2
 	fi
 	if ! tree_output="$(git ls-tree "$head_sha" -- "$relative_path")"; then
@@ -703,14 +717,28 @@ PY
 		fi
 		return 1
 	fi
-	if ! git cat-file -e "$base_sha^{commit}" 2>/dev/null; then
+	if ! is_valid_git_commit_sha "$base_sha"; then
+		if pull_request_head_blob_required; then
+			invalid_pull_request_sha "base"
+			return 2
+		fi
+		return 1
+	fi
+	if ! is_valid_git_commit_sha "$head_sha"; then
+		if pull_request_head_blob_required; then
+			invalid_pull_request_sha "head"
+			return 2
+		fi
+		return 1
+	fi
+	if ! git rev-parse --verify --quiet "$base_sha^{commit}" >/dev/null; then
 		if pull_request_head_blob_required; then
 			echo "ERROR: pull request base commit could not be read; failing closed: $base_sha" >&2
 			return 2
 		fi
 		return 1
 	fi
-	if ! git cat-file -e "$head_sha^{commit}" 2>/dev/null; then
+	if ! git rev-parse --verify --quiet "$head_sha^{commit}" >/dev/null; then
 		if pull_request_head_blob_required; then
 			echo "ERROR: pull request head commit could not be read; failing closed: $head_sha" >&2
 			return 2
@@ -719,7 +747,7 @@ PY
 	fi
 
 	local changed_files_output
-	if ! changed_files_output="$(git diff --name-only "$base_sha...$head_sha")"; then
+	if ! changed_files_output="$(git diff --name-only "$base_sha...$head_sha" --)"; then
 		if pull_request_head_blob_required; then
 			echo "ERROR: pull request changed file list could not be read; failing closed." >&2
 			return 2
@@ -1077,7 +1105,7 @@ PY
 		local copy_rc=1
 		local head_sha_for_copy
 		head_sha_for_copy="$(trim_whitespace "${PR_HEAD_SHA:-}")"
-		if pull_request_head_blob_required || { [ -n "$head_sha_for_copy" ] && git cat-file -e "$head_sha_for_copy^{commit}" 2>/dev/null; }; then
+		if pull_request_head_blob_required || { [ -n "$head_sha_for_copy" ] && is_valid_git_commit_sha "$head_sha_for_copy" && git rev-parse --verify --quiet "$head_sha_for_copy^{commit}" >/dev/null; }; then
 			copy_rc=0
 			copy_pr_head_blob_to_file "$relative_path" "$dst_path" || copy_rc=$?
 		fi
@@ -1126,7 +1154,7 @@ PY
 			local copy_rc=1
 			local head_sha_for_copy
 			head_sha_for_copy="$(trim_whitespace "${PR_HEAD_SHA:-}")"
-			if pull_request_head_blob_required || { [ -n "$head_sha_for_copy" ] && git cat-file -e "$head_sha_for_copy^{commit}" 2>/dev/null; }; then
+			if pull_request_head_blob_required || { [ -n "$head_sha_for_copy" ] && is_valid_git_commit_sha "$head_sha_for_copy" && git rev-parse --verify --quiet "$head_sha_for_copy^{commit}" >/dev/null; }; then
 				copy_rc=0
 				copy_pr_head_blob_to_file "$relative_path" "$dst_path" || copy_rc=$?
 			fi
diff --git a/scripts/ci/test_strix_quick_gate.sh b/scripts/ci/test_strix_quick_gate.sh
index 2fc1e04f8..5d3ac92c3 100644
--- a/scripts/ci/test_strix_quick_gate.sh
+++ b/scripts/ci/test_strix_quick_gate.sh
@@ -3114,6 +3114,98 @@ EOF
 	rm -rf "$tmp_dir"
 }
 
+run_pull_request_target_rejects_invalid_sha_case() {
+	local case_name="$1"
+	local invalid_side="$2"
+
+	local tmp_dir
+	tmp_dir="$(mktemp -d)"
+	local bin_dir="$tmp_dir/bin"
+	local repo_root_dir="$tmp_dir/repo"
+	mkdir -p "$bin_dir" "$repo_root_dir/scripts/ci"
+	cp "$GATE_SCRIPT" "$repo_root_dir/scripts/ci/strix_quick_gate.sh"
+	cp "$REPO_ROOT/scripts/ci/strix_model_utils.sh" "$repo_root_dir/scripts/ci/strix_model_utils.sh"
+	chmod +x "$repo_root_dir/scripts/ci/strix_quick_gate.sh"
+
+	local fake_strix="$bin_dir/strix"
+	local call_log="$tmp_dir/calls.log"
+	local output_log="$tmp_dir/output.log"
+	local strix_llm_file="$tmp_dir/strix_llm.txt"
+	local llm_api_key_file="$tmp_dir/llm_api_key.txt"
+
+	cat >"$fake_strix" <<'EOF'
+#!/usr/bin/env bash
+set -euo pipefail
+printf 'called\n' >> "${FAKE_STRIX_CALL_LOG:?}"
+echo "Error: Strix should not run after invalid pull request SHA metadata" >&2
+exit 67
+EOF
+	chmod +x "$fake_strix"
+	printf '%s' 'gemini/test-model' >"$strix_llm_file"
+	printf '%s' 'dummy' >"$llm_api_key_file"
+
+	(
+		cd "$repo_root_dir"
+		git init -q
+		git config user.name 'Strix Test'
+		git config user.email 'strix-test@example.invalid'
+		echo 'seed' >README.md
+		git add .
+		git commit -qm 'base commit'
+	)
+	local base_sha
+	base_sha="$(git -C "$repo_root_dir" rev-parse HEAD)"
+	(
+		cd "$repo_root_dir"
+		echo 'head' >>README.md
+		git add .
+		git commit -qm 'head commit'
+	)
+	local head_sha
+	head_sha="$(git -C "$repo_root_dir" rev-parse HEAD)"
+	git -C "$repo_root_dir" checkout -q "$base_sha"
+
+	local injection_marker="STRIX_SHA_INJECTION_MARKER"
+	local malicious_sha='0000000000000000000000000000000000000000$(echo STRIX_SHA_INJECTION_MARKER)'
+	local expected_message="pull request $invalid_side commit SHA is invalid; failing closed"
+	if [ "$invalid_side" = "base" ]; then
+		base_sha="$malicious_sha"
+	else
+		head_sha="$malicious_sha"
+	fi
+
+	set +e
+	(
+		cd "$repo_root_dir"
+		env -u GITHUB_EVENT_PATH -u STRIX_TEST_CHANGED_FILES_OVERRIDE \
+			PATH="$bin_dir:$PATH" \
+			STRIX_INPUT_FILE_ROOT="$tmp_dir" \
+			GITHUB_EVENT_NAME="pull_request_target" \
+			PR_BASE_SHA="$base_sha" \
+			PR_HEAD_SHA="$head_sha" \
+			FAKE_STRIX_CALL_LOG="$call_log" \
+			STRIX_DISABLE_PR_SCOPING="0" \
+			STRIX_LLM_FILE="$strix_llm_file" \
+			LLM_API_KEY_FILE="$llm_api_key_file" \
+			STRIX_TARGET_PATH="." \
+			STRIX_REPORTS_DIR="$repo_root_dir/strix_runs" \
+			bash "./scripts/ci/strix_quick_gate.sh" >"$output_log" 2>&1
+	)
+	local rc=$?
+	set -e
+
+	assert_equals "2" "$rc" "case=$case_name invalid PR SHA exits closed"
+	assert_file_contains "$output_log" "$expected_message" "case=$case_name invalid PR SHA output"
+	assert_file_not_contains "$output_log" "$injection_marker" "case=$case_name invalid PR SHA must not echo untrusted value"
+	local call_count="0"
+	if [ -f "$call_log" ]; then
+		call_count="$(wc -l <"$call_log" | tr -d ' ')"
+	fi
+	assert_equals "0" "$call_count" "case=$case_name invalid PR SHA must not invoke Strix"
+
+	rm -rf "$tmp_dir"
+}
+
 run_pull_request_target_irregular_head_entry_fails_closed_case() {
 	local case_name="$1"
 	local changed_file="$2"
@@ -4157,6 +4249,14 @@ run_pull_request_target_aborts_on_pr_head_blob_failure_case \
 	"HEAD_CONTENT_SHOULD_NOT_BECOME_PARTIAL_SCAN_INPUT" \
 	"diff"
 
+run_pull_request_target_rejects_invalid_sha_case \
+	"pull-request-target-invalid-base-sha-fails-closed" \
+	"base"
+
+run_pull_request_target_rejects_invalid_sha_case \
+	"pull-request-target-invalid-head-sha-fails-closed" \
+	"head"
+
 run_pull_request_target_aborts_on_pr_head_blob_failure_case \
 	"pull-request-target-disabled-pr-scope-pr-head-blob-read-failure" \
 	"src/existing.py" \