diff --git a/platform/internal/handlers/activity.go b/platform/internal/handlers/activity.go
index 4699a3a9..6eabf44a 100644
--- a/platform/internal/handlers/activity.go
+++ b/platform/internal/handlers/activity.go
@@ -338,10 +338,12 @@ func (h *ActivityHandler) Report(c *gin.Context) {
 	// Empty source_id falls through to the default-to-self branch below.
 	sourceID := body.SourceID
 	if sourceID != "" && sourceID != workspaceID {
-		// Log the spoof attempt as a security event so an auditor cron can
-		// surface repeat probing. Keep the log line stable (greppable) and
-		// avoid echoing attacker-supplied data verbatim beyond the UUIDs.
-		log.Printf("security: source_id spoof attempt — authed_workspace=%s body_source_id=%s remote=%s",
+		// #234: sanitize attacker-controlled values before logging.
+		// body.SourceID comes from a JSON request, and json.Unmarshal
+		// decodes \n escapes into literal newlines — an authenticated
+		// workspace could otherwise inject fake log lines. Use %q which
+		// emits a Go-quoted string with all control characters escaped.
+		log.Printf("security: source_id spoof attempt — authed_workspace=%s body_source_id=%q remote=%q",
 			workspaceID, sourceID, c.ClientIP())
 		c.JSON(http.StatusForbidden, gin.H{"error": "source_id must match authenticated workspace"})
 		return
diff --git a/platform/internal/handlers/handlers_test.go b/platform/internal/handlers/handlers_test.go
index 20897baf..d6cfca9f 100644
--- a/platform/internal/handlers/handlers_test.go
+++ b/platform/internal/handlers/handlers_test.go
@@ -1131,3 +1131,34 @@ func TestActivityHandler_Report_MatchingSourceIDAccepted(t *testing.T) {
 		t.Errorf("matching source_id: got %d, want 200 (%s)", w.Code, w.Body.String())
 	}
 }
+
+// TestActivityHandler_Report_SourceIDLogInjection — #234 regression guard.
+// The security log line must emit the attacker-supplied source_id through
+// %q so control characters (\n, \r, \t) are escaped instead of splitting
+// the log stream into fake entries. Harder to assert directly without a
+// log capture, so we just exercise the code path with a payload containing
+// newlines and confirm the handler still returns 403 cleanly (no panic,
+// no accidental success).
+func TestActivityHandler_Report_SourceIDLogInjection(t *testing.T) {
+	setupTestDB(t)
+	setupTestRedis(t)
+	broadcaster := newTestBroadcaster()
+	handler := NewActivityHandler(broadcaster)
+
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Params = gin.Params{{Key: "id", Value: "ws-alice"}}
+	// JSON body with explicit \n escapes — json.Unmarshal decodes these
+	// into literal newline bytes before reaching the log call.
+	body := `{"activity_type":"agent_log","summary":"x","source_id":"ws-evil\ntimestamp=FORGED level=INFO msg=fake"}`
+	c.Request = httptest.NewRequest("POST", "/workspaces/ws-alice/activity",
+		bytes.NewBufferString(body))
+	c.Request.Header.Set("Content-Type", "application/json")
+
+	handler.Report(c)
+
+	if w.Code != http.StatusForbidden {
+		t.Errorf("spoof with newline in source_id: got %d, want 403 (%s)",
+			w.Code, w.Body.String())
+	}
+}