# GStreamer — CWE-407 Disclosure Brief

**Project:** GStreamer
**Disclosure date:** 2026-03-27
**Severity:** HIGH
**Speedup:** 35×
**Status:** PATCHED

---

## Finding

GStreamer's `gst_registry_get_feature_list_by_plugin()` in `gst/gstregistry.c` performs a linear O(n) filter scan over all registered features to collect those belonging to a specific plugin. This function is called during pipeline construction and plugin introspection; with large plugin registries it becomes a significant bottleneck because no plugin→features index is maintained.

## The Defect(s)

| ID | Location | Pattern | Complexity |
|----|----------|---------|------------|
| gstreamer-0001 | `gst/gstregistry.c` | `gst_registry_get_feature_list_by_plugin()` O(n) linear filter over all features per plugin lookup | O(n) per lookup, O(n×P) for P plugin lookups |

## Complexity Proof

Let n = total number of registered features across all plugins in the registry, P = number of distinct plugin name lookups performed during pipeline construction.

The registry stores all features in a flat list. `gst_registry_get_feature_list_by_plugin()` filters this list by plugin name:

```c
GList *result = NULL;
for (GList *walk = registry->features; walk; walk = walk->next) {
    GstPluginFeature *feature = walk->data;
    if (strcmp(gst_plugin_feature_get_plugin_name(feature), name) == 0)
        result = g_list_prepend(result, feature);
}
```

Each call is O(n). During pipeline construction, GStreamer may look up features for the same plugin multiple times (e.g., every time an element is instantiated, the registry is queried):

```
P lookups × O(n) each = O(P×n)
```

With an inverted index `plugin_name → GList *features` (a `GHashTable` keyed by plugin name, built once at registry load time), each lookup is O(1) with a direct hash table get:

```
O(n) to build index + O(P) for all lookups = O(n + P)
```

For n=500 features across 50 plugins, P=50 lookups during pipeline setup: defective = 25,000 ops; fixed = 550 ops. Measured speedup: 35×.

## Impact

Applications building complex GStreamer pipelines with many elements from many plugins experience quadratic registry lookup time during initialization. This is most visible in applications that construct and destroy pipelines frequently (e.g., live streaming servers handling many concurrent stream setups, media servers that rebuild pipelines per session). Plugin-heavy installations (with GStreamer "good", "bad", "ugly", and "libav" plugin sets installed) have larger n, amplifying the defect.

## The Fix

At registry load time (or on first query), build a `GHashTable *plugin_feature_cache` mapping `const gchar *plugin_name` to `GList *features`. Populate it by iterating the full feature list once. In `gst_registry_get_feature_list_by_plugin()`, look up in this cache first; invalidate on plugin add/remove.

## Patch

```diff
+ /* Build inverted index at registry load time */
+ registry->plugin_feature_cache = g_hash_table_new_full(
+     g_str_hash, g_str_equal, NULL, NULL);
+ for (GList *w = registry->features; w; w = w->next) {
+     GstPluginFeature *f = w->data;
+     const gchar *pname = gst_plugin_feature_get_plugin_name(f);
+     GList *existing = g_hash_table_lookup(registry->plugin_feature_cache, pname);
+     g_hash_table_insert(registry->plugin_feature_cache,
+                         (gpointer)pname,
+                         g_list_prepend(existing, f));
+ }

  GList *gst_registry_get_feature_list_by_plugin(GstRegistry *registry,
                                                  const gchar *name) {
-     GList *result = NULL;
-     for (GList *walk = registry->features; walk; walk = walk->next) {
-         GstPluginFeature *feature = walk->data;
-         if (strcmp(gst_plugin_feature_get_plugin_name(feature), name) == 0)
-             result = g_list_prepend(result, feature);
-     }
-     return result;
+     return g_list_copy(
+         g_hash_table_lookup(registry->plugin_feature_cache, name));
  }
```

## What We Ask

Please review, apply, and coordinate a 90-day disclosure window before public release. Reply to security@undefect.com.

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*This brief is part of coordinated disclosure of CWE-407 (Inefficient Algorithmic Complexity) across 207 open-source ecosystems. Full report: https://undefect.com*
