gh-145883: Fix two heap-buffer-overflows in _zoneinfo

.gitattributes

@@ -19,6 +19,7 @@
 *.zip binary
 
 # Specific binary files
+Lib/test/test_zoneinfo/data/tzif_* binary
 PC/classicAppCompat.* binary
 
 # Text files that should not be subject to eol conversion

Lib/test/test_zoneinfo/test_zoneinfo.py

@@ -741,6 +741,16 @@ def test_empty_zone(self):
         with self.assertRaises(ValueError):
             self.klass.from_file(zf)
 
+    def test_invalid_transition_index(self):
+        with open(DATA_DIR / "tzif_invalid_trans_idx", "rb") as f:
+            with self.assertRaises(ValueError):
+                self.klass.from_file(f)
+
+    def test_transition_lookahead_out_of_bounds(self):
+        with open(DATA_DIR / "tzif_invalid_lookahead", "rb") as f:
+            zi = self.klass.from_file(f)
+        self.assertIsNotNone(zi)
+
     def test_zone_very_large_timestamp(self):
         """Test when a transition is in the far past or future.
 

Lib/zoneinfo/_common.py

@@ -71,6 +71,10 @@ def load_data(fobj):
         trans_list_utc = ()
         trans_idx = ()
 
+    for idx in trans_idx:
+        if idx >= typecnt:
+            raise ValueError(f"Invalid transition index found while reading TZif: {idx}")
+
     # Read the ttinfo struct, (utoff, isdst, abbrind)
     if typecnt:
         utcoff, isdst, abbrind = zip(

Lib/zoneinfo/_zoneinfo.py

@@ -338,7 +338,7 @@ def _utcoff_to_dstoff(trans_idx, utcoffsets, isdsts):
             if not isdsts[comp_idx]:
                 dstoff = utcoff - utcoffsets[comp_idx]
 
-            if not dstoff and idx < (typecnt - 1):
+            if not dstoff and idx < (typecnt - 1) and i + 1 < len(trans_idx):
                 comp_idx = trans_idx[i + 1]
 
                 # If the following transition is also DST and we couldn't

Misc/NEWS.d/next/Library/2026-03-12-21-01-48.gh-issue-145883.lUvXcc.rst

@@ -0,0 +1,2 @@
+:mod:`zoneinfo`: Fix heap buffer overflow reads from malformed TZif data.
+Found by OSS Fuzz, issues :oss-fuzz:`492245058` and :oss-fuzz:`492230068`.

Modules/_zoneinfo.c

@@ -1065,7 +1065,7 @@ load_data(zoneinfo_state *state, PyZoneInfo_ZoneInfo *self, PyObject *file_obj)
         }
 
         trans_idx[i] = (size_t)cur_trans_idx;
-        if (trans_idx[i] > self->num_ttinfos) {
+        if (trans_idx[i] >= self->num_ttinfos) {
             PyErr_Format(
                 PyExc_ValueError,
                 "Invalid transition index found while reading TZif: %zd",
@@ -2063,7 +2063,7 @@ utcoff_to_dstoff(size_t *trans_idx, long *utcoffs, long *dstoffs,
             dstoff = utcoff - utcoffs[comp_idx];
         }
 
-        if (!dstoff && idx < (num_ttinfos - 1)) {
+        if (!dstoff && idx < (num_ttinfos - 1) && i + 1 < num_transitions) {
             comp_idx = trans_idx[i + 1];
 
             // If the following transition is also DST and we couldn't find

Tools/build/compute-changes.py

@@ -98,6 +98,9 @@
     Path("Modules/pyexpat.c"),
     # zipfile
     Path("Lib/zipfile/"),
+    # zoneinfo
+    Path("Lib/zoneinfo/"),
+    Path("Modules/_zoneinfo.c"),
 })