From bf84156dcab516350d23801f44246369cc9a671b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Maurycy=20Paw=C5=82owski-Wiero=C5=84ski?=
 <maurycy@maurycy.com>
Date: Mon, 4 May 2026 11:40:52 +0200
Subject: [PATCH] gh-148093: Raise binascii.Error from binascii.a2b_uu() on
 empty input (GH-149077)
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Instead of reading past the end of the empty buffer.
(cherry picked from commit 0c6d2f64c0c83e7652760f770ff0c5cdc5040426)

Co-authored-by: Maurycy Pawłowski-Wieroński <maurycy@maurycy.com>
---
 Lib/test/test_binascii.py                                 | 7 +++++++
 .../2026-04-27-22-34-09.gh-issue-148093.9pWceM.rst        | 2 ++
 Modules/binascii.c                                        | 8 ++++++++
 3 files changed, 17 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Library/2026-04-27-22-34-09.gh-issue-148093.9pWceM.rst

diff --git a/Lib/test/test_binascii.py b/Lib/test/test_binascii.py
index 6e75d7d8f28f93..6e397df13c406c 100644
--- a/Lib/test/test_binascii.py
+++ b/Lib/test/test_binascii.py
@@ -240,6 +240,10 @@ def test_uu(self):
         self.assertEqual(binascii.a2b_uu(b"\xff"), b"\x00"*31)
         self.assertRaises(binascii.Error, binascii.a2b_uu, b"\xff\x00")
         self.assertRaises(binascii.Error, binascii.a2b_uu, b"!!!!")
+        self.assertRaises(binascii.Error, binascii.a2b_uu,
+                          self.type2test(b""))
+        self.assertRaises(binascii.Error, binascii.a2b_uu,
+                          self.type2test(b"#86)C")[:0])
         self.assertRaises(binascii.Error, binascii.b2a_uu, 46*b"!")
 
         # Issue #7701 (crash on a pydebug build)
@@ -447,6 +451,9 @@ def test_empty_string(self):
                 binascii.crc_hqx(empty, 0)
                 continue
             f = getattr(binascii, func)
+            if func == 'a2b_uu':
+                self.assertRaises(binascii.Error, f, empty)
+                continue
             try:
                 f(empty)
             except Exception as err:
diff --git a/Misc/NEWS.d/next/Library/2026-04-27-22-34-09.gh-issue-148093.9pWceM.rst b/Misc/NEWS.d/next/Library/2026-04-27-22-34-09.gh-issue-148093.9pWceM.rst
new file mode 100644
index 00000000000000..9418044201f8bd
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-04-27-22-34-09.gh-issue-148093.9pWceM.rst
@@ -0,0 +1,2 @@
+Fix an out-of-bounds read of one byte in :func:`binascii.a2b_uu`. Raise
+:exc:`binascii.Error`, instead of reading past the buffer end.
diff --git a/Modules/binascii.c b/Modules/binascii.c
index 1030eb15f4169c..6b762b809b5989 100644
--- a/Modules/binascii.c
+++ b/Modules/binascii.c
@@ -219,6 +219,14 @@ binascii_a2b_uu_impl(PyObject *module, Py_buffer *data)
     assert(ascii_len >= 0);
 
     /* First byte: binary data length (in bytes) */
+    if (ascii_len == 0) {
+        state = get_binascii_state(module);
+        if (state == NULL) {
+            return NULL;
+        }
+        PyErr_SetString(state->Error, "Missing length byte");
+        return NULL;
+    }
     bin_len = (*ascii_data++ - ' ') & 077;
     ascii_len--;