From ea8d735eb084cf8cc021df1a30e90d10a8f052e3 Mon Sep 17 00:00:00 2001
From: "Miss Islington (bot)"
 <31488909+miss-islington@users.noreply.github.com>
Date: Tue, 4 Aug 2026 11:18:25 +0200
Subject: [PATCH] [3.12] gh-148395: Fix a possible UAF in
 `{LZMA,BZ2,_Zlib}Decompressor` (GH-148396) (#148503)

* gh-148395: Fix a possible UAF in `{LZMA,BZ2,_Zlib}Decompressor` (GH-148396)

Fix dangling input pointer after `MemoryError` in _lzma/_bz2/_ZlibDecompressor.decompress
(cherry picked from commit 8fc66aef6d7b3ae58f43f5c66f9366cc8cbbfcd2)

Co-authored-by: Stan Ulbrych <stan@python.org>
---
 .../Security/2026-04-10-16-28-21.gh-issue-148395.kfzm0G.rst  | 5 +++++
 Modules/_bz2module.c                                         | 1 +
 Modules/_lzmamodule.c                                        | 1 +
 Modules/zlibmodule.c                                         | 1 +
 4 files changed, 8 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Security/2026-04-10-16-28-21.gh-issue-148395.kfzm0G.rst

diff --git a/Misc/NEWS.d/next/Security/2026-04-10-16-28-21.gh-issue-148395.kfzm0G.rst b/Misc/NEWS.d/next/Security/2026-04-10-16-28-21.gh-issue-148395.kfzm0G.rst
new file mode 100644
index 000000000000000..9502189ab199c15
--- /dev/null
+++ b/Misc/NEWS.d/next/Security/2026-04-10-16-28-21.gh-issue-148395.kfzm0G.rst
@@ -0,0 +1,5 @@
+Fix a dangling input pointer in :class:`lzma.LZMADecompressor`,
+:class:`bz2.BZ2Decompressor`, and internal :class:`!zlib._ZlibDecompressor`
+when memory allocation fails with :exc:`MemoryError`, which could let a
+subsequent :meth:`!decompress` call read or write through a stale pointer to
+the already-released caller buffer.
diff --git a/Modules/_bz2module.c b/Modules/_bz2module.c
index 97bd44b4ac96944..a732e89d554448c 100644
--- a/Modules/_bz2module.c
+++ b/Modules/_bz2module.c
@@ -587,6 +587,7 @@ decompress(BZ2Decompressor *d, char *data, size_t len, Py_ssize_t max_length)
     return result;
 
 error:
+    bzs->next_in = NULL;
     Py_XDECREF(result);
     return NULL;
 }
diff --git a/Modules/_lzmamodule.c b/Modules/_lzmamodule.c
index 7bbd6569aa2e449..103a6ef86c0d38f 100644
--- a/Modules/_lzmamodule.c
+++ b/Modules/_lzmamodule.c
@@ -1114,6 +1114,7 @@ decompress(Decompressor *d, uint8_t *data, size_t len, Py_ssize_t max_length)
     return result;
 
 error:
+    lzs->next_in = NULL;
     Py_XDECREF(result);
     return NULL;
 }
diff --git a/Modules/zlibmodule.c b/Modules/zlibmodule.c
index f94c57e4c89c4f7..9759593b6acff4b 100644
--- a/Modules/zlibmodule.c
+++ b/Modules/zlibmodule.c
@@ -1645,6 +1645,7 @@ decompress(ZlibDecompressor *self, uint8_t *data,
     return result;
 
 error:
+    self->zst.next_in = NULL;
     Py_XDECREF(result);
     return NULL;
 }
