diff --git a/contrib/fuzz/mpatch_corpus.py b/contrib/fuzz/mpatch_corpus.py
--- a/contrib/fuzz/mpatch_corpus.py
+++ b/contrib/fuzz/mpatch_corpus.py
@@ -78,6 +78,10 @@
     zf.writestr(
         "mpatch_decode_old_overread", "\x02\x00\x00\x00\x02\x00\x00\x00"
     )
+    # https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=8876
+    zf.writestr(
+        "mpatch_ossfuzz_getbe32_ubsan",
+        "\x02\x00\x00\x00\x0c    \xff\xff\xff\xff    ")
     zf.writestr(
         "mpatch_apply_over_memcpy",
         '\x13\x01\x00\x05\xd0\x00\x00\x00\x00\x00\x00\x00\x00\n \x00\x00\x00'
diff --git a/mercurial/bitmanipulation.h b/mercurial/bitmanipulation.h
--- a/mercurial/bitmanipulation.h
+++ b/mercurial/bitmanipulation.h
@@ -9,7 +9,8 @@
 {
 	const unsigned char *d = (const unsigned char *)c;
 
-	return ((d[0] << 24) | (d[1] << 16) | (d[2] << 8) | (d[3]));
+	return ((((uint32_t)d[0]) << 24) | (((uint32_t)d[1]) << 16) |
+	        (((uint32_t)d[2]) << 8) | (d[3]));
 }
 
 static inline int16_t getbeint16(const char *c)