diff --git a/third_party/libopenjpeg20/0027-undefined-shift-opj_t1_decode_cblk.patch b/third_party/libopenjpeg20/0027-undefined-shift-opj_t1_decode_cblk.patch
new file mode 100644
index 0000000..7ba877a
--- /dev/null
+++ b/third_party/libopenjpeg20/0027-undefined-shift-opj_t1_decode_cblk.patch
@@ -0,0 +1,13 @@
+diff --git a/third_party/libopenjpeg20/t1.c b/third_party/libopenjpeg20/t1.c
+index a119db1f7..1ad850c77 100644
+--- a/third_party/libopenjpeg20/t1.c
++++ b/third_party/libopenjpeg20/t1.c
+@@ -1411,7 +1411,7 @@ static OPJ_BOOL opj_t1_decode_cblk(opj_t1_t *t1,
+             }
+                }
+ 
+-               for (passno = 0; passno < seg->real_num_passes; ++passno) {
++               for (passno = 0; (passno < seg->real_num_passes) && (bpno_plus_one >= 1); ++passno) {
+             switch (passtype) {
+                 case 0:
+                     if (type == T1_TYPE_RAW) {
diff --git a/third_party/libopenjpeg20/README.pdfium b/third_party/libopenjpeg20/README.pdfium
index 8ed6377..ea8f523 100644
--- a/third_party/libopenjpeg20/README.pdfium
+++ b/third_party/libopenjpeg20/README.pdfium
@@ -36,4 +36,5 @@
 0024-l_marker_size_check.patch: Return error before overflow in opj_j2k_read_header_procedure.
 0025-opj_j2k_add_mct_null_data.patch: Check m_data != null before trying to read from it.
 0026-use_opj_uint_ceildiv.patch: Remove (OPJ_UINT32)opj_int_ceildiv((OPJ_INT32)a, (OPJ_INT32) b).
+0027-undefined-shift-opj_t1_decode_cblk.patch: upstream fix for a ubsan bug.
 TODO(thestig): List all the other patches.
diff --git a/third_party/libopenjpeg20/t1.c b/third_party/libopenjpeg20/t1.c
index a119db1..1ad850c 100644
--- a/third_party/libopenjpeg20/t1.c
+++ b/third_party/libopenjpeg20/t1.c
@@ -1411,7 +1411,7 @@
             }
 		}
 
-		for (passno = 0; passno < seg->real_num_passes; ++passno) {
+		for (passno = 0; (passno < seg->real_num_passes) && (bpno_plus_one >= 1); ++passno) {
             switch (passtype) {
                 case 0:
                     if (type == T1_TYPE_RAW) {