--- a/iconvdata/Makefile
+++ b/iconvdata/Makefile
@@ -73,7 +73,7 @@
 ifeq (yes,$(build-shared))
 tests = bug-iconv1 bug-iconv2 tst-loading tst-e2big tst-iconv4 bug-iconv4 \
 	tst-iconv6 bug-iconv5 bug-iconv6 tst-iconv7 bug-iconv8 bug-iconv9 \
-	bug-iconv10 bug-iconv11 bug-iconv12
+	bug-iconv10 bug-iconv11 bug-iconv12 tst-iconv-iso-2022-cn-ext
 ifeq ($(have-thread-library),yes)
 tests += bug-iconv3
 endif
@@ -316,6 +316,8 @@
 			  $(addprefix $(objpfx),$(modules.so))
 $(objpfx)bug-iconv12.out: $(objpfx)gconv-modules \
 			  $(addprefix $(objpfx),$(modules.so))
+$(objpfx)tst-iconv-iso-2022-cn-ext.out: $(addprefix $(objpfx), $(gconv-modules)) \
+					$(addprefix $(objpfx),$(modules.so))

 $(objpfx)iconv-test.out: run-iconv-test.sh $(objpfx)gconv-modules \
 			 $(addprefix $(objpfx),$(modules.so)) \
diff --git a/iconvdata/iso-2022-cn-ext.c b/iconvdata/iso-2022-cn-ext.c
index 947b807421..34e1010bed 100644
--- a/iconvdata/iso-2022-cn-ext.c
+++ b/iconvdata/iso-2022-cn-ext.c
@@ -575,6 +575,12 @@ DIAG_IGNORE_Os_NEEDS_COMMENT (5, "-Wmaybe-uninitialized");
 	      {								      \
 		const char *escseq;					      \
 									      \
+		if (outptr + 4 > outend)				      \
+		  {							      \
+		    result = __GCONV_FULL_OUTPUT;			      \
+		    break;						      \
+		  }							      \
+									      \
 		assert (used == CNS11643_2_set); /* XXX */		      \
 		escseq = "*H";						      \
 		*outptr++ = ESC;					      \
@@ -588,6 +594,12 @@ DIAG_IGNORE_Os_NEEDS_COMMENT (5, "-Wmaybe-uninitialized");
 	      {								      \
 		const char *escseq;					      \
 									      \
+		if (outptr + 4 > outend)				      \
+		  {							      \
+		    result = __GCONV_FULL_OUTPUT;			      \
+		    break;						      \
+		  }							      \
+									      \
 		assert ((used >> 5) >= 3 && (used >> 5) <= 7);		      \
 		escseq = "+I+J+K+L+M" + ((used >> 5) - 3) * 2;		      \
 		*outptr++ = ESC;					      \
diff --git a/iconvdata/tst-iconv-iso-2022-cn-ext.c b/iconvdata/tst-iconv-iso-2022-cn-ext.c
new file mode 100644
index 0000000000..96a8765fd5
--- /dev/null
+++ b/iconvdata/tst-iconv-iso-2022-cn-ext.c
@@ -0,0 +1,128 @@
+/* Verify ISO-2022-CN-EXT does not write out of the bounds.
+   Copyright (C) 2024 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <stdio.h>
+#include <string.h>
+
+#include <errno.h>
+#include <iconv.h>
+#include <sys/mman.h>
+
+#include <support/xunistd.h>
+#include <support/check.h>
+#include <support/support.h>
+
+/* The test sets up a two memory page buffer with the second page marked
+   PROT_NONE to trigger a fault if the conversion writes beyond the exact
+   expected amount.  Then we carry out various conversions and precisely
+   place the start of the output buffer in order to trigger a SIGSEGV if the
+   process writes anywhere between 1 and page sized bytes more (only one
+   PROT_NONE page is setup as a canary) than expected.  These tests exercise
+   all three of the cases in ISO-2022-CN-EXT where the converter must switch
+   character sets and may run out of buffer space while doing the
+   operation.  */
+
+static int
+do_test (void)
+{
+  iconv_t cd = iconv_open ("ISO-2022-CN-EXT", "UTF-8");
+  TEST_VERIFY_EXIT (cd != (iconv_t) -1);
+
+  char *ntf;
+  size_t ntfsize;
+  char *outbufbase;
+  {
+    int pgz = getpagesize ();
+    TEST_VERIFY_EXIT (pgz > 0);
+    ntfsize = 2 * pgz;
+
+    ntf = xmmap (NULL, ntfsize, PROT_READ | PROT_WRITE, MAP_PRIVATE
+		 | MAP_ANONYMOUS, -1);
+    xmprotect (ntf + pgz, pgz, PROT_NONE);
+
+    outbufbase = ntf + pgz;
+  }
+
+  /* Check if SOdesignation escape sequence does not trigger an OOB write.  */
+  {
+    char inbuf[] = "\xe4\xba\xa4\xe6\x8d\xa2";
+
+    for (int i = 0; i < 9; i++)
+      {
+	char *inp = inbuf;
+	size_t inleft = sizeof (inbuf) - 1;
+
+	char *outp = outbufbase - i;
+	size_t outleft = i;
+
+	TEST_VERIFY_EXIT (iconv (cd, &inp, &inleft, &outp, &outleft)
+			  == (size_t) -1);
+	TEST_COMPARE (errno, E2BIG);
+
+	TEST_VERIFY_EXIT (iconv (cd, NULL, NULL, NULL, NULL) == 0);
+      }
+  }
+
+  /* Same as before for SS2designation.  */
+  {
+    char inbuf[] = "ã´½ \xe3\xb4\xbd";
+
+    for (int i = 0; i < 14; i++)
+      {
+	char *inp = inbuf;
+	size_t inleft = sizeof (inbuf) - 1;
+
+	char *outp = outbufbase - i;
+	size_t outleft = i;
+
+	TEST_VERIFY_EXIT (iconv (cd, &inp, &inleft, &outp, &outleft)
+			  == (size_t) -1);
+	TEST_COMPARE (errno, E2BIG);
+
+	TEST_VERIFY_EXIT (iconv (cd, NULL, NULL, NULL, NULL) == 0);
+      }
+  }
+
+  /* Same as before for SS3designation.  */
+  {
+    char inbuf[] = "å \xe5\x8a\x84";
+
+    for (int i = 0; i < 14; i++)
+      {
+	char *inp = inbuf;
+	size_t inleft = sizeof (inbuf) - 1;
+
+	char *outp = outbufbase - i;
+	size_t outleft = i;
+
+	TEST_VERIFY_EXIT (iconv (cd, &inp, &inleft, &outp, &outleft)
+			  == (size_t) -1);
+	TEST_COMPARE (errno, E2BIG);
+
+	TEST_VERIFY_EXIT (iconv (cd, NULL, NULL, NULL, NULL) == 0);
+      }
+  }
+
+  TEST_VERIFY_EXIT (iconv_close (cd) != -1);
+
+  xmunmap (ntf, ntfsize);
+
+  return 0;
+}
+
+#include <support/test-driver.c>
--
2.39.3
