diff -r e8783c581928 Lib/test/test_codecencodings_cn.py
--- a/Lib/test/test_codecencodings_cn.py
+++ b/Lib/test/test_codecencodings_cn.py
@@ -46,6 +46,8 @@ class Test_GB18030(test_multibytecodec_support.Tes
         ("abc\x80\x80\xc1\xc4", "ignore",  u"abc\u804a"),
         ("abc\x84\x39\x84\x39\xc1\xc4", "replace", u"abc\ufffd\u804a"),
         (u"\u30fb", "strict", "\x819\xa79"),
+        # issue24117
+        (b"\x81\x30\xFF\x30", "strict",  None),
     )
     has_iso10646 = True
 
@@ -76,6 +78,10 @@ class Test_HZ(test_multibytecodec_support.TestBase
         (b'ab~cd', 'replace', u'ab\uFFFDd'),
         (b'ab\xffcd', 'replace', u'ab\uFFFDcd'),
         (b'ab~{\x81\x81\x41\x44~}cd', 'replace', u'ab\uFFFD\uFFFD\u804Acd'),
+        # issue24117
+        (u'hi~', 'strict',  b'hi~~'), # escape ~
+        (b'~{Dc~~:C~}', 'strict', None), # ~~ only in ASCII mode
+        (b'~{Dc~\n:C~}', 'strict', None), # ~\n only in ASCII mode
     )
 
 def test_main():
diff -r e8783c581928 Modules/cjkcodecs/_codecs_cn.c
--- a/Modules/cjkcodecs/_codecs_cn.c
+++ b/Modules/cjkcodecs/_codecs_cn.c
@@ -266,7 +266,8 @@ DECODER(gb18030)
             REQUIRE_INBUF(4)
             c3 = IN3;
             c4 = IN4;
-            if (c < 0x81 || c3 < 0x81 || c4 < 0x30 || c4 > 0x39)
+            if (c < 0x81 || c > 0xFE || c3 < 0x81 || c3 > 0xFE ||
+                c4 < 0x30 || c4 > 0x39)
                 return 4;
             c -= 0x81;  c2 -= 0x30;
             c3 -= 0x81; c4 -= 0x30;
@@ -333,14 +334,27 @@ ENCODER(hz)
         DBCHAR code;
 
         if (c < 0x80) {
-            if (state->i == 0) {
-                WRITE1((unsigned char)c)
-                NEXT(1, 1)
+            if (c == '~') {
+                if (state->i == 0) {
+                    WRITE2('~', '~')
+                    NEXT(1, 2)
+                }
+                else {
+                    WRITE4('~', '}', '~', '~')
+                    NEXT(1, 4)
+                    state->i = 0;
+                }
             }
             else {
-                WRITE3('~', '}', (unsigned char)c)
-                NEXT(1, 3)
-                state->i = 0;
+                if (state->i == 0) {
+                    WRITE1((unsigned char)c)
+                    NEXT(1, 1)
+                }
+                else {
+                    WRITE3('~', '}', (unsigned char)c)
+                    NEXT(1, 3)
+                    state->i = 0;
+                }
             }
             continue;
         }
@@ -385,10 +399,11 @@ DECODER(hz)
         unsigned char c = IN1;
 
         if (c == '~') {
-            unsigned char c2 = IN2;
+            unsigned char c2;
 
             REQUIRE_INBUF(2)
-            if (c2 == '~') {
+            c2 = IN2;
+            if (c2 == '~' && state->i == 0) {
                 WRITE1('~')
                 NEXT(2, 1)
                 continue;
@@ -397,7 +412,7 @@ DECODER(hz)
                 state->i = 1; /* set GB */
             else if (c2 == '}' && state->i == 1)
                 state->i = 0; /* set ASCII */
-            else if (c2 == '\n')
+            else if (c2 == '\n' && state->i == 0)
                 ; /* line-continuation */
             else
                 return 2;