diff --git a/ext/lexbor/lexbor/encoding/decode.c b/ext/lexbor/lexbor/encoding/decode.c
index 3e48971e3657..5010e4ed9d04 100644
--- a/ext/lexbor/lexbor/encoding/decode.c
+++ b/ext/lexbor/lexbor/encoding/decode.c
@@ -2907,42 +2907,51 @@ lxb_encoding_decode_valid_utf_8_single(const lxb_char_t **data,
     else if ((*p & 0xe0) == 0xc0) {
         /* 110xxxxx 10xxxxxx */
 
-        if (end - p < 2) {
-            *data = end;
+        if (*p < 0xC2 || end - p < 2 || (p[1] & 0xC0) != 0x80) {
+            (*data) = (end - p < 2) ? end : *data + 1;
             return LXB_ENCODING_DECODE_ERROR;
         }
 
-        cp  = (p[0] ^ (0xC0 & p[0])) << 6;
-        cp |= (p[1] ^ (0x80 & p[1]));
+        cp  = (*p & 0x1F) << 6;
+        cp |= (p[1] & 0x3F);
 
         (*data) += 2;
     }
     else if ((*p & 0xf0) == 0xe0) {
         /* 1110xxxx 10xxxxxx 10xxxxxx */
 
-        if (end - p < 3) {
-            *data = end;
+        if (end - p < 3
+            || (p[1] & 0xC0) != 0x80 || (p[2] & 0xC0) != 0x80
+            || (*p == 0xE0 && p[1] < 0xA0)
+            || (*p == 0xED && p[1] > 0x9F))
+        {
+            (*data) = (end - p < 3) ? end : *data + 1;
             return LXB_ENCODING_DECODE_ERROR;
         }
 
-        cp  = (p[0] ^ (0xE0 & p[0])) << 12;
-        cp |= (p[1] ^ (0x80 & p[1])) << 6;
-        cp |= (p[2] ^ (0x80 & p[2]));
+        cp  = (*p & 0x0F) << 12;
+        cp |= (p[1] & 0x3F) << 6;
+        cp |= (p[2] & 0x3F);
 
         (*data) += 3;
     }
     else if ((*p & 0xf8) == 0xf0) {
         /* 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx */
 
-        if (end - p < 4) {
-            *data = end;
+        if (*p > 0xF4 || end - p < 4
+            || (p[1] & 0xC0) != 0x80 || (p[2] & 0xC0) != 0x80
+            || (p[3] & 0xC0) != 0x80
+            || (*p == 0xF0 && p[1] < 0x90)
+            || (*p == 0xF4 && p[1] > 0x8F))
+        {
+            (*data) = (end - p < 4) ? end : *data + 1;
             return LXB_ENCODING_DECODE_ERROR;
         }
 
-        cp  = (p[0] ^ (0xF0 & p[0])) << 18;
-        cp |= (p[1] ^ (0x80 & p[1])) << 12;
-        cp |= (p[2] ^ (0x80 & p[2])) << 6;
-        cp |= (p[3] ^ (0x80 & p[3]));
+        cp  = (*p & 0x07) << 18;
+        cp |= (p[1] & 0x3F) << 12;
+        cp |= (p[2] & 0x3F) << 6;
+        cp |= (p[3] & 0x3F);
 
         (*data) += 4;
     }
diff --git a/ext/lexbor/patches/0007-Validate-UTF-8-in-decode_valid_utf_8_single.patch b/ext/lexbor/patches/0007-Validate-UTF-8-in-decode_valid_utf_8_single.patch
new file mode 100644
index 000000000000..a33fd738fd42
--- /dev/null
+++ b/ext/lexbor/patches/0007-Validate-UTF-8-in-decode_valid_utf_8_single.patch
@@ -0,0 +1,71 @@
+diff --git a/ext/lexbor/lexbor/encoding/decode.c b/ext/lexbor/lexbor/encoding/decode.c
+index 3e48971e365..5010e4ed9d0 100644
+--- a/ext/lexbor/lexbor/encoding/decode.c
++++ b/ext/lexbor/lexbor/encoding/decode.c
+@@ -2907,42 +2907,51 @@ lxb_encoding_decode_valid_utf_8_single(const lxb_char_t **data,
+     else if ((*p & 0xe0) == 0xc0) {
+         /* 110xxxxx 10xxxxxx */
+ 
+-        if (end - p < 2) {
+-            *data = end;
++        if (*p < 0xC2 || end - p < 2 || (p[1] & 0xC0) != 0x80) {
++            (*data) = (end - p < 2) ? end : *data + 1;
+             return LXB_ENCODING_DECODE_ERROR;
+         }
+ 
+-        cp  = (p[0] ^ (0xC0 & p[0])) << 6;
+-        cp |= (p[1] ^ (0x80 & p[1]));
++        cp  = (*p & 0x1F) << 6;
++        cp |= (p[1] & 0x3F);
+ 
+         (*data) += 2;
+     }
+     else if ((*p & 0xf0) == 0xe0) {
+         /* 1110xxxx 10xxxxxx 10xxxxxx */
+ 
+-        if (end - p < 3) {
+-            *data = end;
++        if (end - p < 3
++            || (p[1] & 0xC0) != 0x80 || (p[2] & 0xC0) != 0x80
++            || (*p == 0xE0 && p[1] < 0xA0)
++            || (*p == 0xED && p[1] > 0x9F))
++        {
++            (*data) = (end - p < 3) ? end : *data + 1;
+             return LXB_ENCODING_DECODE_ERROR;
+         }
+ 
+-        cp  = (p[0] ^ (0xE0 & p[0])) << 12;
+-        cp |= (p[1] ^ (0x80 & p[1])) << 6;
+-        cp |= (p[2] ^ (0x80 & p[2]));
++        cp  = (*p & 0x0F) << 12;
++        cp |= (p[1] & 0x3F) << 6;
++        cp |= (p[2] & 0x3F);
+ 
+         (*data) += 3;
+     }
+     else if ((*p & 0xf8) == 0xf0) {
+         /* 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx */
+ 
+-        if (end - p < 4) {
+-            *data = end;
++        if (*p > 0xF4 || end - p < 4
++            || (p[1] & 0xC0) != 0x80 || (p[2] & 0xC0) != 0x80
++            || (p[3] & 0xC0) != 0x80
++            || (*p == 0xF0 && p[1] < 0x90)
++            || (*p == 0xF4 && p[1] > 0x8F))
++        {
++            (*data) = (end - p < 4) ? end : *data + 1;
+             return LXB_ENCODING_DECODE_ERROR;
+         }
+ 
+-        cp  = (p[0] ^ (0xF0 & p[0])) << 18;
+-        cp |= (p[1] ^ (0x80 & p[1])) << 12;
+-        cp |= (p[2] ^ (0x80 & p[2])) << 6;
+-        cp |= (p[3] ^ (0x80 & p[3]));
++        cp  = (*p & 0x07) << 18;
++        cp |= (p[1] & 0x3F) << 12;
++        cp |= (p[2] & 0x3F) << 6;
++        cp |= (p[3] & 0x3F);
+ 
+         (*data) += 4;
+     }
diff --git a/ext/uri/tests/whatwg/parsing/gh21734_overlong_utf8.phpt b/ext/uri/tests/whatwg/parsing/gh21734_overlong_utf8.phpt
new file mode 100644
index 000000000000..812292071f8b
--- /dev/null
+++ b/ext/uri/tests/whatwg/parsing/gh21734_overlong_utf8.phpt
@@ -0,0 +1,43 @@
+--TEST--
+GH-21734: WHATWG URL parser rejects overlong UTF-8 and invalid continuation bytes
+--FILE--
+<?php
+
+// Overlong hostname: %C1%A5 = overlong 'e', %C1%B6 = overlong 'v', etc.
+// Must fail to parse (browsers reject at the UTF-8 decode step)
+$url = Uri\WhatWg\Url::parse("http://%C1%A5%C1%B6%C1%A9%C1%AC.com/");
+var_dump($url);
+
+// Raw overlong 'a' in hostname
+$url = Uri\WhatWg\Url::parse("http://\xC1\xA1.com/");
+var_dump($url);
+
+// Overlong '/' in path: raw bytes \xC0\xAF should not produce a path separator
+$url = Uri\WhatWg\Url::parse("http://example.com/a\xC0\xAFb");
+var_dump($url->getPath());
+
+// Invalid continuation byte: \xC0 followed by ASCII 'A' (not 0x80-0xBF)
+$url = Uri\WhatWg\Url::parse("http://example.com/\xC0\x41");
+var_dump($url->getPath());
+
+// Surrogate \xED\xA0\x80 = U+D800 (must be rejected)
+$url = Uri\WhatWg\Url::parse("http://example.com/\xED\xA0\x80");
+var_dump($url->getPath());
+
+// Valid UTF-8 still works
+$url = Uri\WhatWg\Url::parse("http://example.com/café");
+var_dump($url->getPath());
+
+// Valid IDN hostname still works
+$url = Uri\WhatWg\Url::parse("http://münchen.de/");
+var_dump($url->getAsciiHost());
+
+?>
+--EXPECT--
+NULL
+NULL
+string(9) "/a%C0%AFb"
+string(5) "/%C0A"
+string(10) "/%ED%A0%80"
+string(10) "/caf%C3%A9"
+string(17) "xn--mnchen-3ya.de"