diff --git a/Lib/email/_header_value_parser.py b/Lib/email/_header_value_parser.py
index f11fa83d45ed2d..235b180473b3b0 100644
--- a/Lib/email/_header_value_parser.py
+++ b/Lib/email/_header_value_parser.py
@@ -82,11 +82,14 @@
 
 WSP = set(' \t')
 CFWS_LEADER = WSP | set('(')
+CFWS_LEADER_WITH_DOT = CFWS_LEADER | set('.')
 SPECIALS = set(r'()<>@,:;.\"[]')
 ATOM_ENDS = SPECIALS | WSP
 DOT_ATOM_ENDS = ATOM_ENDS - set('.')
 # '.', '"', and '(' do not end phrases in order to support obs-phrase
 PHRASE_ENDS = SPECIALS - set('."(')
+PHRASE_ENDS_CHARS = r''.join(PHRASE_ENDS)
+PHRASE_ENDS_CHARS_NO_SEMICOLON = PHRASE_ENDS_CHARS.replace(';', '')
 TSPECIALS = (SPECIALS | set('/?=')) - set('.')
 TOKEN_ENDS = TSPECIALS | WSP
 ASPECIALS = TSPECIALS | set("*'%")
@@ -1300,6 +1303,12 @@ def get_cfws(value):
         cfws.append(token)
     return cfws, value
 
+def get_cfws_digits(value, leader_set):
+    ind = 0
+    while ind < len(value) and value[ind] not in leader_set:
+        ind += 1
+    return value[:ind], value[ind:]
+
 def get_quoted_string(value):
     """quoted-string = [CFWS] <bare-quoted-string> [CFWS]
 
@@ -1443,11 +1452,14 @@ def get_phrase(value):
         phrase.defects.append(errors.InvalidHeaderDefect(
             "phrase does not start with word"))
     while value and value[0] not in PHRASE_ENDS:
-        if value[0]=='.':
-            phrase.append(DOT)
-            phrase.defects.append(errors.ObsoleteHeaderDefect(
-                "period in 'phrase'"))
-            value = value[1:]
+        if value[0] == '.':
+            tmpvalue = value.lstrip('.')
+            n = len(value) - len(tmpvalue)
+            phrase.extend(DOT for _ in range(n))
+            phrase.defects.extend(
+                errors.ObsoleteHeaderDefect("period in 'phrase'")
+                for _ in range(n))
+            value = tmpvalue
         else:
             try:
                 token, value = get_word(value)
@@ -1461,6 +1473,19 @@ def get_phrase(value):
             phrase.append(token)
     return phrase, value
 
+def _find_phrase(reslist, value, phrase_ends, phrase_end_chars, endchars):
+    while value and value[0] not in endchars:
+        if value[0] in phrase_ends:
+            tmpvalue = value.lstrip(phrase_end_chars)
+            reslist.extend(
+                ValueTerminal(value[i], 'misplaced-special')
+                for i in range(len(value) - len(tmpvalue)))
+            value = tmpvalue
+        else:
+            token, value = get_phrase(value)
+            reslist.append(token)
+    return value
+
 def get_local_part(value):
     """ local-part = dot-atom / quoted-string / obs-local-part
 
@@ -1842,14 +1867,10 @@ def get_invalid_mailbox(value, endchars):
 
     """
     invalid_mailbox = InvalidMailbox()
-    while value and value[0] not in endchars:
-        if value[0] in PHRASE_ENDS:
-            invalid_mailbox.append(ValueTerminal(value[0],
-                                                 'misplaced-special'))
-            value = value[1:]
-        else:
-            token, value = get_phrase(value)
-            invalid_mailbox.append(token)
+    # lstrip() should not strip stuff in 'endchars'
+    phrase_end_chars = ''.join(PHRASE_ENDS - set(endchars))
+    value = _find_phrase(invalid_mailbox, value,
+                         PHRASE_ENDS, phrase_end_chars, endchars)
     return invalid_mailbox, value
 
 def get_mailbox_list(value):
@@ -2196,10 +2217,7 @@ def parse_mime_version(value):
         if not value:
             mime_version.defects.append(errors.HeaderMissingRequiredValue(
                 "Expected MIME version number but found only CFWS"))
-    digits = ''
-    while value and value[0] != '.' and value[0] not in CFWS_LEADER:
-        digits += value[0]
-        value = value[1:]
+    digits, value = get_cfws_digits(value, CFWS_LEADER_WITH_DOT)
     if not digits.isdigit():
         mime_version.defects.append(errors.InvalidHeaderDefect(
             "Expected MIME major version number but found {!r}".format(digits)))
@@ -2227,10 +2245,7 @@ def parse_mime_version(value):
             mime_version.defects.append(errors.InvalidHeaderDefect(
                 "Incomplete MIME version; found only major number"))
         return mime_version
-    digits = ''
-    while value and value[0] not in CFWS_LEADER:
-        digits += value[0]
-        value = value[1:]
+    digits, value = get_cfws_digits(value, CFWS_LEADER)
     if not digits.isdigit():
         mime_version.defects.append(errors.InvalidHeaderDefect(
             "Expected MIME minor version number but found {!r}".format(digits)))
@@ -2255,14 +2270,8 @@ def get_invalid_parameter(value):
 
     """
     invalid_parameter = InvalidParameter()
-    while value and value[0] != ';':
-        if value[0] in PHRASE_ENDS:
-            invalid_parameter.append(ValueTerminal(value[0],
-                                                   'misplaced-special'))
-            value = value[1:]
-        else:
-            token, value = get_phrase(value)
-            invalid_parameter.append(token)
+    value = _find_phrase(invalid_parameter, value,
+                         PHRASE_ENDS, PHRASE_ENDS_CHARS_NO_SEMICOLON, ';')
     return invalid_parameter, value
 
 def get_ttext(value):
@@ -2407,10 +2416,8 @@ def get_section(value):
     if not value or not value[0].isdigit():
         raise errors.HeaderParseError("Expected section number but "
                                       "found {}".format(value))
-    digits = ''
-    while value and value[0].isdigit():
-        digits += value[0]
-        value = value[1:]
+    ind = next((i for i, ch in enumerate(value) if not ch.isdigit()), 0)
+    digits, value = value[:ind], value[ind:]
     if digits[0] == '0' and digits != '0':
         section.defects.append(errors.InvalidHeaderDefect(
                 "section number has an invalid leading 0"))
@@ -2567,12 +2574,15 @@ def get_parameter(value):
         while value:
             if value[0] in WSP:
                 token, value = get_fws(value)
+                v.append(token)
             elif value[0] == '"':
-                token = ValueTerminal('"', 'DQUOTE')
-                value = value[1:]
+                tmpvalue = value.lstrip('"')
+                n = len(value) - len(tmpvalue)
+                v.extend((ValueTerminal('"', 'DQUOTE') for _ in range(n)))
+                value = tmpvalue
             else:
                 token, value = get_qcontent(value)
-            v.append(token)
+                v.append(token)
         token = v
     else:
         token, value = get_value(value)
@@ -2638,17 +2648,11 @@ def _find_mime_parameters(tokenlist, value):
     """Do our best to find the parameters in an invalid MIME header
 
     """
-    while value and value[0] != ';':
-        if value[0] in PHRASE_ENDS:
-            tokenlist.append(ValueTerminal(value[0], 'misplaced-special'))
-            value = value[1:]
-        else:
-            token, value = get_phrase(value)
-            tokenlist.append(token)
-    if not value:
-        return
-    tokenlist.append(ValueTerminal(';', 'parameter-separator'))
-    tokenlist.append(parse_mime_parameters(value[1:]))
+    value = _find_phrase(tokenlist, value,
+                         PHRASE_ENDS, PHRASE_ENDS_CHARS_NO_SEMICOLON, ';')
+    if value:
+        tokenlist.append(ValueTerminal(';', 'parameter-separator'))
+        tokenlist.append(parse_mime_parameters(value[1:]))
 
 def parse_content_type_header(value):
     """ maintype "/" subtype *( ";" parameter )
@@ -2757,12 +2761,16 @@ def parse_content_transfer_encoding_header(value):
     if not value:
         return cte_header
     while value:
-        cte_header.defects.append(errors.InvalidHeaderDefect(
-            "Extra text after content transfer encoding"))
         if value[0] in PHRASE_ENDS:
-            cte_header.append(ValueTerminal(value[0], 'misplaced-special'))
-            value = value[1:]
+            tmpvalue = value.lstrip(PHRASE_ENDS_CHARS)
+            for i in range(len(value) - len(tmpvalue)):
+                cte_header.defects.append(errors.InvalidHeaderDefect(
+                    "Extra text after content transfer encoding"))
+                cte_header.append(ValueTerminal(value[i], 'misplaced-special'))
+            value = tmpvalue
         else:
+            cte_header.defects.append(errors.InvalidHeaderDefect(
+                "Extra text after content transfer encoding"))
             token, value = get_phrase(value)
             cte_header.append(token)
     return cte_header
diff --git a/Lib/test/test_email/test__header_value_parser.py b/Lib/test/test_email/test__header_value_parser.py
index fd4ac2c404ce47..3d91537191c69b 100644
--- a/Lib/test/test_email/test__header_value_parser.py
+++ b/Lib/test/test_email/test__header_value_parser.py
@@ -2676,6 +2676,16 @@ def test_invalid_content_transfer_encoding(self):
             ";foo", ";foo", ";foo", [errors.InvalidHeaderDefect]*3
         )
 
+    def test_invalid_content_transfer_encoding_misplaced_special(self):
+        cte = parser.parse_content_transfer_encoding_header("foo;;;;;")
+        self.assertEqual(len(cte), 6)
+        self.assertEqual(cte[0].value, "foo")
+        self.assertEqual(cte[0].token_type, "token")
+        self.assertEqual(cte[0].value, "foo")
+        self.assertEqual(cte[0].token_type, "token")
+        terminal = parser.ValueTerminal(";", "misplaced-special")
+        self.assertEqual(cte[1:], [terminal] * 5)
+
     # get_msg_id
 
     def test_get_msg_id_empty(self):
diff --git a/Misc/NEWS.d/next/Security/2025-05-30-20-08-38.gh-issue-134873.6Z5xUC.rst b/Misc/NEWS.d/next/Security/2025-05-30-20-08-38.gh-issue-134873.6Z5xUC.rst
new file mode 100644
index 00000000000000..93389a64ee2ead
--- /dev/null
+++ b/Misc/NEWS.d/next/Security/2025-05-30-20-08-38.gh-issue-134873.6Z5xUC.rst
@@ -0,0 +1,2 @@
+Fix various HTTP header value parsing routines with worst-case
+quadratic time complexity. Patch by Bénédikt Tran.