Fix CVE-2026-45409: quadratic time complexity in IDNA validation

Backport two upstream commits (c0dda45, e1cb465) fixing
CVE-2026-45409 (GHSA-65pc-fj4g-8rjx). The patch adds early
length checks in encode(), decode(), and check_label() in
idna/core.py to reject oversized inputs before per-label
validation runs, preventing CONTEXTO/CONTEXTJ-heavy inputs
from driving validation into quadratic time complexity.

CVE: CVE-2026-45409
Upstream patches:
 - c0dda4501d.patch
 - e1cb465b63.patch
Resolves: RHEL-215651

This commit was backported by Ymir, a Red Hat Enterprise Linux software maintenance AI agent.

Assisted-by: Ymir
Co-authored-by: Tomáš Hrnčiar <thrnciar@redhat.com>

CVE-2026-45409.patch

@@ -0,0 +1,134 @@
+From 487f90216c00097caa6c5f10eb1f843ec2d8a4b6 Mon Sep 17 00:00:00 2001
+From: Kim Davies <kim@cynosure.com.au>
+Date: Sun, 10 May 2026 08:47:22 -0700
+Subject: [PATCH 1/2] Merge commit from fork
+
+---
+ idna/core.py       | 14 ++++++++++++++
+ tests/test_idna.py | 13 +++++++++++++
+ 2 files changed, 27 insertions(+)
+
+diff --git a/idna/core.py b/idna/core.py
+index 0dae61a..a549326 100644
+--- a/idna/core.py
++++ b/idna/core.py
+@@ -340,6 +340,15 @@ def encode(s: Union[str, bytes, bytearray], strict: bool = False, uts46: bool =
+             raise IDNAError('should pass a unicode string to the function rather than a byte string.')
+     if uts46:
+         s = uts46_remap(s, std3_rules, transitional)
++
++    # Reject inputs that exceed the maximum DNS domain length up-front.
++    # Each codepoint in a U-label contributes at least one octet to its
++    # A-label form, so any input longer than the domain limit cannot
++    # produce a valid A-domain. Short-circuiting here prevents per-label
++    # validation from being driven into quadratic time
++    if len(s) > 254:
++        raise IDNAError("Domain too long")
++
+     trailing_dot = False
+     result = []
+     if strict:
+@@ -373,6 +382,11 @@ def decode(s: Union[str, bytes, bytearray], strict: bool = False, uts46: bool =
+         raise IDNAError('Invalid ASCII in A-label')
+     if uts46:
+         s = uts46_remap(s, std3_rules, False)
++    # See encode() for rationale; the same bound applies because every
++    # legal A-domain is at most 254 octets and every codepoint of a
++    # legal U-domain contributes at least one octet to its A-form.
++    if len(s) > 254:
++        raise IDNAError("Domain too long")
+     trailing_dot = False
+     result = []
+     if not strict:
+diff --git a/tests/test_idna.py b/tests/test_idna.py
+index 81afb32..5001b48 100755
+--- a/tests/test_idna.py
++++ b/tests/test_idna.py
+@@ -78,6 +78,19 @@ class IDNATests(unittest.TestCase):
+         self.assertFalse(idna.valid_label_length('a' * 64))
+         self.assertRaises(idna.IDNAError, idna.encode, 'a' * 64)
+ 
++    def test_oversized_input_rejected_promptly(self):
++        # GHSA-65pc-fj4g-8rjx: encode/decode must reject inputs that
++        # exceed the maximum DNS domain length before per-codepoint
++        # validation runs, so labels dominated by CONTEXTO codepoints
++        # cannot drive validation into quadratic time.
++        import time
++
++        for payload in ("٠" * 8000, "・" * 8000 + "漢"):
++            start = time.perf_counter()
++            self.assertRaises(idna.IDNAError, idna.encode, payload)
++            self.assertRaises(idna.IDNAError, idna.decode, payload)
++            self.assertLess(time.perf_counter() - start, 1.0)
++
+     def test_check_bidi(self):
+ 
+         l = '\u0061'
+
+From 219662569606146771ac69fac5939d3308159064 Mon Sep 17 00:00:00 2001
+From: metsw24-max <metsw24@gmail.com>
+Date: Mon, 11 May 2026 20:59:30 +0530
+Subject: [PATCH 2/2] Enforce early length limits in check_label
+
+---
+ idna/core.py       | 11 +++++++++++
+ tests/test_idna.py | 24 ++++++++++++++++++++++++
+ 2 files changed, 35 insertions(+)
+
+diff --git a/idna/core.py b/idna/core.py
+index a549326..f863141 100644
+--- a/idna/core.py
++++ b/idna/core.py
+@@ -230,6 +230,17 @@ def check_label(label: Union[str, bytes, bytearray]) -> None:
+         label = label.decode('utf-8')
+     if len(label) == 0:
+         raise IDNAError('Empty Label')
++    # Reject oversized labels before per-codepoint validation runs.
++    # CONTEXTJ/CONTEXTO checks scan the whole label per codepoint, so an
++    # uncapped label drives validation into quadratic time
++    # (GHSA-65pc-fj4g-8rjx / CVE-2024-3651). encode()/decode() cap the
++    # whole-domain length; this cap protects direct callers of
++    # alabel/ulabel/check_label and the idna2008 incremental codec.
++    # Use the whole-domain bound rather than the per-label DNS bound so
++    # that UTS #46 lenient decoding of labels longer than 63 chars is
++    # preserved.
++    if not valid_string_length(label, trailing_dot=True):
++        raise IDNAError('Label too long')
+ 
+     check_nfc(label)
+     check_hyphen_ok(label)
+diff --git a/tests/test_idna.py b/tests/test_idna.py
+index 5001b48..2dc0892 100755
+--- a/tests/test_idna.py
++++ b/tests/test_idna.py
+@@ -91,6 +91,30 @@ class IDNATests(unittest.TestCase):
+             self.assertRaises(idna.IDNAError, idna.decode, payload)
+             self.assertLess(time.perf_counter() - start, 1.0)
+ 
++    def test_oversized_label_rejected_promptly(self):
++        # The whole-domain cap in encode()/decode() does not cover direct
++        # callers of alabel/ulabel/check_label, nor the idna2008
++        # incremental codec which calls alabel/ulabel per label. Without a
++        # per-label cap, a single oversized CONTEXTO-heavy label still
++        # drives validation into quadratic time.
++        import codecs
++        import time
++
++        import idna.codec  # noqa: F401  (register the idna2008 codec)
++
++        payload = "・" * 8000 + "漢"
++        start = time.perf_counter()
++        self.assertRaises(idna.IDNAError, idna.check_label, payload)
++        self.assertRaises(idna.IDNAError, idna.alabel, payload)
++        self.assertRaises(idna.IDNAError, idna.ulabel, payload)
++        self.assertRaises(
++            idna.IDNAError,
++            codecs.getincrementalencoder("idna2008")().encode,
++            payload,
++            True,
++        )
++        self.assertLess(time.perf_counter() - start, 1.0)
++
+     def test_check_bidi(self):
+ 
+         l = '\u0061'

python-idna.spec

@@ -8,6 +8,11 @@ Summary:        Internationalized Domain Names in Applications (IDNA)
 License:        BSD-3-Clause
 URL:            https://github.com/kjd/idna
 Source0:        https://pypi.io/packages/source/i/%{srcname}/%{srcname}-%{version}.tar.gz
+
+# https://github.com/kjd/idna/commit/c0dda4501df5d91c3181ce6f962dc5de74e82cc1
+# https://github.com/kjd/idna/commit/e1cb465b6376f33306a26f467d197edbcd01c4b9
+Patch:          CVE-2026-45409.patch
+
 BuildArch:      noarch
 
 BuildRequires:  python3-devel