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python3-twisted: Fix CVE-2026-42304

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This patch applies the upstream 26.4.0rc2 backport for
CVE-2026-42304. The upstream fix merge is referenced in [1],
and the public CVE advisory is referenced in [2]. The individual
backported commit links are recorded in the patch headers.

[1] https://github.com/twisted/twisted/commit/2d196123264efb0027eecfe1b430be4a9babdbd8
[2] https://github.com/advisories/GHSA-grgv-6hw6-v9g4

Signed-off-by: Hetvi Thakar <hthakar@cisco.com>
Signed-off-by: Anuj Mittal <anuj.mittal@oss.qualcomm.com>
This commit is contained in:
Hetvi Thakar authored and Anuj Mittal committed 2026-08-24 12:58:53 +05:30
1 parent 8519105eec
commit 2fb2b9fb1d
7 files changed
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meta-python/recipes-devtools/python/files/CVE-2026-42304_p4.patch
+318
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@@ -0,0 +1,318 @@
From 3cb501679cd7f45ab49d32cf72ec546ef3a64825 Mon Sep 17 00:00:00 2001
From: Tomas Illuminati <tomas.illuminati@owasp.org>
Date: Mon, 20 Apr 2026 10:01:39 -0300
Subject: [PATCH] names: Refactor DNS compression mitigation
CVE: CVE-2026-42304
Upstream-Status: Backport [https://github.com/twisted/twisted/commit/d7d81e08d46b3f266963ea77e5f6b4a333af455f]
Backport Changes:
- Adapted the 25.5.0 imports by moving Sequence from typing to
collections.abc and retaining the target's Optional and Union imports.
(cherry picked from commit d7d81e08d46b3f266963ea77e5f6b4a333af455f)
Signed-off-by: Hetvi Thakar <hthakar@cisco.com>
---
src/twisted/names/dns.py | 113 +++++++++++++------
src/twisted/names/newsfragments/12626.bugfix | 1 +
src/twisted/names/test/test_dns.py | 47 ++++----
3 files changed, 105 insertions(+), 56 deletions(-)
create mode 100644 src/twisted/names/newsfragments/12626.bugfix
diff --git a/src/twisted/names/dns.py b/src/twisted/names/dns.py
index 93e4080bf..869ffec76 100644
--- a/src/twisted/names/dns.py
+++ b/src/twisted/names/dns.py
@@ -16,9 +16,11 @@ import inspect
import random
import socket
import struct
+from collections.abc import Sequence
+from contextlib import contextmanager
from io import BytesIO
from itertools import chain
-from typing import Optional, Sequence, SupportsInt, Union, overload
+from typing import Final, Optional, SupportsInt, Union, overload
from zope.interface import Attribute, Interface, implementer
@@ -446,17 +448,19 @@ def readPrecisely(file, l):
return buff
-# Cap the total number of compression-pointer dereferences performed while
-# decoding a single DNS message. A hostile peer can otherwise craft a packet
-# in which every record name chases a long compression chain, forcing O(N*M)
-# work and stalling the reactor.
-MAX_COMPRESSION_POINTERS_PER_MESSAGE = 1000
+MAX_COMPRESSION_POINTERS_PER_MESSAGE: Final = 1000
+"""
+Cap the total number of compression-pointer dereferences performed while
+decoding a single DNS message. A hostile peer can otherwise craft a packet
+in which every record name chases a long compression chain, forcing
+C{O(N*M)} work and stalling the reactor.
+"""
class DNSDecodeError(ValueError):
"""
Raised when a DNS message cannot be decoded because it violates a
- protocol-level safety limit
+ protocol-level safety limit.
"""
@@ -469,8 +473,12 @@ class _DecodeContext:
jumps taken across every name in the message, defending against packets
that fan out thousands of records pointing to deeply chained pointers.
+ This class is private. External callers must not rely on it; the
+ per-message scope is installed and torn down by L{Message.decode}
+ through L{_decodeContextVar}.
+
@ivar jumps: The number of compression pointers followed so far.
- @ivar maxJumps: The inclusive upper bound on C{jumps}. Exceeding it
+ @ivar maxJumps: The inclusive upper bound on L{jumps}. Exceeding it
causes L{registerJump} to raise L{DNSDecodeError}.
"""
@@ -482,10 +490,14 @@ class _DecodeContext:
def registerJump(self) -> None:
"""
- Record that a compression pointer has been followed
+ Record that a compression pointer has been followed.
+
+ The check is performed before any further bytes are read so the
+ caller fails fast as soon as the aggregate limit is breached, even
+ if additional records remain in the buffer.
@raise DNSDecodeError: if the cumulative number of jumps exceeds
- L{maxJumps}
+ L{maxJumps}.
"""
self.jumps += 1
if self.jumps > self.maxJumps:
@@ -495,15 +507,37 @@ class _DecodeContext:
)
-# Tracks state across nested calls without altering every record's signature.
-# L{Message.decode} manages the lifecycle per-message, while standalone decoders
-# default to a local context when C{_decodeContextVar} is C{None}
-
+# Private module-level L{contextvars.ContextVar} used to share a single
+# L{_DecodeContext} across the re-entrant calls performed while decoding one
+# DNS message. L{contextvars} (rather than a plain module attribute) is used
+# on purpose: although Twisted's reactor is single-threaded, message decoding
+# is re-entrant across many records in a single pass and L{ContextVar}
+# guarantees the scope is restored correctly on exit -- and remains isolated
+# per-task should a future caller decode messages from multiple
+# L{asyncio}-style contexts concurrently.
_decodeContextVar: contextvars.ContextVar[_DecodeContext | None] = (
contextvars.ContextVar("_dnsDecodeContext", default=None)
)
+@contextmanager
+def _installDecodeContext(context: _DecodeContext):
+ """
+ Install C{context} on L{_decodeContextVar} for the duration of the
+ C{with} block and restore the previous value on exit.
+
+ This wraps the L{contextvars.ContextVar.set} / L{contextvars.ContextVar.reset}
+ token dance so call sites can use a plain C{with} statement.
+
+ @param context: The L{_DecodeContext} to install as the active context.
+ """
+ token = _decodeContextVar.set(context)
+ try:
+ yield context
+ finally:
+ _decodeContextVar.reset(token)
+
+
class IEncodable(Interface):
"""
Interface for something which can be encoded to and decoded
@@ -609,8 +643,18 @@ class Name:
@ivar name: A byte string giving the name.
@type name: L{bytes}
+
+ @cvar maxCompressionPointers: Per-message cap on the total number of
+ compression-pointer dereferences L{decode} will follow before
+ raising L{DNSDecodeError}. Defined as a class attribute so
+ subclasses (and, in the future, individual instances) may override
+ it to tune the trade-off between tolerance for legitimately
+ verbose messages and resistance to denial-of-service attacks.
+ @type maxCompressionPointers: L{int}
"""
+ maxCompressionPointers: int = MAX_COMPRESSION_POINTERS_PER_MESSAGE
+
def __init__(self, name: bytes | str = b""):
"""
@param name: A name.
@@ -651,35 +695,37 @@ class Name:
strio.write(label)
strio.write(b"\x00")
- def decode(self, strio, length=None, context=None):
+ def decode(self, strio, length=None):
"""
Decode a byte string into this Name.
+ When invoked from L{Message.decode}, a shared compression-pointer
+ counter is picked up transparently from the private
+ L{_decodeContextVar}. Standalone callers get a fresh per-call
+ counter seeded from L{maxCompressionPointers}, so existing code
+ keeps working unchanged while still being protected against
+ pathological inputs.
+
@type strio: file
@param strio: Bytes will be read from this file until the full Name
- is decoded.
+ is decoded.
- @type context: L{_DecodeContext} or L{None}
- @param context: Shared decoding state used to cap the total number
- of compression-pointer jumps taken while decoding the enclosing
- DNS message. When L{None}, the context installed by
- L{Message.decode} is used if one is active; otherwise a fresh,
- call-local context is created so that direct callers remain
- protected and backwards compatible.
+ @type length: L{int} or L{None}
+ @param length: Present for compatibility with the L{IEncodable}
+ interface; ignored by this decoder.
@raise EOFError: Raised when there are not enough bytes available
- from C{strio}.
+ from C{strio}.
- @raise ValueError: Raised when the name cannot be decoded because it
- contains a compression loop.
+ @raise ValueError: Raised when the name cannot be decoded because
+ it contains a compression loop.
@raise DNSDecodeError: Raised when the cumulative number of
compression-pointer jumps exceeds the configured limit.
"""
+ context = _decodeContextVar.get()
if context is None:
- context = _decodeContextVar.get()
- if context is None:
- context = _DecodeContext()
+ context = _DecodeContext(maxJumps=self.maxCompressionPointers)
visited = set()
self.name = b""
off = 0
@@ -2782,9 +2828,10 @@ class Message(tputil.FancyEqMixin):
# A single shared counter bounds the total compression-pointer work
# performed across every name in this message. It is installed on
- # the context variable so nested record decoders pick it up without
- # needing to thread it through each signature.
- with _decodeContextVar.set(_DecodeContext()):
+ # the private context variable so nested record decoders pick it up
+ # without needing to thread it through each signature.
+ decodeContext = _DecodeContext(maxJumps=Name.maxCompressionPointers)
+ with _installDecodeContext(decodeContext):
self.queries = []
for i in range(nqueries):
q = Query()
@@ -2802,8 +2849,6 @@ class Message(tputil.FancyEqMixin):
for l, n in items:
self.parseRecords(l, n, strio)
- finally:
- _decodeContextVar.reset(token)
def parseRecords(self, list, num, strio):
for i in range(num):
diff --git a/src/twisted/names/newsfragments/12626.bugfix b/src/twisted/names/newsfragments/12626.bugfix
new file mode 100644
index 000000000..44896c3e5
--- /dev/null
+++ b/src/twisted/names/newsfragments/12626.bugfix
@@ -0,0 +1 @@
+twisted.names was fix for Denial of Service (DoS) attack via resource exhaustion during DNS name decompression. CVE-REFERENCE HERE
\ No newline at end of file
diff --git a/src/twisted/names/test/test_dns.py b/src/twisted/names/test/test_dns.py
index 94aa4a802..9626115ab 100644
--- a/src/twisted/names/test/test_dns.py
+++ b/src/twisted/names/test/test_dns.py
@@ -356,48 +356,51 @@ class NameTests(unittest.TestCase):
"""
L{Name.decode} raises L{dns.DNSDecodeError} when the number of
compression-pointer dereferences taken for a single message exceeds
- the limit carried by the shared L{dns._DecodeContext}.
+ the limit carried by the shared L{dns._DecodeContext} installed
+ through the private L{dns._decodeContextVar}.
"""
# Five distinct pointers chained end-to-end, terminated by a zero
# label byte. With a maxJumps of three the fourth dereference must
# trip the safety limit.
payload = b"\xc0\x02\xc0\x04\xc0\x06\xc0\x08\x00"
context = dns._DecodeContext(maxJumps=3)
- self.assertRaises(
- dns.DNSDecodeError,
- dns.Name().decode,
- BytesIO(payload),
- None,
- context,
- )
+ with dns._installDecodeContext(context):
+ self.assertRaises(
+ dns.DNSDecodeError,
+ dns.Name().decode,
+ BytesIO(payload),
+ )
def test_compressionPointerCounterIsShared(self):
"""
The L{dns._DecodeContext} counter accumulates across successive
L{Name.decode} calls, so that a message whose individual names are
each within bounds is still rejected when their aggregate exceeds
- the configured limit.
+ the configured limit. This mirrors production: L{Message.decode}
+ invokes L{Name.decode} many times against the same stream under one
+ shared context.
"""
payload = b"\xc0\x02\xc0\x04\x00"
context = dns._DecodeContext(maxJumps=3)
- stream = BytesIO(payload)
- dns.Name().decode(stream, context=context)
- self.assertEqual(context.jumps, 2)
+ with dns._installDecodeContext(context):
+ stream = BytesIO(payload)
+ dns.Name().decode(stream)
+ self.assertEqual(context.jumps, 2)
- stream.seek(0)
- self.assertRaises(
- dns.DNSDecodeError,
- dns.Name().decode,
- strio=stream,
- length=None,
- context=context,
- )
+ stream.seek(0)
+ self.assertRaises(
+ dns.DNSDecodeError,
+ dns.Name().decode,
+ stream,
+ )
def test_decodeWithoutContextIsBackwardsCompatible(self):
"""
- L{Name.decode} continues to work when called without a context,
- using a fresh per-call counter so existing callers are unaffected.
+ L{Name.decode} continues to work when called with no active
+ L{dns._decodeContextVar}, using a fresh per-call counter seeded
+ from L{dns.Name.maxCompressionPointers} so existing callers are
+ unaffected.
"""
name = dns.Name()
stream = BytesIO()