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EEF-CVE-2026-111101
Summary
Allocation of Resources Without Limits or Throttling vulnerability in the HTTP/1.1 chunked body reader of mtrudel bandit allows an unauthenticated remote attacker to exhaust server memory and CPU via a request body made of many 1-byte chunks.
The chunked reader in lib/bandit/http1/socket.ex keeps every chunk as its own list cell and binary term until IO.iodata_to_binary/1 joins them, and nothing limits the number of chunks. The :length option of read_body/2 counts only data bytes, so tiny chunks cost far more memory than their payload: 48 MB of 1-byte chunks grew one handler process to about 500 MB and took about 6 s of CPU. The route does not need to read the body, because on keep-alive connections ensure_completed/1 drains unread bodies through the same reader.
This issue affects bandit: from 0.1.0 before 1.14.0.
Details
1. Per-chunk accumulation. do_read_chunked_data! in lib/bandit/http1/socket.ex collects chunks as [body | chunk], adding a cons cell and a binary term for every chunk.
2. Byte-only limit. :length limits data bytes, not the number of chunks, so a body of 1\r\nx\r\n chunks becomes an 8-million-element improper list before IO.iodata_to_binary runs. Sending 48 MB of 1-byte chunks grew the handler heap to about 500 MB and used about 6 s of CPU. The same body as one 8 MB chunk peaked at about 16 MB and took 5 ms.
3. No body read needed. On keep-alive connections, ensure_completed/1 drains unread bodies through the same path. Against a Plug that never calls read_body, the attack used about 417 MB.
Proof of concept
- Run Bandit with a Plug that calls
Plug.Conn.read_body(conn, length: 1_000_000, read_length: 1_000_000). - Send
POST /withTransfer-Encoding: chunkedand a body of 1,000,000 chunks of1\r\nA\r\n, followed by0\r\n\r\n. - When
read_bodyreturns the 1,000,000 body bytes, the handler process holds about 48 MB and BEAM memory has grown by about 55 MB, for 6 MB sent.
Impact
An unauthenticated client can make a single HTTP/1.1 request hold many times its body size in server memory (about 48 MB for a 1 MB body in the reporter's test), and keep CPU busy while the chunks are joined. This affects any Bandit server that accepts HTTP/1.1 requests, whether or not its routes read the body.
Workarounds
From 0.7.0, a server whose clients and proxies all speak HTTP/2 can disable HTTP/1 with http_1_options: [enabled: false], so that the vulnerable chunked body reader never runs.
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The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. No user interaction is needed for the attacker to exploit this vulnerability.
Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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