CVE-2026-49755 — req

Summary

Req's default response pipeline auto-decodes archive and compressed bodies based on the server-supplied content-type (or URL extension) and materialises the full decompressed contents in memory with no size cap. An attacker who controls (or can redirect a victim into) an HTTP endpoint reached by Req.get!/1 can return a tiny "decompression bomb" that expands to many gigabytes on the client and exhausts the BEAM's memory.

Details

1. Archive auto-decoding. Req.Steps.decode_body/1 in lib/req/steps.ex dispatches on the response content-type (or URL extension) and calls Erlang's archive libraries with :memory, returning a [{name, bytes}] list of every entry fully decompressed in RAM: application/zip → :zip.extract(body, [:memory]), application/x-tar → :erl_tar.extract({:binary, body}, [:memory]), application/gzip / .tgz → :erl_tar.extract({:binary, body}, [:memory, :compressed]). No byte cap is enforced before decoding and no per-entry size limit is passed to :zip / :erl_tar.

2. content-encoding chaining. Req.Steps.decompress_body/1 walks the content-encoding header and chains :zlib / :brotli / :ezstd decoders, so a response advertising content-encoding: gzip, gzip, gzip, … inflates through multiple layers without bound.

3. Default-on, attacker-chosen decoder. Both steps are part of Req's default pipeline. The caller does not need to opt in, and the attacker chooses which decoder fires by setting content-type and content-encoding on their own server (or on any host reached via Req's automatic redirect following).

PoC

1. Run an HTTP server that responds 200 with content-type: application/zip and a body that is a zip archive whose single entry is ~400 MB of zero bytes (compressed wire payload: a few hundred KB).
2. From the victim process, call Req.get!(url) against that server (no special options, no opt-in to archive decoding).
3. decode_body/1 dispatches on content-type, invokes :zip.extract(body, [:memory]), and the response body becomes [{~c"bomb.bin", <<400 MB of zero bytes>>}]. A sub-MB request produces hundreds of MB resident memory; layering gzip on the content-encoding path or increasing entry size scales arbitrarily.

Impact

Memory-exhaustion denial of service against any Elixir application that uses Req with its default step pipeline to fetch URLs influenced by an untrusted party, including webhook senders, link previews, OAuth/OIDC discovery clients, package mirrors, image proxies, and any Req.get!/1 call that may follow redirects to attacker-controlled hosts. No authentication is required; a single response can crash the BEAM and take down unrelated workloads on the same VM.