Weakness Type (CWE)

CWE-409 — CWE-409 Improper Handling of Highly Compressed Data (Data Amplification)

CAPEC

CVSS 4.0 Score

8.2

HIGH

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

Vulnerability description

Improper Handling of Highly Compressed Data (Data Amplification) vulnerability in wojtekmach Req allows attacker-controlled HTTP servers to exhaust memory in a Req client via decompression-bomb response bodies.

Req's default response pipeline includes 'Elixir.Req.Steps':decode_body/1 and 'Elixir.Req.Steps':decompress_body/1 in lib/req/steps.ex. decode_body/1 dispatches on the server-supplied content-type (or URL extension) and calls :zip.extract(body, [:memory]) for application/zip, :erl_tar.extract({:binary, body}, [:memory]) for application/x-tar, and :erl_tar.extract({:binary, body}, [:memory, :compressed]) for application/gzip / .tgz. Each returns the full decompressed archive contents as a [{name, bytes}] list in memory, with no per-entry or total size cap. 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.

Both steps are enabled by default, no caller opt-in is required, and the attacker controls the content-type and content-encoding headers on their own server (or on any host reached via Req's automatic redirect following). A sub-megabyte response can expand to multiple gigabytes on the victim, crashing the BEAM process.

This issue affects req: from 0.1.0 before 0.6.1.

Affected

pkg:hex/req

Module Source File Routine
Req.Steps lib/req/steps.ex Req.Steps.decode_body/1
Req.Steps.decompress_body/1
Status Type Version Changes / Fixed in
affected semver ⓘ 0.1.0 < 0.6.1

pkg:github/wojtekmach/req

Module Source File Routine
Req.Steps lib/req/steps.ex Req.Steps.decode_body/1
Req.Steps.decompress_body/1
Status Type Version Changes / Fixed in
affected git ⓘ e37753741c < 84977e5b1a

Workarounds

Disable Req's automatic body decoding on requests that fetch attacker-influenced URLs by passing decode_body: false to 'Elixir.Req':new/1 / 'Elixir.Req':get!/1. To also skip the content-encoding decompression pipeline, pass raw: true. Both options leave the response body as the raw on-the-wire bytes, so the caller can size-check before any decompression.

References

Credits

  • Finder: Peter Ullrich
  • Remediation developer: Wojtek Mach
  • Analyst: Jonatan Männchen / EEF

CVE record as JSON:  GET /cves/CVE-2026-49755.json
OSV record as JSON:  GET /osv/EEF-CVE-2026-49755.json