tornado: CurlAsyncHTTPClient enforces no response-size limit — decompression bomb drives unbounded memory accumulation to OOM

Why it is Yellow

• CVSS 7.5 high
• Remotely exploitable without logging in
• On your watchlist: PyPI

Affected packages

References

• https://github.com/advisories/GHSA-chx6-46f5-w4vp
• https://github.com/tornadoweb/tornado/security/advisories/GHSA-chx6-46f5-w4vp
• https://github.com/tornadoweb/tornado/pull/3719
• https://github.com/tornadoweb/tornado/commit/15f056080d8e456f92cca8ad0c33e5a5480414ac
• https://github.com/tornadoweb/tornado/commit/6564e0a0922c16f239d3a18adccd04736e380c89
• https://github.com/tornadoweb/tornado/commit/aa2eb0d989716ac00fe8d7a9919b81c1eccd6ec4
• https://github.com/tornadoweb/tornado/commit/e412435febba4569c552c5c9054f1bf92bd171a9
• https://github.com/tornadoweb/tornado/releases/tag/v6.5.9
• https://github.com/advisories/GHSA-chx6-46f5-w4vp

Original description

An unbounded memory accumulation (decompression bomb) in `tornado.curl_httpclient.CurlAsyncHTTPClient` — the client-side sibling gap of CVE-2026-49855 — verified end-to-end on the 2026-08-15 master snapshot (`6.6.dev1`) and present unchanged in the latest release tag `v6.5.8` and on master (checked 2026-08-17). When a Tornado application configures the curl client (the documented deployment for proxy support / advanced TLS options) and `fetch()`es an attacker-chosen or attacker-compromised URL with default `decompress_response=True`, a malicious server replying `Content-Encoding: gzip` with a ~2.8 MB wire bomb drove the client's RSS from 30,884 kB to **1,032,100 kB (~1008 MB) in 3.18 s** (~350 MB/s, monotonic, no plateau) until the kernel OOM-killed the process (exit 137, cgroup `OOMKilled=true`) — with the transfer only 67% complete and **no client-side size check ever intervening**: `curl_httpclient.py` contains zero occurrences of `max_body_size`/`MAXFILESIZE`. The identical bomb against the default `SimpleAsyncHTTPClient` fails cleanly at ~65 MB, because every size gate CVE-2026-49855 added (compressed CL, chunked total, cumulative decompressed size — `http1connection.py:620,676,742`) lives in code the curl client never executes. This is a distinct component from the published advisory (which fixed `_GzipMessageDelegate`/`SimpleAsyncHTTPClient` only) and from the other curl-client advisories (credential handle-reuse GHSA-pw6j-qg29-8w7f, header CRLF GHSA-w235-7p84-xx57); the file's full commit history (latest 2026-06-17) shows no response-size work. ## Details `tornado/curl_httpclient.py` (line numbers identical on master `6.6.dev1`, `v6.5.8`, and the audited snapshot): ```python "buffer": BytesIO(), # :202 — plain BytesIO, no accounting ... else: write_function = buffer.write # :359 — every decompressed byte lands here curl.setopt(pycurl.WRITEFUNCTION, write_function) # :360 ... if request.decompress_response: # default True (HTTPRequest) curl.setopt(pycurl.ENCODING, "gzip,deflate") # :373-374 — libcurl advertises + auto-decodes ``` libcurl decompresses the response **before** invoking `WRITEFUNCTION`, so the callback receives decompressed bytes, which are appended to an unbounded `BytesIO` until the transfer ends or the process dies. The only ceiling is `request_timeout` (default 20 s) — at zlib's hundreds of MB/s that still permits many GB of accumulation; the PoC raised it to 300 s and the 1 GiB cgroup cap was hit in 3.2 s regardless. There is no `pycurl.MAXFILESIZE`, no `max_buffer_size`/`max_body_size` plumbing (the constructor accepts no body-size option), and `streaming_callback` users fare no better (the callback variant at `:353-356` also performs zero accounting). Contrast — `SimpleAsyncHTTPClient` path (`tornado/http1connection.py`), all absent from the curl path: ```python if cast(int, content_length) > self._max_body_size: # :620 Content-Length gate if total_size > self._max_body_size: # :676 chunked total gate if self._decompressed_body_size > self._max_body_size: # :742 CVE-2026-49855 decompressed gate ``` `SimpleAsyncHTTPClient.initialize()` defaults `max_buffer_size = 104857600` (100 MiB) with `max_body_size` defaulting to it (`simple_httpclient.py:117-121`); `CurlAsyncHTTPClient.initialize()` has no corresponding parameter at all. Attack chain (attacker = malicious HTTP server; victim = any Tornado app doing `fetch()` on attacker-influenced URLs — URL fetchers, webhook processors, link previewers, RSS/probe pollers): 1. App configures `AsyncHTTPClient.configure("tornado.curl_httpclient.CurlAsyncHTTPClient")` (documented for proxy support; proxies are only supported with the curl client). 2. App calls `fetch("http://attacker/...")` with defaults → request advertises `Accept-Encoding: gzip,deflate`. 3. Attacker replies `200`, `Content-Encoding: gzip`, `Transfer-Encoding: chunked`, body = a gzip stream of zeros (4,174,525 wire bytes expanding to 4 GiB, 1029:1, sent in 64 KB chunks). 4. libcurl auto-decodes at ~350 MB/s into `buffer.write` with no size accounting → process RSS climbs linearly until OOM. The response need not complete: the client died with 2,818,048/4,174,525 wire bytes delivered (67%). Variant without compression: `decompress_response=False` plus an endless streaming body (no Content-Length, no final chunk) feeds the same unaccounted `buffer.write` — this client never enforces any cap on any path. ## PoC Verified end-to-end 2026-08-15 in a single container (cgroup `--memory 1g --memory-swap 1g` so the exhaustion endpoint is safe and fast to observe), two processes over a real TCP socket on `127.0.0.1:8081`: a raw-socket malicious server (`evil_server.py`, builds the 4 GiB-of-zeros gzip bomb once, serves it with chunked framing) and the Tornado victim (`victim_curl.py`, configures `CurlAsyncHTTPClient`, `fetch(..., request_timeout=300)`, samples `/proc/self/status` VmRSS every 0.2 s from a monitor thread so the curve survives the OOM kill). ### Build & run the victim ```bash git clone https://github.com/tornadoweb/tornado cd tornado pip install pycurl python evil_server.py & # 127.0.0.1:8081; prints BOMB_BUILT wire_bytes=... ratio=... EVIL_LISTENING python victim_curl.py # victim: CurlAsyncHTTPClient fetch -> expect linear RSS rise -> OOM python victim_simple.py # control: default SimpleAsyncHTTPClient on the same bomb ``` Verified environment: debian:bookworm-slim, Python 3.11.2, python3-pycurl 7.45.2 (libcurl 7.88.1, zlib 1.2.13), tornado master snapshot `6.6.dev1` of 2026-08-15 run from the source tree (`sys.path`), container memory capped at 1 GiB. `curl_httpclient.py` verified byte-equivalent (still zero size-limit references) in tag `v6.5.8` and on master as of 2026-08-17. ### Reproduction steps 1. **Precondition — the documented curl-client deployment fetching a remote URL.** The vulnerabili