CVE-2026-103262VulnerabilityPublished 1 Oct 2026Updated 29 Oct 2026

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

Affects nvcheckerSource GCVE Database

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Mentioned vulnerabilities

CVE-2026-49855

Authoritative record:CVE-2026-103262· High 8.7

Not known to be exploited · CVSS 8.7 High : upgrade to 2.22_1 soon.

Data as of 30 Oct 2026, 07:30 UTC

CVSS 4.0 base score

8.7High

How severe the flaw is on the 0–10 CVSS scale.

Patch: Available – fixed in 2.22_1

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

Attack Vector: Network
Attack Complexity: Low
Attack Requirements: None
Privileges Required: None
User Interaction: None
Vulnerable system confidentiality: None
Vulnerable system integrity: None
Vulnerable system availability: High
Subsequent system confidentiality: None
Subsequent system integrity: None
Subsequent system availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable system is bound to the network stack and the set of possible attackers extends beyond the other options listed below, up to and including the entire Internet. Such a vulnerability is often termed “remotely exploitable” and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: The attacker must take no measurable action to exploit the vulnerability. The attack requires no target-specific circumvention to exploit the vulnerability. An attacker can expect repeatable success against the vulnerable system.
Attack Requirements: None: The successful attack does not depend on the deployment and execution conditions of the vulnerable system. The attacker can expect to be able to reach the vulnerability and execute the exploit under all or most instances of the vulnerability.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without interaction from any human user, other than the attacker.
Vulnerable system confidentiality: None: There is no loss of confidentiality within the Vulnerable System.
Vulnerable system integrity: None: There is no loss of integrity within the Vulnerable System.
Vulnerable system availability: High: There is a total loss of availability, resulting in the attacker being able to fully deny access to resources in the Vulnerable System; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the Vulnerable System (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).
Subsequent system confidentiality: None: There is no loss of confidentiality within the Subsequent System or all confidentiality impact is constrained to the Vulnerable System.
Subsequent system integrity: None: There is no loss of integrity within the Subsequent System or all integrity impact is constrained to the Vulnerable System.
Subsequent system availability: None: There is no impact to availability within the Subsequent System or all availability impact is constrained to the Vulnerable System.

Definitions: FIRST CVSS v4.0 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 1 Oct 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 29 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade nvchecker to 2.22_1.

Affected software

Product	Affected	Fixed in
Homebrewnvchecker`pkg:brew/nvchecker`Other threats	Affected >=2.7 <2.22_1	Fixed in 2.22_1

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

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)`

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