CVE-2026-49855 tornado AsyncHTTPClient accumulates decompressed chunks without size limit (gzip bomb)
A malicious server can cause memory exhaustion in Tornado's AsyncHTTPClient by sending a gzip bomb, leading to denial of service.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-49855?
CVE-2026-49855 is a high-severity denial of service vulnerability in tornadoweb tornado, published on 14 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
Tornado is a Python web framework and asynchronous networking library. Prior to 6.5.6, Tornado gzip decompression routines processed limited-size chunks but did not enforce an overall limit on accumulated decompressed chunks, allowing a malicious server accessed by SimpleAsyncHTTPClient or an HTTPServer configured with decompress_request=True to consume effectively unlimited memory. This issue is fixed in version 6.5.6.
Weakness: CWE-409
Which versions are affected by CVE-2026-49855?
3 open source packages ship an affected version, including tornado.
| Package | Affected | Fixed in |
|---|---|---|
tornado PyPI | All versions | 6.5.6 |
tornado PyPI | All versions | 6.5.6 |
tornado PyPI | All versions | 6.5.6 |
How do I fix CVE-2026-49855?
Upgrade tornado to 6.5.6 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-49855 being exploited?
There is no known exploitation of CVE-2026-49855 in the wild and no public exploit code.
Sources
Also known as AZL-92385, BELL-CVE-2026-49855, CLEANSTART-2026-QK13571, CLSA-2026-1781744125, DEBIAN-CVE-2026-49855, ECHO-7775-eb27-2601 and 13 more.