CVE-2026-91990 Tornado before 6.5.8 Memory Amplification DoS via multipart
Attackers can cause denial of service in Tornado web servers by sending crafted multipart requests that exhaust server memory.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-91990?
CVE-2026-91990 is a high-severity denial of service vulnerability in tornadoweb tornado, published on 15 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
Tornado before 6.5.8 contains a memory amplification vulnerability in parse_multipart_form_data that splits multipart data before validating the max_parts limit. Attackers can send crafted multipart requests with many parts to create large transient lists, exhausting server memory and causing denial of service.
Weakness: CWE-770
Which versions are affected by CVE-2026-91990?
2 open source packages ship an affected version, including tornado.
| Package | Affected | Fixed in |
|---|---|---|
tornado PyPI | All versions | 6.5.8 |
tornado PyPI | All versions | 6.5.8 |
How do I fix CVE-2026-91990?
Upgrade tornado to 6.5.8 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-2026-91990 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-91990 exists.
Sources
Also known as AZL-103107, BELL-CVE-2026-91990, CLEANSTART-2026-NE81748, CLEANSTART-2026-PJ73152, CLEANSTART-2026-SY45614, CLEANSTART-2026-YX36075 and 30 more.