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CVE-2026-91990 – Memory Amplification DoS – Tornado (TornadoServer) before 6.5.8

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Summary

CVE-2026-91990 is a memory amplification denial-of-service vulnerability in the Tornado Python web framework’s multipart form-data parser. It affects all versions of tornado prior to 6.5.8 and is rated HIGH severity (CVSS v3.1 base score 7.5 / CVSS v4.0 8.7). A remote, unauthenticated attacker can crash or degrade a Tornado-based service by sending crafted multipart HTTP requests, without needing valid credentials or any user interaction.

Technical details

  • Root cause: The parse_multipart_form_data function in Tornado’s httputil.py splits the raw multipart request body on the boundary delimiter (data.split(b"--"+boundary+b"rn")) and materializes the full list of parts in memory before it checks the request against the configured max_parts limit.
  • Trigger condition: An attacker sends a multipart/form-data request containing a very large number of parts (e.g., roughly 100,000 parts in a ~600KB body) to any endpoint that parses multipart form data.
  • Attack vector: Network, pre-authentication, no user interaction required (CVSS AV:N/AC:L/PR:N/UI:N).
  • Impact: Because the part list is built before validation, the server allocates memory disproportionate to the request size, exhausting available memory and causing denial of service (CVSS availability impact: HIGH; no confidentiality or integrity impact).

Affected software

  • tornado (tornadoweb/tornado), PyPI package tornado: all versions before 6.5.8
  • Fixed in: 6.5.8 and later

Severity

  • CVSS v3.1 Base Score: 7.5 (HIGH) — CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  • CVSS v4.0 Base Score: 8.7 (HIGH) — 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
  • CWE-770: Allocation of Resources Without Limits or Throttling

Mitigation and recommended actions

  • Immediate: Upgrade tornado to version 6.5.8 or later, which validates/limits the number of multipart parts before building the full parts list.
  • If patching is not immediately possible:
    • Enforce request body size limits at a reverse proxy or load balancer in front of the Tornado application to cap the size of incoming multipart requests.
    • Restrict or rate-limit access to endpoints that accept multipart/form-data uploads.
    • Monitor for abnormal memory consumption or spikes correlated with multipart upload traffic as an indicator of attempted exploitation.

References

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