Memory exhaustion via unbounded cumulative gzip decompression
Published Jul 14, 2026 · Updated Jul 14, 2026
Denial of service in tornadoweb Tornado before 6.5.6 allows remote attackers to exhaust process memory with compressed HTTP bodies. The _GzipMessageDelegate in tornado/http1connection.py capped each decompressed chunk but failed to count cumulative output against max_body_size. SimpleAsyncHTTPClient is exposed by default to malicious server responses, while HTTPServer exposure requires decompress_request=True; the consequence is application-level memory exhaustion and service interruption.
Summary
What happened
Denial of service in tornadoweb Tornado before 6.5.6 allows remote attackers to exhaust process memory with compressed HTTP bodies. The _GzipMessageDelegate in tornado/http1connection.py capped each decompressed chunk but failed to count cumulative output against max_body_size. SimpleAsyncHTTPClient is exposed by default to malicious server responses, while HTTPServer exposure requires decompress_request=True; the consequence is application-level memory exhaustion and service interruption.
The record
- CVE
- CVE-2026-49855
- Published
- Jul 14, 2026
- Updated
- Jul 14, 2026
- Vendor
- tornadoweb
- Product
- Tornado
- Classifications
- CWE-409, T1499.003
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Jul 14, 2026CVE publishedPublication date reported by the CVE source.
- Jul 14, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
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- Affected versionversion=< 6.5.6
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
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Weakness, pattern, technique
Public exploit references
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Technologies
Your stack
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Your stack
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