CVE-2026-49855: Tornado Tornado AsyncHTTPClient Accumulates Decompressed Chunks Vulnerability

HIGH
Advisory Identifier: GHSA-mgf9-4vpg-hj56
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CVSS 7.5
CWE-409
Published
Advisory Summary

CVE-2026-49855 is a HIGH (CVSS 7.5) tornado asynchttpclient accumulates decompressed chunks vulnerability (CWE-409: Improper Handling of Highly Compressed Data (Data Amplification)) affecting Tornado libraries in the PyPI ecosystem. The issue involves tornado asynchttpclient accumulates decompressed chunks without size limit (gzip bomb). Remediation is available by upgrading affected packages to version 6.5.6.

CVE-2026-49855 at a Glance

CVE Identifier CVE-2026-49855
GHSA Identifier GHSA-mgf9-4vpg-hj56
Severity HIGH
CVSS v3.1 Base Score 7.5
CWE Classification CWE-409: Improper Handling of Highly Compressed Data (Data Amplification)
Affected Product Tornado
Package Ecosystem PyPI
Published Date
Last Updated

Security Impact

CVSS: 7.5

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality None (C:N)
Integrity None (I:N)
Availability High (A:H)

How to Fix CVE-2026-49855

To remediate this vulnerability, upgrade all affected Tornado dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

tornado fixed: 6.5.6 
pip install tornado>=6.5.6
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

pip show tornado

How to Check if You Are Affected

To determine whether your Python environment is affected by CVE-2026-49855, inspect your installed packages using pip:

pip show tornado

Analyze the terminal output to locate any version of Tornado in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 6.5.6), your application is affected.

Note: The pip show tornado command checks currently installed packages in your active Python environment.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-409: Improper Handling of Highly Compressed Data (Data Amplification), where the product does not handle or incorrectly handles a compressed input with a very high compression ratio that produces a large output. In Tornado, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

Tornado's gzip decompression routines work in limited-size chunks, but have no overall limit for the total size of decompressed chunks that they will accumulate (There has always been a limit for the total *compressed* size). This allows a malicious server to consume effectively unlimited amounts of memory if it is accessed via SimpleAsyncHTTPClient in its default configuration. `HTTPServer` is not affected in its default configuration, but it is if `decompress_request=True` is set. This bug is fixed in Tornado 6.5.6. `max_body_size` is now checked both for the compressed and cumulative decompressed size of the response. Prior to upgrading, this issue can be mitigated by setting `decompress_response=False` or using `CurlAsyncHTTPClient`.

CVE-2026-49855 Affected Tornado Versions

1 component(s)
Fixed Versions:
6.5.6
Product / Package Ecosystem Affected Versions Fixed Version
tornado
pypi/tornado
PyPI
< 6.5.6
6.5.6

References

GitHub Advisory / Wiki ADVISORY
https://github.com/tornadoweb/tornado/security/advisories/GHSA-mgf9-4vpg-hj56
Tornado Source Repository REPOSITORY
https://github.com/tornadoweb/tornado

Frequently Asked Questions

What is CVE-2026-49855?
CVE-2026-49855 is a security vulnerability in Tornado involving tornado AsyncHTTPClient accumulates decompressed chunks without size limit (gzip bomb) under CWE-409: Improper Handling of Highly Compressed Data (Data Amplification).
Which Tornado versions are affected?
Affected versions include releases prior to 6.5.6 across affected packages: tornado.
What version fixes CVE-2026-49855?
Upgrading to 6.5.6 (or the latest patched release for your respective branch) resolves the vulnerability.
Is tornado affected?
Yes, tornado is affected in releases prior to 6.5.6. Upgrade your dependency to 6.5.6 or later.
What is the CVSS score of CVE-2026-49855?
CVE-2026-49855 has a CVSS v3.1 base score of 7.5 (HIGH) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.
What CWE is associated with CVE-2026-49855?
CVE-2026-49855 is classified under CWE-409: Improper Handling of Highly Compressed Data (Data Amplification) (The product does not handle or incorrectly handles a compressed input with a very high compression ratio that produces a large output.).
How can I check PyPI dependencies for CVE-2026-49855?
Run the dependency tree command: 'pip show tornado'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-mgf9-4vpg-hj56",
  "modified" : "2026-06-15T20:19:28Z",
  "published" : "2026-06-15T20:19:28Z",
  "aliases" : [ "CVE-2026-49855" ],
  "summary" : "tornado AsyncHTTPClient accumulates decompressed chunks without size limit (gzip bomb)",
  "details" : "Tornado's gzip decompression routines work in limited-size chunks, but have no overall limit for the total size of decompressed chunks that they will accumulate (There has always been a limit for the total *compressed* size). This allows a malicious server to consume effectively unlimited amounts of memory if it is accessed via SimpleAsyncHTTPClient in its default configuration. `HTTPServer` is not affected in its default configuration, but it is if `decompress_request=True` is set.\n\nThis bug is fixed in Tornado 6.5.6. `max_body_size` is now checked both for the compressed and cumulative decompressed size of the response.\n\nPrior to upgrading, this issue can be mitigated by setting `decompress_response=False` or using `CurlAsyncHTTPClient`.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
  } ],
  "affected" : [ {
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      "name" : "tornado"
    },
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      "type" : "ECOSYSTEM",
      "events" : [ {
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      }, {
        "fixed" : "6.5.6"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/tornadoweb/tornado/security/advisories/GHSA-mgf9-4vpg-hj56"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/tornadoweb/tornado"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-409" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-06-15T20:19:28Z",
    "nvd_published_at" : null
  }
}