Improper handling of highly compressed data in Tornado - CVE-2026-49855

Vulnerability identifier: #VU149935

CSH Severity: Medium

CVSS v4: 8.7 [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]

CVE-ID: CVE-2026-49855

CWE-ID: CWE-409

Exploitation vector: Remote access

Exploit availability: No public exploit available

The vulnerability allows a remote attacker to cause a denial of service.

The vulnerability exists due to improper handling of highly compressed data in Tornado gzip decompression routines when SimpleAsyncHTTPClient processes a response from a malicious server. A remote attacker can provide a compressed response that expands to consume effectively unlimited memory to cause a denial of service.

SimpleAsyncHTTPClient is affected in its default configuration. HTTPServer is affected only when decompress_request=True is set.

Tornado
SUSE Linux Enterprise Server 15 SP4
SUSE Linux Enterprise Server 15 SP5
SUSE Linux Enterprise Server 15 SP6
SUSE Linux Enterprise Server 12
SUSE Linux Enterprise Server for SAP Applications 12
SUSE Linux Enterprise Desktop 12
SUSE Linux Enterprise High Performance Computing 12
SUSE Linux Enterprise Server for SAP Applications 15
SUSE Linux Enterprise Server 15
SUSE Linux Enterprise High Performance Computing LTSS 15
SUSE Linux Enterprise High Performance Computing ESPOS 15
SUSE Linux Enterprise High Performance Computing 15
SUSE Linux Enterprise Desktop 15
SUSE Linux Enterprise Real Time 15
SUSE Linux Enterprise Micro
SUSE Linux Enterprise Micro for Rancher
Red Hat Enterprise Linux for IBM z Systems
Red Hat Enterprise Linux for Power, little endian
Red Hat Enterprise Linux for x86_64
Red Hat Enterprise Linux for ARM 64
Red Hat Enterprise Linux for ARM 64 - Extended Update Support
Red Hat Enterprise Linux for x86_64 - Extended Update Support
Red Hat Enterprise Linux for IBM z Systems - Extended Update Support
Red Hat Enterprise Linux for Power, little endian - Extended Update Support
SUSE Manager Client Tools for SLE
SUSE Linux Enterprise Server for the Raspberry Pi
SUSE Linux Enterprise Server
SUSE Linux Enterprise Server for SAP Applications
SUSE Linux Enterprise Desktop
Basesystem Module
Python 3 Module
openSUSE Leap
openEuler
python3-tornado
python-tornado
python-tornado-debuginfo
python-tornado-debugsource
python3-tornado-debuginfo
python311-tornado6
python311-tornado6-debuginfo
python-tornado6-debugsource
python-tornado-help
python-tornado (Red Hat package)

Install security update from vendor's website.

Tornado - update to 6.5.6
python3-tornado - addressed in versions 4.2.1-17.21.1, 4.5.3-150000.3.22.1
python-tornado - update to 4.2.1-17.21.1
python-tornado-debuginfo - addressed in versions 4.2.1-17.21.1, 4.5.3-150000.3.22.1
python-tornado-debugsource - addressed in versions 4.2.1-17.21.1, 4.5.3-150000.3.22.1
python3-tornado-debuginfo - update to 4.5.3-150000.3.22.1
python311-tornado6 - update to 6.3.2-150400.9.18.1
python311-tornado6-debuginfo - update to 6.3.2-150400.9.18.1
python-tornado6-debugsource - update to 6.3.2-150400.9.18.1
python-tornado-debuginfo - update to 6.5-5
python3-tornado - update to 6.5-5
python-tornado-help - update to 6.5-5
python-tornado-debugsource - update to 6.5-5
python-tornado - update to 6.5-5
python-tornado (Red Hat package) - addressed in versions 6.5.8-0.el9_8.1, 6.5.8-0.el10_0.1

• https://github.com/tornadoweb/tornado/security/advisories/GHSA-mgf9-4vpg-hj56

• Multiple vulnerabilities in Tornado
• openEuler 24.03 LTS SP3 update for python-tornado
• openEuler 24.03 LTS SP1 update for python-tornado
• SUSE update for python-tornado6
• SUSE update for python-tornado
• SUSE update for python-tornado
• Red Hat Enterprise Linux 9 update for python-tornado
• Red Hat Enterprise Linux 10 update for python-tornado