CVE-2026-49855
tornado AsyncHTTPClient accumulates decompressed chunks without size limit (gzip bomb)
Description
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`.
How to fix CVE-2026-49855
To remediate CVE-2026-49855, upgrade the affected package to a fixed version below.
- —upgrade to 6.5.6 or later
Is CVE-2026-49855 being exploited?
No exploitation signal available. Neither CISA KEV nor a current EPSS score has been published for CVE-2026-49855.
Affected packages (1)
- from 0, < 6.5.6
CVSS scores
| Source | Version | Severity | Vector |
|---|---|---|---|
| osv | CVSS 3.1 | HIGH7.5 | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H |