Description
h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit 9265bdd, there is an HTTP/2 state amplification issue that combines HPACK decompression amplification with Slowloris-style stream stalling. Amplified decoded header state can be retained by stalled HTTP/2 streams, and depending on the configuration, additional limits are needed to bound decoded header state and prevent attack. This issue has been fixed by commit 9265bdd.
References (2)
Core 2
Core References
X_Refsource_Confirm x_refsource_confirm
https://github.com/h2o/h2o/security/advisories/GHSA-qcrr-wrhc-pgq9
X_Refsource_Misc x_refsource_misc
https://github.com/h2o/h2o/commit/9265bdd9a996ed992681055e3996baf3e09d2063
Scores
CVSS v3
7.5
EPSS
0.0028
EPSS Percentile
20.1%
Attack Vector
NETWORK
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
CISA SSVC
Vulnrichment
Exploitation
none
Automatable
yes
Technical Impact
partial
Details
CWE
CWE-400
CWE-770
Status
published
Products (1)
h2o/h2o
< 9265bdd
Published
Jul 17, 2026
Tracked Since
Jul 17, 2026