High severity · published Jul 16, 2026
CVE-2026-54340
h2o has HTTP/2 state amplification
7.5
CVSS
0.5%
EPSS
No
On KEV
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.
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Confidentiality
None
Integrity
None
Availability
High
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.5% · higher than 42% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
H2
h2o · h2o
< 9265bdd · source: vendor, NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-54340
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-54340
Patch
https://github.com/h2o/h2o/commit/9265bdd9a996ed992681055e3996baf3e09d2063
Third Party Advisory
https://github.com/h2o/h2o/security/advisories/GHSA-qcrr-wrhc-pgq9
More from h2o
9.8
CVE-2026-8751
h2oai h2o-3 JAR Model.java importBinaryModel deserialization
9.8
CVE-2026-3960
Remote Code Execution in h2oai/h2o-3
9.8
CVE-2025-10769
h2oai h2o-3 H2 JDBC Driver ImportSQLTable deserialization
9.8
CVE-2025-10768
h2oai h2o-3 IBMDB2 JDBC Driver ImportSQLTable deserialization
9.8
CVE-2025-6544
Deserialization Vulnerability in h2oai/h2o-3