CVE-2026-54340: h2o has HTTP/2 state amplification
Vulnerability summary
CVE-2026-54340 is a high (CVSS 7.5) Uncontrolled Resource Consumption vulnerability in h2o. EPSS estimates a 0.49% chance of exploitation in the next 30 days, higher than 42.4% of scored vulnerabilities.
Severe, CVSS 7.5 (CVE.org); no exploitation reported by the 5 checked sources; EPSS 0% (FIRST, 03-11-2026)
Source freshness unavailable
How is this decided?Timeline
- 16-07-2026PublishedVulnerability first disclosed.
- 17-07-2026Last modifiedVulnerability information updated.
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 metrics
4 scores · 4 sources · 1 version- OSV · UbuntuEcosystem advisory · calculated7.5HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- OSV · DebianEcosystem advisory · calculated7.5HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- NVDNIST7.5HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
EPSS trend
Probability
0.49%
Chance of exploitation in the next 30 days
Percentile
42%
Versus all other CVEs
30-day change
No change 0.00 pp
From 0.49% on 04-10-2026
Known exploited
Compare KEV catalogs
CISA KEVNot in the CISA KEV catalog.
ENISA CNWNot in the ENISA CNW catalog.
CIRCLNot in the CIRCL catalog.
Public exploits
0 foundNo public exploits indexed.
Weaknesses
CWE · root cause- CWE-400Uncontrolled Resource ConsumptionThe product does not properly control the allocation and maintenance of a limited resource.
- CWE-770Allocation of Resources Without Limits or ThrottlingThe product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated.
Affected systems
Known conditions: network-reachable, no authentication, no user interaction (CVSS vector). Other conditions: not stated in public data.
Ubuntu2 products
- dnsdistAll versions (No fix recorded)
- h2oAll versions (No fix recorded)
Debian1 product
- h2oAll versions (No fix recorded)
H2o1 product
- h2o< 9265bdd | ≥ 2026-05-29, < 2026-06-04
3 vendors · 4 products
References
5 links- github.com/h2o/h2o/security/advisories/GHSA-qcrr-wrhc-pgq9X Refsource CONFIRMREPORTThird Party Advisory
