CVE-2023-26964 is a high-severity (CVSS 7.5) CWE-770 vulnerability in h2. 1 public exploit reference exists, so weaponization risk is real. A fix is available for h2 — see the affected versions and patch details below.
h2 vulnerable to denial of service
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
Exploitation and automatability from CISA’s SSVC triage for CVE-2023-26964.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2023-26964 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 378,567 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
h2Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects crates.io packages — download data is not available via public APIs for these ecosystems.
Description
Hyper is an HTTP library for Rust and h2 is an HTTP 2.0 client & server implementation for Rust. An issue was discovered in h2 v0.2.4 when processing header frames. It incorrectly processes the HTTP2 RST_STREAM frames by not always releasing the memory immediately upon receiving the reset frame, leading to stream stacking. As a result, the memory and CPU usage are high which can lead to a Denial of Service (DoS).
This issue affects users only when dealing with http2 connections.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🦀crates.io | h2 | all versions | 0.3.17cargo update -p h2 --precise 0.3.17 |
Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for h2, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update h2 to 0.3.17 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2023-26964 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2023-26964 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2023-26964. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2023-26964 in your dependencies?
O3 Security finds CVE-2023-26964 across crates.io dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.