CVE-2023-33953 — grpcio
HIGHCVE-2023-33953 is a high-severity (CVSS 7.5) CWE-789 vulnerability in grpcio. A fix is available for grpcio — see the affected versions and patch details below.
Denial-of-Service in gRPC
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2023-33953.
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-2023-33953 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 377,333 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
grpcio🐍grpcio🐍grpcio🐍grpcio💎grpc💎grpc💎grpc💎grpcReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
gRPC contains a vulnerability that allows hpack table accounting errors could lead to unwanted disconnects between clients and servers in exceptional cases/ Three vectors were found that allow the following DOS attacks:
- Unbounded memory buffering in the HPACK parser
- Unbounded CPU consumption in the HPACK parser
The unbounded CPU consumption is down to a copy that occurred per-input-block in the parser, and because that could be unbounded due to the memory copy bug we end up with an O(n^2) parsing loop, with n selected by the client.
The unbounded memory buffering bugs:
- The header size limit check was behind the string reading code, so we needed to first buffer up to a 4 gigabyte string before rejecting it as longer than 8 or 16kb.
- HPACK varints have an encoding quirk whereby an infinite number of 0’s can be added at the start of an integer. gRPC’s hpack parser needed to read all of them before concluding a parse.
- gRPC’s metadata overflow check was performed per frame, so that the following sequence of frames could cause infinite buffering: HEADERS: containing a: 1 CONTINUATION: containing a: 2 CONTINUATION: containing a: 3 etc…
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | grpcio | all versions | 1.53.2pip install --upgrade 'grpcio==1.53.2' |
| 🐍PyPI | grpcio | ≥ 1.54.0&&< 1.54.3 | 1.54.3pip install --upgrade 'grpcio==1.54.3' |
| 🐍PyPI | grpcio | ≥ 1.55.0&&< 1.55.2 | 1.55.2pip install --upgrade 'grpcio==1.55.2' |
| 🐍PyPI | grpcio | ≥ 1.56.0&&< 1.56.2 | 1.56.2pip install --upgrade 'grpcio==1.56.2' |
| 💎RubyGems | grpc | all versions | 1.53.2bundle update grpc --conservative |
| 💎RubyGems | grpc | ≥ 1.54.0&&< 1.54.3 | 1.54.3bundle update grpc --conservative |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for grpcio, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update grpcio to 1.53.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2023-33953 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-2023-33953 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2023-33953. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
This vulnerability is specific to C++ implementations of gRPC prior to the 1.57 release. The gRPC library was packaged with Openshift via the Kuryr component. However, Kuryr was never configured to run code using the gRPC library and has since been removed.
| Product | Fixed in | Advisory |
|---|---|---|
| Red Hat Enterprise Linux 9 | rhc-worker-playbook-0:0.1.10-1.el9_5 | RHSA-2024:10761 |
Frequently Asked Questions
Is CVE-2023-33953 in your dependencies?
O3 Security finds CVE-2023-33953 across PyPI, RubyGems dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.