CVE-2026-40898 is a medium-severity (CVSS 5.3) CWE-770 vulnerability in github.com/quic-go/quic-go. A fix is available for github.com/quic-go/quic-go — see the affected versions and patch details below.
quic-go: HTTP/3 QPACK Trailer Expansion Memory Exhaustion
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-2026-40898.
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-2026-40898 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 379,145 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
github.com/quic-go/quic-goReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.
Description
Summary
An attacker can cause excessive memory allocation in quic-go's HTTP/3 client and server implementations by sending a QPACK-encoded HEADERS frame that decodes into a large trailer field section with many unique field names and/or large values. The implementation builds an http.Header for the corresponding http.Request or http.Response, while only enforcing limits on the size of the QPACK-compressed HEADERS frame, not on the decoded field section. This can lead to memory exhaustion.
This is very similar to CVE-2025-64702. The difference is that this issue uses HTTP trailers, rather than HTTP headers, as the attack vector.
Impact
A misbehaving or malicious peer can cause a denial-of-service (DoS) attack against quic-go's HTTP/3 servers or clients by triggering excessive memory allocation, potentially leading to crashes or resource exhaustion. This affects both servers and clients due to symmetric header construction.
Details
In HTTP/3, field sections are compressed using QPACK (RFC 9204). Field sections are used for both HTTP headers and trailers. quic-go's HTTP/3 server and client decode the QPACK-encoded HEADERS frame into header fields, then construct an http.Request or http.Response.
http3.Server.MaxHeaderBytes and http3.Transport.MaxResponseHeaderBytes limit the encoded HEADERS frame size, with defaults of 1 MB for servers and 10 MB for clients. However, they did not limit the decoded field section size. A maliciously crafted HEADERS frame carrying trailers can expand to about 50x the encoded size using QPACK static table entries with long names and/or values.
RFC 9114 requires endpoints to enforce decoded field section size limits via SETTINGS, which quic-go did not do for trailers.
The Fix
quic-go now enforces RFC 9114 decoded field section size limits for trailers as well. It incrementally decodes QPACK entries and checks the field section size after each entry, aborting the stream if an entry causes the limit to be exceeded.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/quic-go/quic-go | all versions | 0.59.1go get github.com/quic-go/quic-go@v0.59.1 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/quic-go/quic-go, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/quic-go/quic-go to 0.59.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-40898 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-2026-40898 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-40898. 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 is an Important denial-of-service flaw in `quic-go` that allows a remote attacker to exhaust system memory by sending specially crafted HTTP/3 trailer fields. This can lead to crashes or resource unavailability in affected Red Hat products utilizing `quic-go` for HTTP/3 communication, impacting both client and…
| Product | Fixed in | Advisory |
|---|---|---|
| Red Hat Ansible Automation Platform 2.5 for RHEL 8 | receptor-0:1.6.7-1.el8ap | RHSA-2026:50319 |
| Red Hat Ansible Automation Platform 2.6 for RHEL 10 | receptor-0:1.6.7-1.el10ap | RHSA-2026:50336 |
| Red Hat Ansible Automation Platform 2.7 for RHEL 10 | receptor-0:1.6.7-1.el10ap | RHSA-2026:50318 |
| Red Hat Ansible Automation Platform 2.6 | ansible-automation-platform-26/receptor-rhel9:1785632117 | RHSA-2026:50479 |
| Red Hat Ansible Automation Platform 2.7 | ansible-automation-platform-27/receptor-rhel9:1785389482 | RHSA-2026:50340 |
| Red Hat Container Native Virtualization 4.21 | container-native-virtualization/kubevirt-apiserver-proxy-rhel9:1786125762 | RHSA-2026:53734 |
| Red Hat Hardened Images | trivy-main-0.69.3-1.2.hum1 | RHSA-2026:15862 |
| Red Hat Hardened Images | caddy-main-2.11.4-0.2.hum1 | RHSA-2026:40223 |
Frequently Asked Questions
Is CVE-2026-40898 in your dependencies?
O3 Security finds CVE-2026-40898 across Go dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.