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
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
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 |
Affected Products
quic-goquic-go_projectDetection & 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
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.
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…
Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.Source: Red Hat security advisory for CVE-2026-40898 (CC BY 4.0)
| 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?
Find it across Go, including transitive dependencies.