CVE-2026-26278 is a high-severity (CVSS 7.5) CWE-776 vulnerability in fast-xml-parser. A fix is available for fast-xml-parser — see the affected versions and patch details below.
fast-xml-parser affected by DoS through entity expansion in DOCTYPE (no expansion limit)
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-2026-26278.
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-2026-26278 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 377,166 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
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
fast-xml-parsernpmDescription
fast-xml-parser allows users to validate XML, parse XML to JS object, or build XML from JS object without C/C++ based libraries and no callback. In versions 4.1.3 through 5.3.5, the XML parser can be forced to do an unlimited amount of entity expansion. With a very small XML input, it’s possible to make the parser spend seconds or even minutes processing a single request, effectively freezing the application. Version 5.3.6 fixes the issue. As a workaround, avoid using DOCTYPE parsing by processEntities: false option.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | fast-xml-parser | ≥ 4.1.3&&< 4.5.4 | 4.5.4npm install fast-xml-parser@4.5.4 |
| 📦npm | fast-xml-parser | ≥ 5.0.0&&< 5.3.6 | 5.3.6npm install fast-xml-parser@5.3.6 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for fast-xml-parser, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update fast-xml-parser to 4.5.4 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-26278 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-2026-26278 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-26278. 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.
| Product | Fixed in | Advisory |
|---|---|---|
| Red Hat Advanced Cluster Security 4.8 | advanced-cluster-security/rhacs-main-rhel8:1775594119 | RHSA-2026:7110 |
| Red Hat Advanced Cluster Security 4.9 | advanced-cluster-security/rhacs-main-rhel8:1775594284 | RHSA-2026:7128 |
| Red Hat Developer Hub 1.8 | rhdh/rhdh-hub-rhel9:1774545605 | RHSA-2026:6174 |
| Red Hat Developer Hub 1.9 | rhdh/rhdh-hub-rhel9:1775140647 | RHSA-2026:6802 |
| Red Hat Openshift Data Foundation 4.18 | odf4/cephcsi-rhel9:1783676191 | RHSA-2026:41941 |
| Red Hat Openshift Data Foundation 4.19 | odf4/cephcsi-rhel9:1783684360 | RHSA-2026:41944 |
| Red Hat Openshift Data Foundation 4.20 | odf4/cephcsi-rhel9:1783667125 | RHSA-2026:40984 |
| Red Hat Satellite 6.18 | satellite/iop-host-inventory-frontend-rhel9:1785922764 | RHSA-2026:51349 |
Frequently Asked Questions
Is CVE-2026-26278 in your dependencies?
O3 Security finds CVE-2026-26278 across npm dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.