How severe is CVE-2019-8400?

CVE-2019-8400 has a CVSS score of 6.1/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2019-8400?

CVE-2019-8400 affects the following packages: github.com/ory/hydra (Go). Ecosystems affected: Go.

How do I fix CVE-2019-8400?

Update github.com/ory/hydra to 1.4.8 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2019-8400 is resolved across your whole dependency graph.

How do I detect CVE-2019-8400 in my Go dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/ory/hydra, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2019-8400 if there is no patch (or I can't update yet)?

Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Is CVE-2019-8400 actively exploited in the wild?

Yes. There are 2 known exploit references for CVE-2019-8400, including 2 in-the-wild exploitations observed on GitHub and other sources. Treat this as actively exploitable and prioritize patching immediately. All exploit code should only be run in an isolated sandbox environment for research or authorized testing — never against production systems without explicit written authorization.

What is the EPSS score for CVE-2019-8400?

CVE-2019-8400 has an EPSS (Exploit Prediction Scoring System) score of 1.3%, placing it in the 71th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2019-8400?

CVE-2019-8400 is classified as Cross-site Scripting (XSS) (CWE-79). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation. This is a high-impact weakness class that often enables remote code execution or data exposure.

When was CVE-2019-8400 published, and has it been updated?

CVE-2019-8400 was published on February 17, 2019 and was last updated on October 20, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐹

🐹 Go

Not in CISA KEV

MEDIUM severity

CVE-2019-8400 — hydra

MEDIUMFix: ory/hydra@9b5bbd4

CVE-2019-8400 is a medium-severity (CVSS 6.1) Cross-site Scripting (XSS) vulnerability in github.com/ory/hydra. 2 public exploit or proof-of-concept references are indexed, so weaponization risk is real. A fix is available for github.com/ory/hydra — see the affected versions and patch details below.

Hydra has Reflected XSS via error_hint parameter

Also known asGHSA-7v6r-w4r6-mhchGO-2026-4861

Published

Feb 17, 2019

Updated

Oct 20, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

2 known

Exploitation data as of Oct 27, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

1.3%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs71th percentile — riskier than 71% of all scored CVEsHighest risk

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

1 pkg affected

🐹github.com/ory/hydra

Real-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

ORY Hydra before v1.0.0-rc.3+oryOS.9 has Reflected XSS via the oauth2/fallbacks/error error_hint parameter.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐹Go	`github.com/ory/hydra`	all versions	1.4.8`go get github.com/ory/hydra@v1.4.8`

Affected Products

1 product · 129 configurations

Application

hydraory

90 versions

0.10.2.00.3.00.3.10.4.00.4.10.4.20.4.30.5.00.5.10.5.20.5.3

Exploits & PoCs

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.

ORY Hydra before v1.0.0-rc.3+oryOS.9 has Reflected XSS via the oauth2/fa…

youtube.comNVDFeb 2019

Report on hackerone.com

hackerone.comNVDFeb 2019

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/ory/hydra, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/ory/hydra to 1.4.8 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2019-8400 is resolved across your whole dependency graph.
Workarounds
Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Frequently Asked Questions

ORY Hydra before v1.0.0-rc.3+oryOS.9 has Reflected XSS via the oauth2/fallbacks/error error_hint parameter.

O3 Security · Impact-Aware SCA

Is CVE-2019-8400 in your dependencies?

Find it across Go, including transitive dependencies.

Scan my dependencies How O3 SCA works

CVSS Score

6.1/10CVSS v3 base

Vector breakdown

Attack Vector

Network

Attack Complexity

Low

Privileges Required

None

User Interaction

Required

Scope

Changed

Confidentiality

Low

Integrity

Low

Availability

None

Raw vector

CVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

CWE Classification

CWE-79Cross-site Scripting (XSS)

Weaponization

No Nuclei template or Metasploit module yet. 2 public proof-of-concept references are listed on this page.

OpenSSF Scorecard

Package health score

hydra

5.4/10

Maintained6/10

Code-Review0/10

Branch-Protection4/10

Assessed Oct 26, 2026

Timeline

Disclosed

Feb 17, 2019

First exploit

Feb 20, 2019

3 days after disclosure — rapid exploitation

Last updated

Oct 20, 2026

Identifiers

CVE-2019-8400

GHSA-7v6r-w4r6-mhch

GO-2026-4861

References

Advisorydrive.google.com · view Advisorygithub.com · CHANGELOG.md Advisorynvd.nist.gov · CVE 2019 8400 Fixgithub.com · 9b5bbd48a72096930af08402c5e07fce7dd770f3 Reporthackerone.com Packagegithub.com · hydra Evidenceyoutube.com · watch

Related vulnerabilities

Also affects hydra

This CVE is medium severity; 2 of 2 other CVEs affecting hydra are rated as high or higher (top: high).

CVE-2026-33504CVE-2020-5300

Same weakness: CWE-79

This CVE is medium severity; 6 of 6 other CVEs with CWE-79 are rated as high or higher (top: critical).

CVE-2026-44267CVE-2026-96632CVE-2026-9086CVE-2026-118246CVE-2026-118251CVE-2026-118253Browse all CWE-79

Provenance

Sources

OSV.devNVDFIRST.org (EPSS)

Last verifiedOct 20, 2026