How severe is CVE-2026-27959?

CVE-2026-27959 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity high, and availability none.

Is CVE-2026-27959 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (25th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-27959?

CVE-2026-27959 affects Koajs Koa. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-27959?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.

Does CVE-2026-27959 have an EU (EUVD) identifier?

Yes. CVE-2026-27959 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-8816.

When was CVE-2026-27959 published?

CVE-2026-27959 was published on 2026-02-26 and last updated on 2026-06-30.

CVE-2026-27959

CVE-2026-27959 is a high-severity vulnerability in Koajs Koa with a CVSS 3.x base score of 7.5. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-20.

Key facts

Severity: High (CVSS 3.x base score 7.5)
EPSS exploit prediction: 0% (25th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-8816
Weakness: CWE-20
Affected product: Koajs Koa
Published: 2026-02-26
Last modified: 2026-06-30

Description

Koa is middleware for Node.js using ES2017 async functions. Prior to versions 3.1.2 and 2.16.4, Koa's `ctx.hostname` API performs naive parsing of the HTTP Host header, extracting everything before the first colon without validating the input conforms to RFC 3986 hostname syntax. When a malformed Host header containing a `@` symbol is received, `ctx.hostname` returns `evil[.]com` - an attacker-controlled value. Applications using `ctx.hostname` for URL generation, password reset links, email verification URLs, or routing decisions are vulnerable to Host header injection attacks. Versions 3.1.2 and 2.16.4 fix the issue.

Frequently asked questions

What is CVE-2026-27959?: Koa is middleware for Node.js using ES2017 async functions. Prior to versions 3.1.2 and 2.16.4, Koa's `ctx.hostname` API performs naive parsing of the HTTP Host header, extracting everything before the first colon without validating the input conforms to RFC 3986 hostname syntax. When a malformed Host header containing a `@` symbol is received, `ctx.hostname` returns `evil[.]com` - an attacker-controlled value. Applications using `ctx.hostname` for URL generation, password reset links, email verification URLs, or routing decisions are vulnerable to Host header injection attacks. Versions 3.1.2 and 2.16.4 fix the issue.
How severe is CVE-2026-27959?: CVE-2026-27959 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity high, and availability none.
Is CVE-2026-27959 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (25th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-27959?: CVE-2026-27959 affects Koajs Koa. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-27959?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
Does CVE-2026-27959 have an EU (EUVD) identifier?: Yes. CVE-2026-27959 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-8816.
When was CVE-2026-27959 published?: CVE-2026-27959 was published on 2026-02-26 and last updated on 2026-06-30.

References

Affected products (1)

cpe:2.3:a:koajs:koa:*:*:*:*:*:node.js:*:*

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