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CVE-2026-27959

Koajs Koa vulnerability

7.5 HIGHPublic exploitEPSS 0.52%
  1. Published26 Feb 2026

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.

CVSS 3.1 breakdown

Attack vectorNetwork
Attack complexityLow
Privileges requiredNone
User interactionNone
ScopeUnchanged
ConfidentialityNone
IntegrityHigh
AvailabilityNone
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

Affected products

Koajs Koa≥ 3.0.0 < 3.1.2 · < 2.16.4

Remediation

Fixed in
Koajs Koa≥ 3.1.2

References

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Frequently asked

Is CVE-2026-27959 being actively exploited?

CVE-2026-27959 is not in the CISA KEV catalog. A public exploit reference exists, so treat it as exploitable. Its EPSS exploit probability is 0.52%.

How severe is CVE-2026-27959?

CVE-2026-27959 has a CVSS 3.1 base score of 7.5, rated high. EPSS estimates a 0.52% probability of exploitation.

Which products are affected by CVE-2026-27959?

Affected products include Koajs Koa.

How do I fix CVE-2026-27959?

Upgrade affected software to a fixed version, for example Koajs Koa 3.1.2 or later, then confirm no affected versions remain in your inventory.

How Secuno handles this

Secuno correlates vulnerabilities like CVE-2026-27959 against the real software inventory on every managed device, every day. Rather than a static severity, each match is scored in context: exploit availability, EPSS, the asset's exposure, and business criticality decide where it lands in the queue. If an affected version is present anywhere across your portfolio, it surfaces automatically with a prioritised remediation path.

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Data from the NVD, CISA KEV catalog and FIRST EPSS. CVSS 3.1 base score shown where available.