CVE-2026-27959
Koajs Koa ≤ 2.16.4
Raw vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:NCVSS and EPSS are reproduced from their sources (NVD, FIRST EPSS). Risk Priority is our own derived reading, not an NVD score.
Summary
CVE-2026-27959 is a high-severity Improper Input Validation (CWE-20) vulnerability in Koajs Koa. Its CVSS base score is 7.5 (High).
Operationally, ranked at the 44th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog; a public proof-of-concept is referenced.
The strongest mitigations our analysis identified map to SA-11 (Developer Testing and Evaluation) and SI-10 (Information Input Validation) — see the control section below for these in your framework.
Deeper analysis AI-assisted summary
Synthesised by an AI model from the NVD description and linked references — a reading aid, not an authoritative source.
CVE-2026-27959 is a Host header injection vulnerability in Koa, a middleware framework for Node.js that uses ES2017 async functions. The issue affects Koa versions prior to 3.1.2 and 2.16.4, specifically in the `ctx.hostname` API, which naively parses the HTTP Host header by extracting everything before the first colon without validating conformance to RFC 3986 hostname syntax. A malformed Host header containing an `@` symbol causes `ctx.hostname` to return an attacker-controlled value, such as `evil[.]com`. The vulnerability is rated 7.5 on the CVSS v3.1 scale (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N) and is associated with CWE-20 (Improper Input Validation).
Remote attackers without authentication can exploit this vulnerability by sending a specially crafted HTTP request with a malformed Host header. Exploitation succeeds if the affected application relies on `ctx.hostname` for generating URLs, password reset links, email verification URLs, or routing decisions, enabling Host header injection attacks. This allows attackers to manipulate outbound links or redirects to point to malicious domains, potentially leading to phishing, open redirects, or unauthorized actions, with high integrity impact but no confidentiality or availability disruption.
The Koa security advisory (GHSA-7gcc-r8m5-44qm) and related GitHub commits detail the fix, recommending an upgrade to Koa versions 3.1.2 or 2.16.4, which implement proper validation of the Host header per RFC 3986 syntax. No additional mitigations are specified in the provided references.
OWASP Top 10 for Web (2025)
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-8816
Vulnerability Data
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…
more
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.
- CWE(s)
Related Threats
CVEs Like This One
Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
Developer testing and evaluation can discover missing input validation through analysis or test cases.
SI-10 directly requires validity checks on information inputs, structurally preventing improper or missing validation.
Requiring documented development standards and tools can embed input-validation practices into the engineering process.
Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.
Testing against a defined set of requirements and using code review plus vulnerability scanning forces validation of inputs and handling of unanticipated conditions, reducing the chance that malformed data will be accepted.
Requiring language-specific secure coding standards, peer review, SAST and documented mitigation of common programming errors forces validation of all inputs before they are trusted.
Hardening callouts derived
DISA STIG baselines are authored against specific weaknesses. Where this CVE cites a broad weakness class, the mapping is not asserted at that abstraction (owner ruling, 2026-09-25: grade only at Base and below), so no rule is shown here rather than a rule shown against the wrong grain.
- No fine-grained STIG row for CWE-20, a class-level weakness, and none of its children carries one either.