ghsa-4pc9-x2fx-p7vj
Vulnerability from github
Summary
PKCE was implemented in the OAuth implementation in workers-oauth-provider that is part of MCP framework. However, it was found that an attacker could cause the check to be skipped.
Impact
Under certain circumstances (see below), if a victim had previously authorized with a server built on workers-oath-provider, and an attacker could later trick the victim into visiting a malicious web site, then attacker could potentially steal the victim's credentials to the same OAuth server and subsequently impersonate them.
In order for the attack to be possible, the OAuth server's authorized callback must be designed to auto-approve authorizations that appear to come from an OAuth client that the victim has authorized previously. The authorization flow is not implemented by workers-oauth-provider; it is up to the application built on top to decide whether to implement such automatic re-authorization. However, many applications do implement such logic.
Patches
Fixed in: https://github.com/cloudflare/workers-oauth-provider/pull/26
We patched up the vulnerabilities in the latest version, v 0.0.5 of the Workers OAuth provider (https://www.npmjs.com/package/@cloudflare/workers-oauth-provider). You'll need to update your MCP servers to use that version to resolve the vulnerability.
Workarounds
None
Note
It is a basic, well-known requirement that OAuth servers should verify that the redirect URI is among the allowed list for the client, both during the authorization flow and subsequently when exchanging the authorization code for an access token. workers-oauth-provider implemented only the latter check, not the former. Unfortunately, the former is the much more important check.
Readers who are familiar with OAuth may recognize that failing to check redirect URIs against the allowed list is a well-known, basic mistake, covered extensively in the RFC and elsewhere. The author of this library would like everyone to know that he was, in fact, well-aware of this requirement, thought about it a lot while designing the library, and then, somehow, forgot to actually make sure the check was in the code. That is, it's not that he didn't know what he was doing, it's that he knew what he was doing but flubbed it.
{
"affected": [
{
"package": {
"ecosystem": "npm",
"name": "@cloudflare/workers-oauth-provider"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "0.0.5"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2025-4143"
],
"database_specific": {
"cwe_ids": [
"CWE-601"
],
"github_reviewed": true,
"github_reviewed_at": "2025-05-01T17:00:29Z",
"nvd_published_at": null,
"severity": "MODERATE"
},
"details": "### Summary\nPKCE was implemented in the OAuth implementation in workers-oauth-provider that is part of[ MCP framework](https://github.com/cloudflare/workers-mcp). However, it was found that an attacker could cause the check to be skipped.\n\n### Impact\nUnder certain circumstances (see below), if a victim had previously authorized with a server built on workers-oath-provider, and an attacker could later trick the victim into visiting a malicious web site, then attacker could potentially steal the victim\u0027s credentials to the same OAuth server and subsequently impersonate them.\n\nIn order for the attack to be possible, the OAuth server\u0027s authorized callback must be designed to auto-approve authorizations that appear to come from an OAuth client that the victim has authorized previously. The authorization flow is not implemented by workers-oauth-provider; it is up to the application built on top to decide whether to implement such automatic re-authorization. However, many applications do implement such logic.\n\n\n### Patches\nFixed in: https://github.com/cloudflare/workers-oauth-provider/pull/26\n\nWe patched up the vulnerabilities in the latest version, v 0.0.5 of the Workers OAuth provider (https://www.npmjs.com/package/@cloudflare/workers-oauth-provider). You\u0027ll need to update your MCP servers to use that version to resolve the vulnerability.\n\n\n### Workarounds\nNone\n\n### Note\n\nIt is a basic, well-known requirement that OAuth servers should verify that the redirect URI is among the allowed list for the client, both during the authorization flow and subsequently when exchanging the authorization code for an access token. workers-oauth-provider implemented only the latter check, not the former. Unfortunately, the former is the much more important check.\n\nReaders who are familiar with OAuth may recognize that failing to check redirect URIs against the allowed list is a well-known, basic mistake, covered extensively in the RFC and elsewhere. The author of this library would like everyone to know that he was, in fact, well-aware of this requirement, thought about it a lot while designing the library, and then, somehow, forgot to actually make sure the check was in the code. That is, it\u0027s not that he didn\u0027t know what he was doing, it\u0027s that he knew what he was doing but flubbed it.",
"id": "GHSA-4pc9-x2fx-p7vj",
"modified": "2025-05-01T17:00:29Z",
"published": "2025-05-01T17:00:29Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/cloudflare/workers-oauth-provider/security/advisories/GHSA-4pc9-x2fx-p7vj"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2025-4143"
},
{
"type": "WEB",
"url": "https://github.com/cloudflare/workers-oauth-provider/pull/26"
},
{
"type": "PACKAGE",
"url": "https://github.com/cloudflare/workers-oauth-provider"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:P/VC:H/VI:N/VA:N/SC:L/SI:N/SA:N",
"type": "CVSS_V4"
}
],
"summary": "@cloudflare/workers-oauth-provider missing validation of redirect_uri on authorize endpoint"
}
- Seen: The vulnerability was mentioned, discussed, or seen somewhere by the user.
- Confirmed: The vulnerability is confirmed from an analyst perspective.
- Exploited: This vulnerability was exploited and seen by the user reporting the sighting.
- Patched: This vulnerability was successfully patched by the user reporting the sighting.
- Not exploited: This vulnerability was not exploited or seen by the user reporting the sighting.
- Not confirmed: The user expresses doubt about the veracity of the vulnerability.
- Not patched: This vulnerability was not successfully patched by the user reporting the sighting.