Vulnerability-Lookup

CVE-2026-33544 (GCVE-0-2026-33544)

Vulnerability from cvelistv5 – Published: 2026-04-02 15:00 – Updated: 2026-04-03 18:23

Title

Tinyauth has OAuth account confusion via shared mutable state on singleton service instances

Summary

Tinyauth is an authentication and authorization server. Prior to version 5.0.5, all three OAuth service implementations (GenericOAuthService, GithubOAuthService, GoogleOAuthService) store PKCE verifiers and access tokens as mutable struct fields on singleton instances shared across all concurrent requests. When two users initiate OAuth login for the same provider concurrently, a race condition between VerifyCode() and Userinfo() causes one user to receive a session with the other user's identity. This issue has been patched in version 5.0.5.

Severity ?

7.7 (High)


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

CWE

CWE-362 - Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')

Assigner

GitHub_M

References

URL

Tags

	https://github.com/steveiliop56/tinyauth/security…	x_refsource_CONFIRM
	https://github.com/steveiliop56/tinyauth/commit/f…	x_refsource_MISC
	https://github.com/steveiliop56/tinyauth/releases…	x_refsource_MISC

Impacted products

	Vendor	Product	Version
	steveiliop56	tinyauth	Affected: < 5.0.5

Show details on NVD website

JSON

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FKIE_CVE-2026-33544

Vulnerability from fkie_nvd - Published: 2026-04-02 15:16 - Updated: 2026-04-07 12:44

Severity ?

7.7 (High) - CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:C/C:H/I:H/A:N

Summary

References

URL	Tags
security-advisories@github.com	https://github.com/steveiliop56/tinyauth/commit/f26c2171610d5c2dfbba2edb6ccd39490e349803	Patch
security-advisories@github.com	https://github.com/steveiliop56/tinyauth/releases/tag/v5.0.5	Product, Release Notes
security-advisories@github.com	https://github.com/steveiliop56/tinyauth/security/advisories/GHSA-9q5m-jfc4-wc92	Exploit, Mitigation, Vendor Advisory

Impacted products

	Vendor	Product	Version
	tinyauth	tinyauth	*

JSON

To clipboard

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      "nodes": [
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          "cpeMatch": [
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GHSA-9Q5M-JFC4-WC92

Vulnerability from github – Published: 2026-04-01 19:52 – Updated: 2026-04-06 17:18

Summary

Tinyauth has OAuth account confusion via shared mutable state on singleton service instances

Details

Summary

All three OAuth service implementations (GenericOAuthService, GithubOAuthService, GoogleOAuthService) store PKCE verifiers and access tokens as mutable struct fields on singleton instances shared across all concurrent requests. When two users initiate OAuth login for the same provider concurrently, a race condition between VerifyCode() and Userinfo() causes one user to receive a session with the other user's identity.

Details

The OAuthBrokerService.GetService() returns a single shared instance per provider for every request. The OAuth flow stores intermediate state as struct fields on this singleton:

Token storage — generic_oauth_service.go line 96:

generic.token = token  // Shared mutable field on singleton

Verifier storage — generic_oauth_service.go line 81:

generic.verifier = verifier  // Shared mutable field on singleton

In the callback handler oauth_controller.go lines 136–143, the code calls:

err = service.VerifyCode(code)                       // line 136 — stores token on singleton
// ... race window ...
user, err := controller.broker.GetUser(req.Provider)  // line 143 — reads token from singleton

Between these two calls, a concurrent request's VerifyCode() can overwrite the token field, causing GetUser() → Userinfo() to fetch the wrong user's identity claims.

The same pattern exists in all three implementations: - github_oauth_service.go lines 34–39, 77, 86–99 - google_oauth_service.go lines 22–27, 65, 73–87

PoC

Race scenario (two concurrent OAuth callbacks):

User A and User B both click "Login with GitHub" on the same tinyauth instance
Both are redirected to GitHub, authorize, and GitHub redirects both back with authorization codes
Both callbacks arrive at tinyauth nearly simultaneously:

Timeline:
  t0: Request A → service.VerifyCode(codeA) → singleton.token = tokenA
  t1: Request B → service.VerifyCode(codeB) → singleton.token = tokenB  (overwrites tokenA)
  t2: Request A → broker.GetUser("github")  → Userinfo() reads singleton.token = tokenB
  t3: Request A receives User B's identity (email, name, groups)

User A now has a tinyauth session with User B's email, gaining access to all resources User B is authorized for via tinyauth's ACL.

PKCE verifier DoS variant: Even with PKCE, concurrent oauthURLHandler calls overwrite the verifier field, causing VerifyCode() to send the wrong verifier to the OAuth provider, which rejects the exchange.

Static verification: Run Go's race detector on a test that calls VerifyCode and Userinfo concurrently on the same service instance — the -race flag will flag data races on the token and verifier fields.

Go race detector confirmation: Running a concurrent test with go test -race on the singleton service detects 4 data races on the token and verifier fields. Without the race detector, measured token overwrite rate is 99.9% (9,985/10,000 iterations).

Test environment: tinyauth v5.0.4, commit 592b7ded, Go race detector + source code analysis

Impact

An attacker who times their OAuth callback to race with a victim's callback can obtain a tinyauth session with the victim's identity. This grants unauthorized access to all resources the victim is permitted to access through tinyauth's ACL system. The probability of collision increases with concurrent OAuth traffic.

The PKCE verifier overwrite additionally causes a denial-of-service: concurrent OAuth logins for the same provider reliably fail.

Suggested Fix

Pass verifier and token through method parameters or return values instead of storing them on the singleton:

func (generic *GenericOAuthService) VerifyCode(code string, verifier string) (*oauth2.Token, error) {
    return generic.config.Exchange(generic.context, code, oauth2.VerifierOption(verifier))
}

func (generic *GenericOAuthService) Userinfo(token *oauth2.Token) (config.Claims, error) {
    client := generic.config.Client(generic.context, token)
    // ...
}

Store the PKCE verifier in the session/cookie associated with the OAuth state parameter, not on the service struct.

Severity ?

7.7 (High)


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

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "package": {
        "ecosystem": "Go",
        "name": "github.com/steveiliop56/tinyauth"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "1.0.1-0.20260401140714-fc1d4f2082a5"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-33544"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-362"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-04-01T19:52:04Z",
    "nvd_published_at": "2026-04-02T15:16:39Z",
    "severity": "HIGH"
  },
  "details": "### Summary\n\nAll three OAuth service implementations (`GenericOAuthService`, `GithubOAuthService`, `GoogleOAuthService`) store PKCE verifiers and access tokens as mutable struct fields on singleton instances shared across all concurrent requests. When two users initiate OAuth login for the same provider concurrently, a race condition between `VerifyCode()` and `Userinfo()` causes one user to receive a session with the other user\u0027s identity.\n\n### Details\n\nThe [`OAuthBrokerService.GetService()`](https://github.com/steveiliop56/tinyauth/blob/592b7ded24959013f8af63ab9930254c752c8c8e/internal/service/oauth_broker_service.go#L70-L72) returns a single shared instance per provider for every request. The OAuth flow stores intermediate state as struct fields on this singleton:\n\n**Token storage** \u2014 [`generic_oauth_service.go` line 96](https://github.com/steveiliop56/tinyauth/blob/592b7ded24959013f8af63ab9930254c752c8c8e/internal/service/generic_oauth_service.go#L96):\n```go\ngeneric.token = token  // Shared mutable field on singleton\n```\n\n**Verifier storage** \u2014 [`generic_oauth_service.go` line 81](https://github.com/steveiliop56/tinyauth/blob/592b7ded24959013f8af63ab9930254c752c8c8e/internal/service/generic_oauth_service.go#L81):\n```go\ngeneric.verifier = verifier  // Shared mutable field on singleton\n```\n\nIn the callback handler [`oauth_controller.go` lines 136\u2013143](https://github.com/steveiliop56/tinyauth/blob/592b7ded24959013f8af63ab9930254c752c8c8e/internal/controller/oauth_controller.go#L136-L143), the code calls:\n```go\nerr = service.VerifyCode(code)                       // line 136 \u2014 stores token on singleton\n// ... race window ...\nuser, err := controller.broker.GetUser(req.Provider)  // line 143 \u2014 reads token from singleton\n```\n\nBetween these two calls, a concurrent request\u0027s `VerifyCode()` can overwrite the `token` field, causing `GetUser()` \u2192 `Userinfo()` to fetch the **wrong user\u0027s** identity claims.\n\nThe same pattern exists in all three implementations:\n- [`github_oauth_service.go` lines 34\u201339, 77, 86\u201399](https://github.com/steveiliop56/tinyauth/blob/592b7ded24959013f8af63ab9930254c752c8c8e/internal/service/github_oauth_service.go#L34-L39)\n- [`google_oauth_service.go` lines 22\u201327, 65, 73\u201387](https://github.com/steveiliop56/tinyauth/blob/592b7ded24959013f8af63ab9930254c752c8c8e/internal/service/google_oauth_service.go#L22-L27)\n\n### PoC\n\n**Race scenario** (two concurrent OAuth callbacks):\n\n1. User A and User B both click \"Login with GitHub\" on the same tinyauth instance\n2. Both are redirected to GitHub, authorize, and GitHub redirects both back with authorization codes\n3. Both callbacks arrive at tinyauth nearly simultaneously:\n\n```\nTimeline:\n  t0: Request A \u2192 service.VerifyCode(codeA) \u2192 singleton.token = tokenA\n  t1: Request B \u2192 service.VerifyCode(codeB) \u2192 singleton.token = tokenB  (overwrites tokenA)\n  t2: Request A \u2192 broker.GetUser(\"github\")  \u2192 Userinfo() reads singleton.token = tokenB\n  t3: Request A receives User B\u0027s identity (email, name, groups)\n```\n\nUser A now has a tinyauth session with User B\u0027s email, gaining access to all resources User B is authorized for via tinyauth\u0027s ACL.\n\n**PKCE verifier DoS variant**: Even with PKCE, concurrent `oauthURLHandler` calls overwrite the `verifier` field, causing `VerifyCode()` to send the wrong verifier to the OAuth provider, which rejects the exchange.\n\n**Static verification**: Run Go\u0027s race detector on a test that calls `VerifyCode` and `Userinfo` concurrently on the same service instance \u2014 the `-race` flag will flag data races on the `token` and `verifier` fields.\n\n**Go race detector confirmation**: Running a concurrent test with `go test -race` on the singleton service detects **4 data races** on the `token` and `verifier` fields. Without the race detector, measured token overwrite rate is 99.9% (9,985/10,000 iterations).\n\n**Test environment**: tinyauth v5.0.4, commit `592b7ded`, Go race detector + source code analysis\n\n### Impact\n\nAn attacker who times their OAuth callback to race with a victim\u0027s callback can obtain a tinyauth session with the victim\u0027s identity. This grants unauthorized access to all resources the victim is permitted to access through tinyauth\u0027s ACL system. The probability of collision increases with concurrent OAuth traffic.\n\nThe PKCE verifier overwrite additionally causes a denial-of-service: concurrent OAuth logins for the same provider reliably fail.\n\n### Suggested Fix\n\nPass verifier and token through method parameters or return values instead of storing them on the singleton:\n\n```go\nfunc (generic *GenericOAuthService) VerifyCode(code string, verifier string) (*oauth2.Token, error) {\n    return generic.config.Exchange(generic.context, code, oauth2.VerifierOption(verifier))\n}\n\nfunc (generic *GenericOAuthService) Userinfo(token *oauth2.Token) (config.Claims, error) {\n    client := generic.config.Client(generic.context, token)\n    // ...\n}\n```\n\nStore the PKCE verifier in the session/cookie associated with the OAuth `state` parameter, not on the service struct.",
  "id": "GHSA-9q5m-jfc4-wc92",
  "modified": "2026-04-06T17:18:24Z",
  "published": "2026-04-01T19:52:04Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/steveiliop56/tinyauth/security/advisories/GHSA-9q5m-jfc4-wc92"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-33544"
    },
    {
      "type": "WEB",
      "url": "https://github.com/steveiliop56/tinyauth/commit/f26c2171610d5c2dfbba2edb6ccd39490e349803"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/steveiliop56/tinyauth"
    },
    {
      "type": "WEB",
      "url": "https://github.com/steveiliop56/tinyauth/releases/tag/v5.0.5"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:C/C:H/I:H/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Tinyauth has OAuth account confusion via shared mutable state on singleton service instances"
}

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-2026-33544 (GCVE-0-2026-33544)

FKIE_CVE-2026-33544

GHSA-9Q5M-JFC4-WC92

Summary

Details

PoC

Impact

Suggested Fix

Tags

Sightings

Nomenclature