GHSA-2gx3-rcp4-g85q CVE-2026-101917 PYSEC-2026-4140
PyJWT: PyJWKClient still amplifies unauthenticated JWKS fetches on unknown kid values (incomplete fix of CVE-2026-48524)
- Severity
- 5.3 / 10
- High · CVSS 3.1
- Exploit chance (EPSS)
- 0.43%
- Chance of exploitation in the next 30 days. Higher than 37.05% of scored vulnerabilities.
- Known exploited
- No
- Not in CISA's catalog
- Weakness
- CWE-770 Allocation of Resources Without Limits or Throttling
- Fix
2.14.0- Upgrade pyjwt to this version or later
Description
Summary CVE-2026-48524 (GHSA-fhv5-28vv-h8m8, "PyJWKClient unbounded JWKS endpoint requests via attacker-controlled kid values (DoS)") was fixed in 2.13.0 by stopping fetch_data() from clearing the cache on a fetch error. That closed one amplification path but did not add the mitigation the advisory's title implies: there is still no rate-limit, negative-cache, or minimum-refresh-interval for unknown kids.
At HEAD, get_signing_key(kid) (jwt/jwks_client.py:185-211), on any unknown kid, calls get_signing_keys(refresh=True), and refresh=True bypasses jwk_set_cache unconditionally and forces a fresh fetch_data(). The kid is read from the unverified token header (get_signing_key_from_jwt decodes with verify_signature=False), so no valid token and no authentication is required. lru_cache does not cache the raised exception, so even the same unknown kid repeated re-fetches on every call.
Affected pyjwt <= 2.13.0 (the latest release; the patched release for CVE-2026-48524). No fixed version yet.
Proof of concept (verified on 2.13.0, cache enabled = realistic prod config) import threading, http.server, socketserver, json from jwt import PyJWKClient hits = {'n': 0} JWKS = json.dumps({"keys":[{"kty":"oct","kid":"real","k":"AAAA"}]}).encode() class H(http.server.BaseHTTPRequestHandler): def do_GET(self): hits['n'] += 1 self.send_response(200); self.send_header('Content-Type','application/json'); self.end_headers() self.wfile.write(JWKS) def log_message(self,*a): pass srv = socketserver.TCPServer(('127.0.0.1',0), H); port = srv.server_address[1] threading.Thread(target=srv.serve_forever, daemon=True).start() c = PyJWKClient(f'http://127.0.0.1/:{port}/jwks.json', cache_keys=True, lifespan=3600) for i in range(8): try: c.get_signing_key(f'attacker-unknown-kid-{i}') except Exception: pass before = hits['n'] for _ in range(5): try: c.get_signing_key('same-unknown') except Exception: pass print('distinct unknown kids: 8 -> fetches:', hits['n']) print('same unknown kid x5 -> extra fetches:', hits['n'] - before)
Output: distinct unknown kids: 8 -> fetches: 9 same unknown kid x5 -> extra fetches: 5 Each unknown kid forces a fresh JWKS fetch; a repeated identical unknown kid still re-fetches every time against an unexpired cache. No rate-limit or negative-cache.
Impact One unauthenticated request -> one outbound JWKS HTTP fetch + full JSON parse on the victim server. An attacker floods tokens carrying junk kids, so the victim hammers its own JWKS/IdP endpoint (amplification: attacker -> victim -> IdP), exhausting victim CPU/sockets and potentially tripping the JWKS provider's rate-limit, causing an application-wide auth outage. This is the unauthenticated DoS the parent advisory is named for, still reachable after the 2.13.0 fix.
Suggested fix Guard the forced refresh on unknown kids: negative-cache unknown kids for a short TTL, or enforce a minimum interval between forced JWKS refreshes, so a repeated or unknown kid cannot force unbounded fetches.
Note: the same-kid-repeated result (5 identical unknown kids producing 5 fetches against an unexpired cache) shows this is request amplification, not legitimate key-rotation handling, since that refresh can never succeed.
Reported by Babakizo (Securva).
Maintainer update — 2026-09-10
The maintainer confirmed the reported behavior against PyJWT 2.13.0. With JWKS caching
enabled, an unknown kid previously forced an unconditional JWKS refresh,
including when the same unknown value was repeated while the cached key set was
still valid. This allowed unauthenticated token headers to cause unnecessary
outbound JWKS requests and repeated parsing work.
The fix is now on master in commit ba4853a. PyJWKClient now applies a
30-second cooldown after successful JWKS fetches before permitting another
unknown-kid refresh, serializes concurrent refresh decisions per client, and
allows callers to configure or disable the cooldown. Cache-disabled behavior
and immediate retry after failed fetches remain unchanged.
Regression tests cover repeated unknown kids, cooldown expiry, concurrent misses, cache-disabled operation, and invalid cooldown values. The available full tox matrix, Ruff, and mypy checks pass. The fix will be included in the next released 2.x version.
Maintainer update — 2026-09-11
The verified fix for this advisory is included in PyJWT 2.14.0, released on 2026-09-11 and available on PyPI. PyJWT 2.14.0 is the first release containing the fix. This advisory is now published with 2.14.0 recorded as the patched version.
Affected versions
PyPI
| Package | Vulnerable | Fixed in |
|---|---|---|
| pyjwt | < 2.14.0 |
2.14.0 |
How to fix
Move to the fixed version or later with your package manager, update the lockfile, and run your tests. If the package only arrives through another dependency, upgrade that dependency first. If none of its releases pulls a fixed version yet, force the version with your package manager's override.
pyjwt PyPI → 2.14.0
- pip
pip install --upgrade "pyjwt>=2.14.0"- poetry
poetry add "pyjwt>=2.14.0"- uv
uv add "pyjwt>=2.14.0"
Severity breakdown
CVSS 3.1 5.3
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
- Attack vector
- Network
- Attack complexity
- Low
- Privileges required
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- None
- Availability
- Low
Weakness
CWE-770: Allocation of Resources Without Limits or Throttling
The product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated.
How to prevent it
- Requirements: Clearly specify the minimum and maximum expectations for capabilities, and dictate which behaviors are acceptable when resource allocation reaches limits.
- Architecture and Design: Limit the amount of resources that are accessible to unprivileged users. Set per-user limits for resources. Allow the system administrator to define these limits. Be careful to avoid CWE-410.
- Architecture and Design: Design throttling mechanisms into the system architecture. The best protection is to limit the amount of resources that an unauthorized user can cause to be expended. A strong authentication and access control model will help prevent such attacks from occurring in the first place, and it will help the administrator to identify who is committing the abuse. The login application should be protected against DoS attacks as much as possible. Limiting the database access, perhaps by caching result sets, can help minimize the resources expended. To further limit the potential for a DoS attack, consider tracking the rate of requests received from users and blocking requests that exceed a defined rate threshold.
- Implementation: Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue." Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
- Architecture and Design: For any security checks that are performed on the client side, ensure that these checks are duplicated on the server side, in order to avoid CWE-602. Attackers can bypass the client-side checks by modifying values after the checks have been performed, or by changing the client to remove the client-side checks entirely. Then, these modified values would be submitted to the server.
- Architecture and Design: Mitigation of resource exhaustion attacks requires that the target system either: recognizes the attack and denies that user further access for a given amount of time, typically by using increasing time delays uniformly throttles all requests in order to make it more difficult to consume resources more quickly than they can again be freed. The first of these solutions is an issue in itself though, since it may allow attackers to prevent the use of the system by a particular valid user. If the attacker impersonates the valid user, they may be able to prevent the user from accessing the server in question. The second solution can be difficult to effectively institute -- and even when properly done, it does not provide a full solution. It simply requires more resources on the part of the attacker.
- Architecture and Design: Ensure that protocols have specific limits of scale placed on them.
- Architecture and Design, Implementation: If the program must fail, ensure that it fails gracefully (fails closed). There may be a temptation to simply let the program fail poorly in cases such as low memory conditions, but an attacker may be able to assert control before the software has fully exited. Alternately, an uncontrolled failure could cause cascading problems with other downstream components; for example, the program could send a signal to a downstream process so the process immediately knows that a problem has occurred and has a better chance of recovery. Ensure that all failures in resource allocation place the system into a safe posture.
- Operation, Architecture and Design: Use quotas or other resource-limiting settings provided by the operating system or environment. For example, when managing system resources in POSIX, setrlimit() can be used to set limits for certain types of resources, and getrlimit() can determine how many resources are available. However, these functions are not available on all operating systems. When the current levels get close to the maximum that is defined for the application (see CWE-770), then limit the allocation of further resources to privileged users; alternately, begin releasing resources for less-privileged users. While this mitigation may protect the system from attack, it will not necessarily stop attackers from adversely impacting other users. Ensure that the application performs the appropriate error checks and error handling in case resources become unavailable (CWE-703).
References
Advisories
Packages
Other links
Timeline
- Published
- Last updated by the source
- Latest EPSS score
Related advisories
| Advisory | Severity | Why it's related |
|---|---|---|
| GHSA-pxh4-856f-4h89 PyJWT: HS256/384/512 verify accepts empty HMAC key via oct JWK (PyJWK/PyJWKClient) enabling token forgery | High | Also affects pyjwt |
| GHSA-gvp8-978c-rx2q PyJWT.decode() reintroduces options-dict mutation, enabling silent claim-verification bypass on dict reuse | Medium | Also affects pyjwt |
| GHSA-x33g-cr3x-6449 PyJWT accepts inconsistent OKP x/d JWKs, causing public/private key identity confusion | High | Also affects pyjwt |
| GHSA-w6j9-cwv2-h6wq PyJWT: Malformed RSA JWK aborts parsing of an entire JWK Set | High | Also affects pyjwt |
| GHSA-w2cx-738m-mc7w PyJWT accepts public JWK containers as HMAC secrets | High | Also affects pyjwt |
| GHSA-r6x4-923q-g947 PyJWT BOM Bypass | High | Also affects pyjwt |
| GHSA-p4g4-x82p-q773 PyJWT: Public keys in DER form are accepted as HMAC secrets, bypassing the CVE-2022-29217 guard | High | Also affects pyjwt |
| GHSA-jwrc-g2q2-pq5p PyJWT: ReDoS vulnerability when calling the `is_pem_format` function. | Medium | Also affects pyjwt |
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