CVE-2026-102265

PyJWT is a Python implementation of JSON Web Token standards. From 2.13.0 until 2.14.0, PyJWS._load in jwt/api_jws.py is affected because parser catches ValueError but not RecursionError. This occurs when a deeply nested token header reaches json.loads. As a result, RecursionError escapes the documented PyJWT error hierarchy. Consequently, an unauthenticated malformed token can cause a request-level failure and HTTP 500. This issue is fixed in version 2.14.0.

Package

pyjwt (PyPI)

Affected versions

tested & verified on: 2.13.0. Every version whose PyJWS._load() translates only ValueError is affected

Description

PyJWS._looad() (jwt/api_jws.py:337) splits the compact token, base64url decodes the header segment and hands it to json.loads() before _verify_signature() runs. The parse is wrapped in except ValueError as e: raise DecodeError(...). That is what makes the documented contract sufficient for untrusted input: except jwt.PyJWTError (or InvalidTokenError) around jwt.decode(token, key, algorithms=[...]).

CPython's JSON decoder raises RecursionError (a RuntimeError subclass, not a ValueError) once nesting crosses the native stack guard. That exception leaves _load(), then jwt.decode(), and matches no PyJWTError handlerr at all. No key and no valid signature are needed: the header is parsed first, so a single unauthenticated request with an unsigned token suffices.

Scope: the recursion threshold is ~65,000 nesting levels; below it the same input yields a clean DecodeError (a 346,800-byte flat header is rejected normally, so depth, not size, is the trigger). A token this large cannot ride in an Authorization header (rejected at HTTP field limits,431 under gunicorn), but body transport (RFC 7662 introspection style) is a standard pattern. Under Flask 3.1.3 / gunicorn 26.2.0 each crafted request returns HTTP 500 instead of 401 and writes a full traceback to the error log; the sync worker survives.

Proof of concept

Against any endpoint that validates a token from its JSON body with the documented pattern:

import base64, json, http.client

def b64url(b): return base64.urlsafe_b64encode(b).rstrip(b"=")

# unsigned token: the header segment is 200,000 nested '[' (266,681 bytes total)
token = (b64url(b"[" * 200_000) + b"." + b64url(b'{"a":1}') + b"." + b64url(b"x")).decode()

conn = http.client.HTTPConnection("127.0.0.1", 8125, timeout=30)
conn.request("POST", "/api/protected", body=json.dumps({"token": token}),
             headers={"Content-Type": "application/json"})
print(conn.getresponse().status)   # 500, expected 401

Determinism: 20/20 crafted requests returned 500 against the reference container (PyJWT 2.13.0, Flask 3.1.3, gunicorn 26.2.0, Python 3.14.7) in 0.12 s (about 6 ms per request in a tight loop, 14 ms for a cold request), each leaving a RecursionError: Stack overflow ... while decoding a JSON array traceback in the error log.

Impact

An unauthenticated attacker can turn every request to a JWT validating endpoint into a server error with a full traceback logged per request, a cheap and repeatable denial of service on the authentication path (no crash, no data exposure).

Fix

Catch RecursionError alongside ValueError in _load() and raise DecodeError, or parse the header with a nonrecursive, depthbounded parser.

Reproduction

A standalone package (docker-compose.yml, Dockerfile, requirements.txt, target app, driver) accompanies this report:

cd <package dir>
docker compose up -d --build
python3 poc_check_then_attack.py
docker compose down -v

poc_pyjwt_recursion.zip

Exit 0= reproduced (control checks green, exactly one crafted request returns 500, traceback verified in docker logs, service alive afterwards); 1 = not reproduced; 2 = checks failed, no attack sent. The package's run_log.txt documents a complete proof run.

Credits

• @Nivid42

Maintainer update — 2026-09-09

We reproduced the reported behavior on PyJWT 2.13.0: a deeply nested JSON array in the untrusted compact-JWS header reaches json.loads() before signature verification and raises RecursionError, which is not a PyJWTError. The same input is accepted by the public jwt.decode() path and can escape an application's normal except PyJWTError handling.

The fix is committed as…

setup.py test %endif

Security Fix(es):

PyJWT is a Python implementation of JSON Web Token standards. Prior to 2.14.0, PyJWT get_signing_key_from_jwt is affected because unknown kid misses force refreshes without a negative cache or minimum refresh interval. This occurs when unauthenticated tokens repeatedly use the same unknown kid or varying kid values absent from the cached JWKS. As a result, each cache miss causes PyJWKClient to refresh the JWKS. Consequently, attacker traffic can amplify outbound requests to the configured JWKS endpoint. This issue is fixed in version 2.14.0.(CVE-2026-101917)

PyJWT is a Python implementation of JSON Web Token standards. From 2.0.0a1 until 2.15.0, PyJWT PyJWKClient.get_signing_key_from_jwt is affected because payload parser catches ValueError but not RecursionError. This occurs when an attacker-controlled recursively nested payload reaches json.loads. As a result, documented PyJWT exception handling does not contain the failure. Consequently, an unauthenticated request can raise an exception that may produce an HTTP 500 response. The advisory-defined affected implementation also includes jwt/api_jwt.py, verify_signature=False. This issue is fixed in version 2.15.0.(CVE-2026-101918)

PyJWT is a Python implementation of JSON Web Token standards. From 2.13.0 until 2.14.0, PyJWS._load in jwt/api_jws.py is affected because parser catches ValueError but not RecursionError. This occurs when a deeply nested token header reaches json.loads. As a result, RecursionError escapes the documented PyJWT error hierarchy. Consequently, an unauthenticated malformed token can cause a request-level failure and HTTP 500. This issue is fixed in version 2.14.0.(CVE-2026-102265)

PyJWT is a Python implementation of JSON Web Token standards. Prior to 2.14.0, PyJWT signature segment is affected because signature segment decoding accepts characters outside the canonical Base64URL representation. This occurs when non-Base64URL characters are appended to a valid compact JWS signature segment. As a result, base64url_decode produces the same signature bytes for different serialized segments. Consequently, raw-token revocation checks can fail to recognize an equivalent modified token. This issue is fixed in version 2.14.0.(CVE-2026-102269)

PyJWT is a Python implementation of JSON Web Token standards. From 2.1.0 until 2.15.0, PyJWT OKPAlgorithm.from_jwk in jwt/algorithms.py is affected because private-JWK import path does not compare the public key derived from d with x. This occurs when an OKP private JWK supplies non-corresponding x and d components. As a result, identity derived from x can differ from operations performed with d. Consequently, if an integration also accepts private key parameters from a proof header without rejecting them, an attacker may use a stolen sender-constrained token without the legitimate private key. This issue is fixed in version 2.15.0.(CVE-2026-102275)

setup.py test %endif

Security Fix(es):

PyJWT is a Python implementation of JSON Web Token standards. From 2.0.0a1 until 2.15.0, PyJWT PyJWKClient.get_signing_key_from_jwt is affected because payload parser catches ValueError but not RecursionError. This occurs when an attacker-controlled recursively nested payload reaches json.loads. As a result, documented PyJWT exception handling does not contain the failure. Consequently, an unauthenticated request can raise an exception that may produce an HTTP 500 response. The advisory-defined affected implementation also includes jwt/api_jwt.py, verify_signature=False. This issue is fixed in version 2.15.0.(CVE-2026-101918)

PyJWT is a Python implementation of JSON Web Token standards. From 2.13.0 until 2.14.0, PyJWS._load in jwt/api_jws.py is affected because parser catches ValueError but not RecursionError. This occurs when a deeply nested token header reaches json.loads. As a result, RecursionError escapes the documented PyJWT error hierarchy. Consequently, an unauthenticated malformed token can cause a request-level failure and HTTP 500. This issue is fixed in version 2.14.0.(CVE-2026-102265)

PyJWT is a Python implementation of JSON Web Token standards. Prior to 2.14.0, PyJWT signature segment is affected because signature segment decoding accepts characters outside the canonical Base64URL representation. This occurs when non-Base64URL characters are appended to a valid compact JWS signature segment. As a result, base64url_decode produces the same signature bytes for different serialized segments. Consequently, raw-token revocation checks can fail to recognize an equivalent modified token. This issue is fixed in version 2.14.0.(CVE-2026-102269)

PyJWT is a Python implementation of JSON Web Token standards. From 2.1.0 until 2.15.0, PyJWT OKPAlgorithm.from_jwk in jwt/algorithms.py is affected because private-JWK import path does not compare the public key derived from d with x. This occurs when an OKP private JWK supplies non-corresponding x and d components. As a result, identity derived from x can differ from operations performed with d. Consequently, if an integration also accepts private key parameters from a proof header without rejecting them, an attacker may use a stolen sender-constrained token without the legitimate private key. This issue is fixed in version 2.15.0.(CVE-2026-102275)

setup.py test %endif

Security Fix(es):

PyJWT is a Python implementation of JSON Web Token standards. Prior to 2.14.0, PyJWT get_signing_key_from_jwt is affected because unknown kid misses force refreshes without a negative cache or minimum refresh interval. This occurs when unauthenticated tokens repeatedly use the same unknown kid or varying kid values absent from the cached JWKS. As a result, each cache miss causes PyJWKClient to refresh the JWKS. Consequently, attacker traffic can amplify outbound requests to the configured JWKS endpoint. This issue is fixed in version 2.14.0.(CVE-2026-101917)

PyJWT is a Python implementation of JSON Web Token standards. From 2.0.0a1 until 2.15.0, PyJWT PyJWKClient.get_signing_key_from_jwt is affected because payload parser catches ValueError but not RecursionError. This occurs when an attacker-controlled recursively nested payload reaches json.loads. As a result, documented PyJWT exception handling does not contain the failure. Consequently, an unauthenticated request can raise an exception that may produce an HTTP 500 response. The advisory-defined affected implementation also includes jwt/api_jwt.py, verify_signature=False. This issue is fixed in version 2.15.0.(CVE-2026-101918)

PyJWT is a Python implementation of JSON Web Token standards. From 2.13.0 until 2.14.0, PyJWS._load in jwt/api_jws.py is affected because parser catches ValueError but not RecursionError. This occurs when a deeply nested token header reaches json.loads. As a result, RecursionError escapes the documented PyJWT error hierarchy. Consequently, an unauthenticated malformed token can cause a request-level failure and HTTP 500. This issue is fixed in version 2.14.0.(CVE-2026-102265)

PyJWT is a Python implementation of JSON Web Token standards. Prior to 2.14.0, PyJWT signature segment is affected because signature segment decoding accepts characters outside the canonical Base64URL representation. This occurs when non-Base64URL characters are appended to a valid compact JWS signature segment. As a result, base64url_decode produces the same signature bytes for different serialized segments. Consequently, raw-token revocation checks can fail to recognize an equivalent modified token. This issue is fixed in version 2.14.0.(CVE-2026-102269)

PyJWT is a Python implementation of JSON Web Token standards. From 2.1.0 until 2.15.0, PyJWT OKPAlgorithm.from_jwk in jwt/algorithms.py is affected because private-JWK import path does not compare the public key derived from d with x. This occurs when an OKP private JWK supplies non-corresponding x and d components. As a result, identity derived from x can differ from operations performed with d. Consequently, if an integration also accepts private key parameters from a proof header without rejecting them, an attacker may use a stolen sender-constrained token without the legitimate private key. This issue is fixed in version 2.15.0.(CVE-2026-102275)

setup.py test %endif

Security Fix(es):

PyJWT is a Python implementation of JSON Web Token standards. Prior to 2.14.0, PyJWT get_signing_key_from_jwt is affected because unknown kid misses force refreshes without a negative cache or minimum refresh interval. This occurs when unauthenticated tokens repeatedly use the same unknown kid or varying kid values absent from the cached JWKS. As a result, each cache miss causes PyJWKClient to refresh the JWKS. Consequently, attacker traffic can amplify outbound requests to the configured JWKS endpoint. This issue is fixed in version 2.14.0.(CVE-2026-101917)

PyJWT is a Python implementation of JSON Web Token standards. From 2.0.0a1 until 2.15.0, PyJWT PyJWKClient.get_signing_key_from_jwt is affected because payload parser catches ValueError but not RecursionError. This occurs when an attacker-controlled recursively nested payload reaches json.loads. As a result, documented PyJWT exception handling does not contain the failure. Consequently, an unauthenticated request can raise an exception that may produce an HTTP 500 response. The advisory-defined affected implementation also includes jwt/api_jwt.py, verify_signature=False. This issue is fixed in version 2.15.0.(CVE-2026-101918)

PyJWT is a Python implementation of JSON Web Token standards. From 2.13.0 until 2.14.0, PyJWS._load in jwt/api_jws.py is affected because parser catches ValueError but not RecursionError. This occurs when a deeply nested token header reaches json.loads. As a result, RecursionError escapes the documented PyJWT error hierarchy. Consequently, an unauthenticated malformed token can cause a request-level failure and HTTP 500. This issue is fixed in version 2.14.0.(CVE-2026-102265)

PyJWT is a Python implementation of JSON Web Token standards. Prior to 2.14.0, PyJWT signature segment is affected because signature segment decoding accepts characters outside the canonical Base64URL representation. This occurs when non-Base64URL characters are appended to a valid compact JWS signature segment. As a result, base64url_decode produces the same signature bytes for different serialized segments. Consequently, raw-token revocation checks can fail to recognize an equivalent modified token. This issue is fixed in version 2.14.0.(CVE-2026-102269)

PyJWT is a Python implementation of JSON Web Token standards. From 2.1.0 until 2.15.0, PyJWT OKPAlgorithm.from_jwk in jwt/algorithms.py is affected because private-JWK import path does not compare the public key derived from d with x. This occurs when an OKP private JWK supplies non-corresponding x and d components. As a result, identity derived from x can differ from operations performed with d. Consequently, if an integration also accepts private key parameters from a proof header without rejecting them, an attacker may use a stolen sender-constrained token without the legitimate private key. This issue is fixed in version 2.15.0.(CVE-2026-102275)

Package

pyjwt (PyPI)

Affected versions

tested & verified on: 2.13.0. Every version whose PyJWS._load() translates only ValueError is affected

Description

PyJWS._looad() (jwt/api_jws.py:337) splits the compact token, base64url decodes the header segment and hands it to json.loads() before _verify_signature() runs. The parse is wrapped in except ValueError as e: raise DecodeError(...). That is what makes the documented contract sufficient for untrusted input: except jwt.PyJWTError (or InvalidTokenError) around jwt.decode(token, key, algorithms=[...]).

CPython's JSON decoder raises RecursionError (a RuntimeError subclass, not a ValueError) once nesting crosses the native stack guard. That exception leaves _load(), then jwt.decode(), and matches no PyJWTError handlerr at all. No key and no valid signature are needed: the header is parsed first, so a single unauthenticated request with an unsigned token suffices.

Scope: the recursion threshold is ~65,000 nesting levels; below it the same input yields a clean DecodeError (a 346,800-byte flat header is rejected normally, so depth, not size, is the trigger). A token this large cannot ride in an Authorization header (rejected at HTTP field limits,431 under gunicorn), but body transport (RFC 7662 introspection style) is a standard pattern. Under Flask 3.1.3 / gunicorn 26.2.0 each crafted request returns HTTP 500 instead of 401 and writes a full traceback to the error log; the sync worker survives.

Proof of concept

Against any endpoint that validates a token from its JSON body with the documented pattern:

import base64, json, http.client

def b64url(b): return base64.urlsafe_b64encode(b).rstrip(b"=")

# unsigned token: the header segment is 200,000 nested '[' (266,681 bytes total)
token = (b64url(b"[" * 200_000) + b"." + b64url(b'{"a":1}') + b"." + b64url(b"x")).decode()

conn = http.client.HTTPConnection("127.0.0.1", 8125, timeout=30)
conn.request("POST", "/api/protected", body=json.dumps({"token": token}),
             headers={"Content-Type": "application/json"})
print(conn.getresponse().status)   # 500, expected 401

Determinism: 20/20 crafted requests returned 500 against the reference container (PyJWT 2.13.0, Flask 3.1.3, gunicorn 26.2.0, Python 3.14.7) in 0.12 s (about 6 ms per request in a tight loop, 14 ms for a cold request), each leaving a RecursionError: Stack overflow ... while decoding a JSON array traceback in the error log.

Impact

An unauthenticated attacker can turn every request to a JWT validating endpoint into a server error with a full traceback logged per request, a cheap and repeatable denial of service on the authentication path (no crash, no data exposure).

Fix

Catch RecursionError alongside ValueError in _load() and raise DecodeError, or parse the header with a nonrecursive, depthbounded parser.

Reproduction

A standalone package (docker-compose.yml, Dockerfile, requirements.txt, target app, driver) accompanies this report:

cd <package dir>
docker compose up -d --build
python3 poc_check_then_attack.py
docker compose down -v

poc_pyjwt_recursion.zip

Exit 0= reproduced (control checks green, exactly one crafted request returns 500, traceback verified in docker logs, service alive afterwards); 1 = not reproduced; 2 = checks failed, no attack sent. The package's run_log.txt documents a complete proof run.

Credits

• @Nivid42

Maintainer update — 2026-09-09

We reproduced the reported behavior on PyJWT 2.13.0: a deeply nested JSON array in the untrusted compact-JWS header reaches json.loads() before signature verification and raises RecursionError, which is not a PyJWTError. The same input is accepted by the public jwt.decode() path and can escape an application's normal except PyJWTError handling.

The fix is committed as…

PyJWT is a Python implementation of JSON Web Token standards. From 2.13.0 until 2.14.0, PyJWS._load in jwt/api_jws.py is affected because parser catches ValueError but not RecursionError. This occurs when a deeply nested token header reaches json.loads. As a result, RecursionError escapes the documented PyJWT error hierarchy. Consequently, an unauthenticated malformed token can cause a request-level failure and HTTP 500. This issue is fixed in version 2.14.0.

PyJWT is a Python implementation of JSON Web Token standards. From 2.13.0 until 2.14.0, PyJWS._load in jwt/api_jws.py is affected because parser catches ValueError but not RecursionError. This occurs when a deeply nested token header reaches json.loads. As a result, RecursionError escapes the documented PyJWT error hierarchy. Consequently, an unauthenticated malformed token can cause a request-level failure and HTTP 500. This issue is fixed in version 2.14.0.

TuxCare rebuilt pyjwt to address 3 CVEs. Fixed in 2.10.1.post8+tuxcare.

TuxCare rebuilt pyjwt to address 4 CVEs. Fixed in 2.8.0.post6+tuxcare.

TuxCare rebuilt pyjwt to address 3 CVEs. Fixed in 2.3.0.post6+tuxcare.