CVE-2026-62995

Improper padding validation in joserfc leads to JWT token malleability.

• CVSS 2.3
• 345
• Input Validation and Sanitization
• Remote

joserfc is a Python library that provides an implementation of several JSON Object Signing and Encryption (JOSE) standards. in versions 1.7.1 and prior, joserfc accepts JWTs with trailing padding (==) which are not conforming to the JOSE specifications. This leads to malleability of the JWTs when consumed by joserfc. Depending on this application this might or not be an issue. This could lead to bypass of token revocation or anti-replay protection when implemented as a deny list of tokens or a deny list of token hashes. Note that ECDSA JWS are always malleable because of the malleability of ECDSA signatures (first test case in the code bellow). This makes a scheme which assumes that JWTs are not malleable brittle. However for other signatures (or MAC) schemes it might make sense to assume non malleability of the token. This issue has been fixed in version 1.7.2.

CVE

CVE-2026-62995

CWE

345

CVSS base score

2.3

Published

2026-07-29

OWASP

A08 Software and Data Integrity Failures

Orthogonal defect classification

Checking

Code defect classification

Incorrect Check

Category

Input Validation and Sanitization

Subcategory

Cryptographic Implementation Error

Accessibility scope

Remote

Impact

Unauthorized Access

Affected component

joserfc

Fixed by upgrading

Yes

Solution

Upgrade `joserfc` to version 1.7.2 or later.

import base64
import binascii
from typing import Union


def to_bytes(s: Union[str, bytes], encoding: str = 'utf-8') -> bytes:
    if isinstance(s, bytes):
        return s
    return s.encode(encoding)


def urlsafe_b64encode(s: bytes) -> bytes:
    return base64.urlsafe_b64encode(s).rstrip(b'=')


# VULNERABLE: Padding is added to the input, allowing non-conformant base64url values.
def urlsafe_b64decode(s: bytes) -> bytes:
    return base64.urlsafe_b64decode(s + b'=' * (-len(s) % 4))


def json_b64encode(data: bytes) -> str:
    return urlsafe_b64encode(data).decode('utf-8')


def json_b64decode(s: str) -> bytes:
    return urlsafe_b64decode(to_bytes(s))

import base64
import binascii
from typing import Union


def to_bytes(s: Union[str, bytes], encoding: str = 'utf-8') -> bytes:
    if isinstance(s, bytes):
        return s
    return s.encode(encoding)


def urlsafe_b64encode(s: bytes) -> bytes:
    return base64.urlsafe_b64encode(s).rstrip(b'=')


# FIX: Validate that the base64url input has no padding, rejecting non-conformant values.
def urlsafe_b64decode(s: bytes) -> bytes:
    """Decodes a base64 string, raising an error if it is incorrectly
    padded.
    """
    rem = len(s) % 4
    if rem > 0:
        # The input is not padded correctly.
        raise binascii.Error('Incorrect padding')
    return base64.urlsafe_b64decode(s)


def json_b64encode(data: bytes) -> str:
    return urlsafe_b64encode(data).decode('utf-8')


def json_b64decode(s: str) -> bytes:
    return urlsafe_b64decode(to_bytes(s))

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@misc{vaitp:cve202662995,
  title        = {{Improper padding validation in joserfc leads to JWT token malleability.}},
  author       = {Bogaerts, Fr\'ed\'eric and Ivaki, Naghmeh and Fonseca, Jos\'e},
  year         = {2026},
  note         = {VAITP Python Vulnerability Dataset, entry CVE-2026-62995},
  howpublished = {\url{https://netpack.pt/vaitp/vulnerability/CVE-2026-62995/}}
}