PYSEC-2026-4148: PyJWT: ReDoS vulnerability when calling the `is_pem_format` function.

Q: What conditions must be met for exploitation?

An attacker must be able to cause the application to call is_pem_format with a certificate-like input containing repeated BEGIN markers and no matching END marker. Exploitation causes intensive CPU consumption through unbounded regular-expression backtracking.

Q: Which deployments are affected?

PyJWT versions prior to 2.14.0 are affected when they invoke is_pem_format on attacker-controlled or otherwise untrusted certificate-like input. The provided information does not establish that every default PyJWT use reaches this function with such input.

Q: What is the remediation?

Upgrade PyJWT to version 2.14.0, which fixes the issue. If an immediate upgrade is not possible, avoid passing untrusted certificate-like input to is_pem_format and apply input validation or size and processing limits before that call.

Published Sep 28, 2026

Updated

PyJWT is a Python implementation of JSON Web Token standards. Prior to 2.14.0, PyJWT ispemformat is affected because lazy PEM regular expression backtracks extensively. This occurs when a certificate-like input contains repeated BEGIN markers without a matching END marker. As a result, ispemformat performs unbounded backtracking while searching for a PEM end marker. Consequently, an attacker can cause intensive CPU consumption. This issue is fixed in version 2.14.0.

Affected Software

1 affected component

pypi/pyjwt<2.14.0

Remediation

Recommended actions to resolve this vulnerability, in priority order.

Upgrade
Upgrade PyJWT to a version that resolves this vulnerability.
Fixed in 2.14.0

Event History

Sep 28, 2026

CVE Published

via OSV·08:29 PM

Data Sourced

via OSV·08:29 PM

DescriptionSeverityWeakness

Frequently Asked Questions

What conditions must be met for exploitation?

An attacker must be able to cause the application to call is_pem_format with a certificate-like input containing repeated BEGIN markers and no matching END marker. Exploitation causes intensive CPU consumption through unbounded regular-expression backtracking.

Which deployments are affected?

PyJWT versions prior to 2.14.0 are affected when they invoke is_pem_format on attacker-controlled or otherwise untrusted certificate-like input. The provided information does not establish that every default PyJWT use reaches this function with such input.

What is the remediation?

Upgrade PyJWT to version 2.14.0, which fixes the issue. If an immediate upgrade is not possible, avoid passing untrusted certificate-like input to is_pem_format and apply input validation or size and processing limits before that call.

PYSEC-2026-4148: PyJWT: ReDoS vulnerability when calling the `is_pem_format` function.

Affected Software

Remediation

Event History

Frequently Asked Questions

What conditions must be met for exploitation?

Which deployments are affected?

What is the remediation?

Company

Resources

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Frequently Asked Questions

What conditions must be met for exploitation?

Which deployments are affected?

What is the remediation?