CVE-2026-34950

CriticalPublic PoC

Published: 06 April 2026

Published

06 April 2026

Modified

22 April 2026

KEV Added

—

Patch

—

CVSS Score 9.1 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

EPSS Score 0.0002 5.1th percentile

Risk Priority 18 60% EPSS · 20% KEV · 20% CVSS

Summary

CVE-2026-34950 is a critical-severity Use of a Broken or Risky Cryptographic Algorithm (CWE-327) vulnerability in Nearform Fast-Jwt. Its CVSS base score is 9.1 (Critical).

Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked at the 5.1th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog; a public proof-of-concept is referenced.

The strongest mitigations our analysis identified are NIST 800-53 RA-5 (Vulnerability Monitoring and Scanning) and SI-2 (Flaw Remediation).

Threat & Defense at a Glance

What attackers do: exploitation maps to Exploit Public-Facing Application (T1190) and 1 other technique. What defenders deploy: see the NIST 800-53 controls recommended below.

Threat & Defense Details

Mitigating Controls (NIST 800-53 r5)AI

SI-2 Flaw Remediation good match

prevent

Directly mitigates CVE-2026-34950 by requiring identification, prioritization, and timely patching of the vulnerable fast-jwt library versions affected by the regex flaw.

RA-5 Vulnerability Monitoring and Scanning good match

detect

Enables proactive detection of the vulnerable fast-jwt library through continuous vulnerability scanning of software components and dependencies.

SC-13 Cryptographic Protection partial match

prevent

Addresses JWT algorithm confusion risks by mandating implementation of approved cryptographic protections that limit use of weak or unintended algorithms during token verification.

MITRE ATT&CK Enterprise TechniquesAI

T1190 Exploit Public-Facing Application Initial Access

Adversaries may attempt to exploit a weakness in an Internet-facing host or system to initially access a network.

attack.mitre.org →

T1606 Forge Web Credentials Credential Access

Adversaries may forge credential materials that can be used to gain access to web applications or Internet services.

attack.mitre.org →

Why these techniques?

The vulnerability enables remote exploitation of JWT verification in public-facing applications via algorithm confusion (T1190) and facilitates forging of web credentials/tokens to bypass authentication (T1606).

Confidence: MEDIUM · MITRE ATT&CK Enterprise v18.1

NVD Description

fast-jwt provides fast JSON Web Token (JWT) implementation. In 6.1.0 and earlier, the publicKeyPemMatcher regex in fast-jwt/src/crypto.js uses a ^ anchor that is defeated by any leading whitespace in the key string, re-enabling the exact same JWT algorithm confusion attack…

that CVE-2023-48223 patched.

Deeper analysisAI

CVE-2026-34950 is a vulnerability in the fast-jwt library, which provides a fast JSON Web Token (JWT) implementation. It affects versions 6.1.0 and earlier, specifically due to a flaw in the publicKeyPemMatcher regex located in fast-jwt/src/crypto.js. The regex uses a ^ anchor that can be bypassed by any leading whitespace in the key string, thereby re-enabling the JWT algorithm confusion attack that was previously addressed by CVE-2023-48223. This issue is classified under CWE-327 (Broken or Risky Cryptographic Algorithm) and carries a CVSS v3.1 base score of 9.1 (AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N).

Remote attackers can exploit this vulnerability over the network with low attack complexity, requiring no privileges, user interaction, or special conditions. By crafting a JWT with leading whitespace in the key, an attacker can confuse the algorithm selection during verification, potentially leading to the acceptance of tokens signed with weaker or unintended algorithms. This enables high-impact compromise of confidentiality and integrity, such as unauthorized access or data tampering in applications relying on fast-jwt for JWT handling.

Mitigation details are available in the GitHub security advisory at https://github.com/nearform/fast-jwt/security/advisories/GHSA-mvf2-f6gm-w987.