CVE Intelligence
Medium Input validation exploit reference

CVE-2023-48223 — fast-jwt JWT Algorithm Confusion

fast-jwt provides fast JSON Web Token (JWT) implementation. Prior to version 3.3.2, the fast-jwt library does not properly prevent JWT algorithm confusion for all public key types. The 'publicKeyPemMatcher' in 'fast-jwt/src/crypto.js' does not properly match all common PEM formats for public keys. To exploit this vulnerability, an attacker needs to craft a malicious JWT token containing the HS256 algorithm, signed with the public RSA key of the victim application. This attack will only work if the victim application utilizes a public key containing the `BEGIN RSA PUBLIC KEY` header. Applications using the RS256 algorithm, a public key with a `BEGIN RSA PUBLIC KEY` header, and calling the verify function without explicitly providing an algorithm, are vulnerable to this algorithm confusion attack which allows attackers to sign arbitrary payloads which will be accepted by the verifier. Version 3.3.2 contains a patch for this issue. As a workaround, change line 29 of `blob/master/src/crypto.js` to include a regular expression.

Published Nov 20, 2023 · Updated Oct 26, 2026 · Sources: NIST National Vulnerability Database, CVE Program (cve.org) e CISA ADP, GitHub Security Advisories · CNA: GitHub_M

Who exploits it and how urgent it is

A 0–100 score combining confirmed exploitation, ransomware and APT use, public exploits, EPSS and severity.

No exploitation signal in the sources consulted so far.
How the score was calculated
  • EPSS 0.7% (30-day exploitation probability)+8
  • Technical severity (medium)+3

The score orders the patch queue; it does not replace your asset context (internet exposure, criticality, compensating controls).

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the sources call it.

Exploitation
Not confirmed
Not in CISA KEV — an absence of record, not proof it is unexploited.
EPSS FIRST
0.68%
ahead of 52% of scored CVEs · Oct 30, 2026
Affects
Nearform
Fast-jwt
CVSS base
5.9
Medium · 4 source(s)

Remediation

Fixes and mitigations documented by the sources — linked to the original advisory.

  • Npm Fast-jwt: fixed in version 3.3.2 — github.com (GitHub Security Advisories)

First 24 hours

Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.

  1. Identify exposed assets running the affected vendor/product/version combinations (Nearform Fast-jwt, Npm Fast-jwt).
  2. The attack is network-based and needs no privileges: restrict exposure (ACLs, VPN, WAF) until patched.
  3. Apply the fixed version or the patch documented under Remediation.
  4. Search available logs for exploit probes matching the vulnerability class (input validation), errors and authentication anomalies.

Affected scope

Vendor, product and versions as the sources word them.

VendorProductVersionsStatusSource
Nearform Fast-jwt < 3.3.2 Vulnerable NIST National Vulnerability Database
Nearform Fast-jwt version < 3.3.2 Vulnerable CVE Program (cve.org) e CISA ADP
Npm Fast-jwt < 3.3.2 Vulnerable GitHub Security Advisories

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Attack vector
Network
Attack complexity
High
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
High
Availability
None

Every base score collected

Sources score independently and may disagree; each row says who scored it and under which version.

ScoreVersionSeverityExpl.ImpactScored by
5.9 CVSS 3.1 Medium 2.2 3.6 security-advisories@github.com via NIST National Vulnerability Database
5.9 CVSS 3.1 Medium 2.2 3.6 nvd@nist.gov via NIST National Vulnerability Database
5.9 CVSS 3.1 Medium — — GitHub, Inc. via GitHub Security Advisories
5.9 CVSS 3.1 Medium — — GitHub_M via CVE Program (cve.org) e CISA ADP
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N (score shown at the top: nvd@nist.gov)

Weakness MITRE

Weakness class according to MITRE’s CWE catalog.

CWE-20 · Improper Input Validation

Other CVEs with this weakness →

EPSS over time FIRST

Oct 26, 2026 → Oct 30, 2026 · 5 reading(s) · max 0.68%

Detection

Read off the CVSS vector and the weakness class. Starting points, not tested rules.

  • Prioritize edge telemetry for the network-reachable component (proxy, WAF, IDS/IPS).
web/proxy/WAF logsnetwork IDS/IPS

Timeline

What happened to this CVE, newest first.

  • 2026
  • Fix documented
    Oct 26, 2026 · GitHub Security Advisories
  • Affected products added · npm/fast-jwt
    Oct 26, 2026 · GitHub Security Advisories
  • Affected products added · nearform/fast-jwt
    Oct 26, 2026 · CVE Program (cve.org) e CISA ADP
  • Record published
    Oct 26, 2026
  • Affected products added · nearform/fast-jwt
    Oct 26, 2026 · NIST National Vulnerability Database
  • CVSS changed · 5.9
    Oct 26, 2026 · NIST National Vulnerability Database
  • Source status changed · → Modified
    Oct 26, 2026 · NIST National Vulnerability Database
  • Record created in this database
    Oct 26, 2026 · Alertas do Telegram
  • 2023
  • CVE published
    Nov 20, 2023 · GitHub_M

Source activity

Repeated readings are counted rather than listed.

  • 2× References added · Oct 26, 2026 – Oct 26, 2026
  • 1× Official description updated · Oct 26, 2026 – Oct 26, 2026

References

6 on the record

Elsewhere on this site

Nearform
every CVE for this vendor
Input validation
same class
CWE-20
other pages naming this weakness

Related CVEs (same products)

  • CVE-2026-107722 fast-jwt: Incomplete patch of CVE-2026-34950: Non-whitespace key-prefix re-enables RSA→HS256 algorithm confusion 22d
  • CVE-2026-107723 fast-jwt : Silent claim-validator bypass when JWT payload is a JSON array 22d
  • CVE-2026-107721 fast-jwt clockTolerance: Infinity silently bypasses both exp and nbf validation (and persists in the verifier cache) 22d
  • CVE-2026-107724 fast-jwt treats raw public JWK JSON as an HMAC secret, enabling HS256 token forgery 22d
  • CVE-2026-107720 fast-jwt: createVerifier accepts unsigned JWTs when key is '' or null and algorithms is explicitly set 22d
  • CVE-2026-107719 fast-jwt: Verifier cache accepts expired JWTs without iat. 22d

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

  • No CISA SSVC assessment.
  • No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
  • No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
  • No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record

Text assembled only from the fields above — no additional claims.

What should defenders know first?

CVE-2023-48223 is "fast-jwt JWT Algorithm Confusion", rated Medium. It affects Fast-jwt (Nearform); versions are in the scope table. It is not in CISA KEV, which does not rule out exploitation. CVSS 5.9 describes technical severity. EPSS 0.68% estimates the probability of exploitation in 30 days; neither replaces your asset context. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.

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