How should we interpret CVSS severity scores?

CVSS estimates technical severity, but it does not automatically equal business risk. Prioritize using context like internet exposure, asset criticality, known exploitation, and whether compensating controls exist. A Medium CVSS on an exposed production system can be more urgent than a Critical on an isolated non-production host.

Vulnerability Database

Q: What is a Vulnerability (CVE) and why does it matter?

A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.

Q: What's the difference between a vulnerability, an exploit, and a zero-day?

A vulnerability is the underlying weakness. An exploit is the method or code used to take advantage of it. A zero-day is a vulnerability that is unknown to the vendor or has no publicly available fix when attackers begin using it. Risk increases sharply when exploitation becomes reliable or widespread.

Q: Why do vulnerabilities keep reappearing in our environment?

Recurring findings usually come from incomplete asset discovery, inconsistent patch management, inherited images, and configuration drift. In modern environments, you also need to watch the software supply chain: dependencies, containers, build pipelines, and third-party services can reintroduce the same weakness even after you patch a single host.

Q: How do we prioritize remediation without burning out the team?

Use a repeatable triage model: focus first on externally exposed assets, high-value systems (identity, VPN, email, production), vulnerabilities with known exploits, and issues that enable remote code execution or privilege escalation. Then enforce patch SLAs and track progress so remediation is steady, not reactive.

Q: How can SynScan help reduce vulnerability risk over time?

SynScan combines attack surface monitoring and continuous security auditing to keep your inventory current, flag high-impact vulnerabilities early, and help you turn raw findings into a practical remediation plan.

358,842

Total vulnerabilities in the database

PHP JWT Framework: JWSVerifier uses algorithm from unprotected header, enabling algorithm confusion attacks — web-token / jwt-library

Insufficient Verification of Data Authenticity

Summary

JWSVerifier::getAlgorithm() in src/Library/Signature/JWSVerifier.php (line 144) merges protected and unprotected headers using PHP's spread operator:

$completeHeader = [...$signature-&gt;getProtectedHeader(), ...$signature-&gt;getHeader()];

In PHP, when spreading arrays with duplicate string keys, the last array's values take precedence. Since the unprotected header (getHeader()) is spread second, an attacker can override the integrity-protected alg parameter by placing a different value in the unprotected header.

This creates a Time-of-Check/Time-of-Use (TOCTOU) vulnerability:

HeaderCheckerManager validates alg from the protected header
JWSVerifier uses alg from the unprotected header for actual verification

The same issue exists in JWEDecrypter.php (lines 120-124) where array_merge() exhibits the same last-wins behavior for alg and enc.

Affected Code

JWSVerifier.php line 144 — Spread operator merge order allows unprotected header to override alg:

$completeHeader = [...$signature-&gt;getProtectedHeader(), ...$signature-&gt;getHeader()];

JWEDecrypter.php lines 120-124 — array_merge() with same last-wins behavior:

$completeHeader = array_merge(
    $jwe-&gt;getSharedProtectedHeader(),
    $jwe-&gt;getSharedHeader(),
    $recipient-&gt;getHeader()
);

Attack Vectors

Vector A — Mixed key sets (HIGH probability)

If the application uses a JWKSet containing keys of different types (common in multi-tenant or federation scenarios), the JWSVerifier iterates all keys (line 86). An attacker can force a different algorithm that matches a different key in the set.

Vector B — alg ONLY in unprotected header (HIGH probability)

If alg is placed EXCLUSIVELY in the unprotected header (not in the protected header at all), HeaderCheckerManager::checkDuplicatedHeaderParameters() does NOT trigger. The JSON Flattened/General serializers allow tokens with no protected header or a protected header without alg. RFC 7515 Section 4.1.1 states alg MUST be integrity-protected, but the library does not enforce this.

Vector C — Direct JWSVerifier usage (HIGH probability)

JWSLoader takes ?HeaderCheckerManager (nullable). If developers use JWSVerifier directly or create JWSLoader without a HeaderCheckerManager, the duplicate header check never runs.

Contrast with JWSBuilder (safe)

JWSBuilder::findSignatureAlgorithm() (line 196) uses [...$header, ...$protectedHeader] where protected wins. It also has checkDuplicatedHeaderParameters() (line 218). The JWSVerifier has neither safeguard.

Proof of Concept

&lt;?php
// Demonstrate algorithm override via unprotected header
$protected = [&quot;alg&quot; =&gt; &quot;RS256&quot;, &quot;typ&quot; =&gt; &quot;JWT&quot;];
$unprotected = [&quot;alg&quot; =&gt; &quot;HS256&quot;];
$merged = [...$protected, ...$unprotected];
// $merged[&quot;alg&quot;] === &quot;HS256&quot; — unprotected wins!

// JSON Flattened JWS with algorithm override:
$maliciousJws = json_encode([
    &#039;payload&#039; =&gt; base64url_encode($payload),
    &#039;protected&#039; =&gt; base64url_encode(&#039;{&quot;alg&quot;:&quot;RS256&quot;}&#039;),
    &#039;header&#039; =&gt; [&#039;alg&#039; =&gt; &#039;HS256&#039;],  // OVERRIDE
    &#039;signature&#039; =&gt; base64url_encode($sig),
]);
// HeaderCheckerManager validates RS256 from protected header -&gt; PASS
// JWSVerifier uses HS256 from unprotected header -&gt; attacker&#039;s algorithm choice

A full working PoC demonstrating HS512-to-HS256 downgrade with mixed keysets is available upon request.

Suggested Fix

In JWSVerifier::getAlgorithm(), read alg exclusively from the protected header:

private function getAlgorithm(Signature $signature): Algorithm
{
    $protectedHeader = $signature-&gt;getProtectedHeader();
    if (! isset($protectedHeader[&#039;alg&#039;])) {
        throw new InvalidArgumentException(&#039;The &quot;alg&quot; parameter must be in the protected header.&#039;);
    }
    return $this-&gt;signatureAlgorithmManager-&gt;get($protectedHeader[&#039;alg&#039;]);
}

For JWEDecrypter, reverse the merge order so protected header wins, or extract alg/enc exclusively from the protected header.

Résolution

Un correctif a été préparé sur une branche dédiée basée sur 3.4.x, avec des tests anti-régression dédiés (fork privé temporaire de cette advisory, PR #1).

JWS algorithm confusion — JWSVerifier lit le paramètre alg exclusivement dans le header protégé en intégrité (RFC 7515 §4.1.1) ; un alg placé dans le header non protégé ne peut plus surcharger l'algorithme signé.

Validation : php -l OK, PHPUnit vert, aucune nouvelle erreur PHPStan introduite (différentiel nul vs 3.4.x), aucun commentaire ajouté dans le code source. Après merge, cascade prévue 3.4.x → 4.0.x → 4.1.x.

Published: Jun 18, 2026
Updated: Jun 19, 2026
GHSA: GHSA-jc38-x7x8-2xc8
Severity: High
Exploit:
CISA KEV:

No technical information available.

CWEs:

CWE-345

Affected Software
References

Software	From	Fixed in
web-token / jwt-library	-	3.4.10
web-token / jwt-library	4.0.0	4.0.7
web-token / jwt-library	4.1.0	4.1.7
web-token / jwt-bundle	-	3.4.10
web-token / jwt-bundle	4.0.0	4.0.7
web-token / jwt-bundle	4.1.0	4.1.7
web-token / jwt-experimental	-	3.4.10
web-token / jwt-experimental	4.0.0	4.0.7
web-token / jwt-experimental	4.1.0	4.1.7
web-token / jwt-framework	-	3.4.10
web-token / jwt-framework	4.0.0	4.0.7
web-token / jwt-framework	4.1.0	4.1.7

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Frequently Asked Questions

What is a Vulnerability (CVE) and why does it matter?

A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.

CVSS (Common Vulnerability Scoring System) estimates technical severity, but it doesn't automatically equal business risk. Prioritize using context like internet exposure, affected asset criticality, known exploitation (proof-of-concept or in-the-wild), and whether compensating controls exist. A "Medium" CVSS on an exposed, production system can be more urgent than a "Critical" on an isolated, non-production host.

A vulnerability is the underlying weakness. An exploit is the method or code used to take advantage of it. A zero-day is a vulnerability that is unknown to the vendor or has no publicly available fix when attackers begin using it. In practice, risk increases sharply when exploitation becomes reliable or widespread.

Recurring findings usually come from incomplete Asset Discovery, inconsistent patch management, inherited images, and configuration drift. In modern environments, you also need to watch the software supply chain: dependencies, containers, build pipelines, and third-party services can reintroduce the same weakness even after you patch a single host. Unknown or unmanaged assets (often called Shadow IT) are a common reason the same issues resurface.

Use a simple, repeatable triage model: focus first on externally exposed assets, high-value systems (identity, VPN, email, production), vulnerabilities with known exploits, and issues that enable remote code execution or privilege escalation. Then enforce patch SLAs and track progress using consistent metrics so remediation is steady, not reactive.

SynScan combines attack surface monitoring and continuous security auditing to keep your inventory current, flag high-impact vulnerabilities early, and help you turn raw findings into a practical remediation plan.