PHP JWT Framework: JWSVerifier uses algorithm from unprotected header, enabling algorithm confusion attacks
The advisory, in context
Summary
JWSVerifier::getAlgorithm() in src/Library/Signature/JWSVerifier.php (line 144) merges protected and unprotected headers using PHP's spread operator:
$completeHeader = [...$signature->getProtectedHeader(), ...$signature->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:
HeaderCheckerManagervalidatesalgfrom the protected headerJWSVerifierusesalgfrom 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->getProtectedHeader(), ...$signature->getHeader()];
JWEDecrypter.php lines 120-124 — array_merge() with same last-wins behavior:
$completeHeader = array_merge(
$jwe->getSharedProtectedHeader(),
$jwe->getSharedHeader(),
$recipient->getHeader()
);
Attack Vectors
Vector A — Mixed key sets (HIGH probability)
If the application uses a JWKSet containing keys of different types (common in multi-tenant or federation scenarios), the JWSVerifier iterates all keys (line 86). An attacker can force a different algorithm that matches a different key in the set.
Vector B — alg ONLY in unprotected header (HIGH probability)
If alg is placed EXCLUSIVELY in the unprotected header (not in the protected header at all), HeaderCheckerManager::checkDuplicatedHeaderParameters() does NOT trigger. The JSON Flattened/General serializers allow tokens with no protected header or a protected header without alg. RFC 7515 Section 4.1.1 states alg MUST be integrity-protected, but the library does not enforce this.
Vector C — Direct JWSVerifier usage (HIGH probability)
JWSLoader takes ?HeaderCheckerManager (nullable). If developers use JWSVerifier directly or create JWSLoader without a HeaderCheckerManager, the duplicate header check never runs.
Contrast with JWSBuilder (safe)
JWSBuilder::findSignatureAlgorithm() (line 196) uses [...$header, ...$protectedHeader] where protected wins. It also has checkDuplicatedHeaderParameters() (line 218). The JWSVerifier has neither safeguard.
Proof of Concept
<?php
// Demonstrate algorithm override via unprotected header
$protected = ["alg" => "RS256", "typ" => "JWT"];
$unprotected = ["alg" => "HS256"];
$merged = [...$protected, ...$unprotected];
// $merged["alg"] === "HS256" — unprotected wins!
// JSON Flattened JWS with algorithm override:
$maliciousJws = json_encode([
'payload' => base64url_encode($payload),
'protected' => base64url_encode('{"alg":"RS256"}'),
'header' => ['alg' => 'HS256'], // OVERRIDE
'signature' => base64url_encode($sig),
]);
// HeaderCheckerManager validates RS256 from protected header -> PASS
// JWSVerifier uses HS256 from unprotected header -> attacker'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 = $signature->getProtectedHeader();
if (! isset($protectedHeader['alg'])) {
throw new InvalidArgumentException('The "alg" parameter must be in the protected header.');
}
return $this->signatureAlgorithmManager->get($protectedHeader['alg']);
}
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.
Affected packages
Start with the recorded fix for your release branch. Check your installed version and test compatibility before upgrading. Fixing this advisory does not rule out other vulnerabilities in the release.
Earlier releases before 3.4.10
Fixed in 3.4.10
16 explicitly affected versions
3.3.0, 3.3.1, 3.3.2, 3.3.3, 3.3.4, 3.3.5, 3.4.0, 3.4.1, 3.4.2, 3.4.3, 3.4.4, 3.4.5, 3.4.6, 3.4.7, 3.4.8, 3.4.9
View affected-version evidence
{
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "3.4.10"
}
],
"type": "ECOSYSTEM"
}
],
"versions": [
"3.3.0",
"3.3.1",
"3.3.2",
"3.3.3",
"3.3.4",
"3.3.5",
"3.4.0",
"3.4.1",
"3.4.2",
"3.4.3",
"3.4.4",
"3.4.5",
"3.4.6",
"3.4.7",
"3.4.8",
"3.4.9"
]
}From 4.0.0 before 4.0.7
Fixed in 4.0.7
7 explicitly affected versions
4.0.0, 4.0.1, 4.0.2, 4.0.3, 4.0.4, 4.0.5, 4.0.6
View affected-version evidence
{
"ranges": [
{
"events": [
{
"introduced": "4.0.0"
},
{
"fixed": "4.0.7"
}
],
"type": "ECOSYSTEM"
}
],
"versions": [
"4.0.0",
"4.0.1",
"4.0.2",
"4.0.3",
"4.0.4",
"4.0.5",
"4.0.6"
]
}From 4.1.0 before 4.1.7
Fixed in 4.1.7
7 explicitly affected versions
4.1.0, 4.1.1, 4.1.2, 4.1.3, 4.1.4, 4.1.5, 4.1.6
View affected-version evidence
{
"ranges": [
{
"events": [
{
"introduced": "4.1.0"
},
{
"fixed": "4.1.7"
}
],
"type": "ECOSYSTEM"
}
],
"versions": [
"4.1.0",
"4.1.1",
"4.1.2",
"4.1.3",
"4.1.4",
"4.1.5",
"4.1.6"
]
}Earlier releases before 3.4.10
Fixed in 3.4.10
110 explicitly affected versions
3.3.0, 3.3.1, 3.3.2, 3.3.3, 3.3.4, 3.3.5, 3.4.0, 3.4.1, 3.4.2, 3.4.3, 3.4.4, 3.4.5, 3.4.6, 3.4.7, 3.4.8, 3.4.9, v0.0.0, v0.0.1, v0.0.2, v0.1.0, v0.2.0, v0.2.1, v0.2.2, v0.2.3, v0.3.0, v0.3.1, v0.3.2, v0.4.0, v0.4.1, v0.5.0, v0.5.1, v0.5.2, v1.0.0, v1.0.1, v1.0.2, v1.0.3, v1.0.4, v1.0.5, v1.0.6, v1.1.0, v1.1.0-beta1, v1.1.0-beta2, v1.1.0-beta3, v1.1.0-beta4, v1.1.1, v1.1.2, v1.1.3, v1.1.4, v1.2.0, v1.2.1, v1.2.2, v1.2.3, v1.2.4, v1.3.0, v1.3.1, v1.3.10, v1.3.2, v1.3.3, v1.3.4, v1.3.5, v1.3.6, v1.3.7, v1.3.8, v1.3.9, v2.0.0, v2.0.0-alpha1, v2.0.0-alpha2, v2.0.0-alpha3, v2.0.1, v2.0.10, v2.0.11, v2.0.12, v2.0.13, v2.0.2, v2.0.3, v2.0.4, v2.0.5, v2.0.6, v2.0.7, v2.0.8, v2.0.9, v2.1.0, v2.1.1, v2.1.2, v2.1.3, v2.1.4, v2.1.5, v2.1.6, v2.1.7, v2.1.8, v2.1.9, v2.2.0, v2.2.1, v2.2.10, v2.2.11, v2.2.2, v2.2.3, v2.2.4, v2.2.5, v2.2.6, v2.2.7, v2.2.8, v2.2.9, v3.0.1, v3.0.2, v3.0.3, v3.0.4, v3.0.5, v3.0.6, v3.0.7
View affected-version evidence
{
"ranges": [
{
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{
"introduced": "0"
},
{
"fixed": "3.4.10"
}
],
"type": "ECOSYSTEM"
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"versions": [
"3.3.0",
"3.3.1",
"3.3.2",
"3.3.3",
"3.3.4",
"3.3.5",
"3.4.0",
"3.4.1",
"3.4.2",
"3.4.3",
"3.4.4",
"3.4.5",
"3.4.6",
"3.4.7",
"3.4.8",
"3.4.9",
"v0.0.0",
"v0.0.1",
"v0.0.2",
"v0.1.0",
"v0.2.0",
"v0.2.1",
"v0.2.2",
"v0.2.3",
"v0.3.0",
"v0.3.1",
"v0.3.2",
"v0.4.0",
"v0.4.1",
"v0.5.0",
"v0.5.1",
"v0.5.2",
"v1.0.0",
"v1.0.1",
"v1.0.2",
"v1.0.3",
"v1.0.4",
"v1.0.5",
"v1.0.6",
"v1.1.0",
"v1.1.0-beta1",
"v1.1.0-beta2",
"v1.1.0-beta3",
"v1.1.0-beta4",
"v1.1.1",
"v1.1.2",
"v1.1.3",
"v1.1.4",
"v1.2.0",
"v1.2.1",
"v1.2.2",
"v1.2.3",
"v1.2.4",
"v1.3.0",
"v1.3.1",
"v1.3.10",
"v1.3.2",
"v1.3.3",
"v1.3.4",
"v1.3.5",
"v1.3.6",
"v1.3.7",
"v1.3.8",
"v1.3.9",
"v2.0.0",
"v2.0.0-alpha1",
"v2.0.0-alpha2",
"v2.0.0-alpha3",
"v2.0.1",
"v2.0.10",
"v2.0.11",
"v2.0.12",
"v2.0.13",
"v2.0.2",
"v2.0.3",
"v2.0.4",
"v2.0.5",
"v2.0.6",
"v2.0.7",
"v2.0.8",
"v2.0.9",
"v2.1.0",
"v2.1.1",
"v2.1.2",
"v2.1.3",
"v2.1.4",
"v2.1.5",
"v2.1.6",
"v2.1.7",
"v2.1.8",
"v2.1.9",
"v2.2.0",
"v2.2.1",
"v2.2.10",
"v2.2.11",
"v2.2.2",
"v2.2.3",
"v2.2.4",
"v2.2.5",
"v2.2.6",
"v2.2.7",
"v2.2.8",
"v2.2.9",
"v3.0.1",
"v3.0.2",
"v3.0.3",
"v3.0.4",
"v3.0.5",
"v3.0.6",
"v3.0.7"
]
}From 4.0.0 before 4.0.7
Fixed in 4.0.7
7 explicitly affected versions
4.0.0, 4.0.1, 4.0.2, 4.0.3, 4.0.4, 4.0.5, 4.0.6
View affected-version evidence
{
"ranges": [
{
"events": [
{
"introduced": "4.0.0"
},
{
"fixed": "4.0.7"
}
],
"type": "ECOSYSTEM"
}
],
"versions": [
"4.0.0",
"4.0.1",
"4.0.2",
"4.0.3",
"4.0.4",
"4.0.5",
"4.0.6"
]
}From 4.1.0 before 4.1.7
Fixed in 4.1.7
7 explicitly affected versions
4.1.0, 4.1.1, 4.1.2, 4.1.3, 4.1.4, 4.1.5, 4.1.6
View affected-version evidence
{
"ranges": [
{
"events": [
{
"introduced": "4.1.0"
},
{
"fixed": "4.1.7"
}
],
"type": "ECOSYSTEM"
}
],
"versions": [
"4.1.0",
"4.1.1",
"4.1.2",
"4.1.3",
"4.1.4",
"4.1.5",
"4.1.6"
]
}Earlier releases before 3.4.10
Fixed in 3.4.10
16 explicitly affected versions
3.3.0, 3.3.1, 3.3.2, 3.3.3, 3.3.4, 3.3.5, 3.4.0, 3.4.1, 3.4.2, 3.4.3, 3.4.4, 3.4.5, 3.4.6, 3.4.7, 3.4.8, 3.4.9
View affected-version evidence
{
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "3.4.10"
}
],
"type": "ECOSYSTEM"
}
],
"versions": [
"3.3.0",
"3.3.1",
"3.3.2",
"3.3.3",
"3.3.4",
"3.3.5",
"3.4.0",
"3.4.1",
"3.4.2",
"3.4.3",
"3.4.4",
"3.4.5",
"3.4.6",
"3.4.7",
"3.4.8",
"3.4.9"
]
}From 4.0.0 before 4.0.7
Fixed in 4.0.7
7 explicitly affected versions
4.0.0, 4.0.1, 4.0.2, 4.0.3, 4.0.4, 4.0.5, 4.0.6
View affected-version evidence
{
"ranges": [
{
"events": [
{
"introduced": "4.0.0"
},
{
"fixed": "4.0.7"
}
],
"type": "ECOSYSTEM"
}
],
"versions": [
"4.0.0",
"4.0.1",
"4.0.2",
"4.0.3",
"4.0.4",
"4.0.5",
"4.0.6"
]
}From 4.1.0 before 4.1.7
Fixed in 4.1.7
7 explicitly affected versions
4.1.0, 4.1.1, 4.1.2, 4.1.3, 4.1.4, 4.1.5, 4.1.6
View affected-version evidence
{
"ranges": [
{
"events": [
{
"introduced": "4.1.0"
},
{
"fixed": "4.1.7"
}
],
"type": "ECOSYSTEM"
}
],
"versions": [
"4.1.0",
"4.1.1",
"4.1.2",
"4.1.3",
"4.1.4",
"4.1.5",
"4.1.6"
]
}Earlier releases before 3.4.10
Fixed in 3.4.10
131 explicitly affected versions
3.1.0, 3.1.1, 3.1.2, 3.1.3, 3.1.4, 3.1.5, 3.1.6, 3.1.7, 3.2.0, 3.2.1, 3.2.10, 3.2.2, 3.2.3, 3.2.4, 3.2.5, 3.2.6, 3.2.7, 3.2.8, 3.2.9, 3.3.0, 3.3.0-alpha1, 3.3.0-alpha2, 3.3.0-alpha3, 3.3.1, 3.3.2, 3.3.3, 3.3.4, 3.3.5, 3.4.0, 3.4.1, 3.4.2, 3.4.3, 3.4.4, 3.4.5, 3.4.6, 3.4.7, 3.4.8, 3.4.9, v0.0.0, v0.0.1, v0.0.2, v0.1.0, v0.2.0, v0.2.1, v0.2.2, v0.2.3, v0.3.0, v0.3.1, v0.3.2, v0.4.0, v0.4.1, v0.5.0, v0.5.1, v0.5.2, v1.0.0, v1.0.1, v1.0.2, v1.0.3, v1.0.4, v1.0.5, v1.0.6, v1.1.0, v1.1.0-beta1, v1.1.0-beta2, v1.1.0-beta3, v1.1.0-beta4, v1.1.1, v1.1.2, v1.1.3, v1.1.4, v1.2.0, v1.2.1, v1.2.2, v1.2.3, v1.2.4, v1.3.0, v1.3.1, v1.3.10, v1.3.2, v1.3.3, v1.3.4, v1.3.5, v1.3.6, v1.3.7, v1.3.8, v1.3.9, v2.0.0, v2.0.0-alpha1, v2.0.0-alpha2, v2.0.0-alpha3, v2.0.1, v2.0.11, v2.0.12, v2.0.13, v2.0.2, v2.0.3, v2.0.4, v2.0.5, v2.0.6, v2.0.7, v2.0.8, v2.0.9, v2.1.0, v2.1.1, v2.1.2, v2.1.3, v2.1.4, v2.1.5, v2.1.6, v2.1.7, v2.1.8, v2.1.9, v2.2.0, v2.2.1, v2.2.10, v2.2.11, v2.2.2, v2.2.3, v2.2.4, v2.2.5, v2.2.6, v2.2.7, v2.2.8, v2.2.9, v3.0.1, v3.0.2, v3.0.3, v3.0.4, v3.0.5, v3.0.6, v3.0.7
View affected-version evidence
{
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
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}
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"type": "ECOSYSTEM"
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"versions": [
"3.1.0",
"3.1.1",
"3.1.2",
"3.1.3",
"3.1.4",
"3.1.5",
"3.1.6",
"3.1.7",
"3.2.0",
"3.2.1",
"3.2.10",
"3.2.2",
"3.2.3",
"3.2.4",
"3.2.5",
"3.2.6",
"3.2.7",
"3.2.8",
"3.2.9",
"3.3.0",
"3.3.0-alpha1",
"3.3.0-alpha2",
"3.3.0-alpha3",
"3.3.1",
"3.3.2",
"3.3.3",
"3.3.4",
"3.3.5",
"3.4.0",
"3.4.1",
"3.4.2",
"3.4.3",
"3.4.4",
"3.4.5",
"3.4.6",
"3.4.7",
"3.4.8",
"3.4.9",
"v0.0.0",
"v0.0.1",
"v0.0.2",
"v0.1.0",
"v0.2.0",
"v0.2.1",
"v0.2.2",
"v0.2.3",
"v0.3.0",
"v0.3.1",
"v0.3.2",
"v0.4.0",
"v0.4.1",
"v0.5.0",
"v0.5.1",
"v0.5.2",
"v1.0.0",
"v1.0.1",
"v1.0.2",
"v1.0.3",
"v1.0.4",
"v1.0.5",
"v1.0.6",
"v1.1.0",
"v1.1.0-beta1",
"v1.1.0-beta2",
"v1.1.0-beta3",
"v1.1.0-beta4",
"v1.1.1",
"v1.1.2",
"v1.1.3",
"v1.1.4",
"v1.2.0",
"v1.2.1",
"v1.2.2",
"v1.2.3",
"v1.2.4",
"v1.3.0",
"v1.3.1",
"v1.3.10",
"v1.3.2",
"v1.3.3",
"v1.3.4",
"v1.3.5",
"v1.3.6",
"v1.3.7",
"v1.3.8",
"v1.3.9",
"v2.0.0",
"v2.0.0-alpha1",
"v2.0.0-alpha2",
"v2.0.0-alpha3",
"v2.0.1",
"v2.0.11",
"v2.0.12",
"v2.0.13",
"v2.0.2",
"v2.0.3",
"v2.0.4",
"v2.0.5",
"v2.0.6",
"v2.0.7",
"v2.0.8",
"v2.0.9",
"v2.1.0",
"v2.1.1",
"v2.1.2",
"v2.1.3",
"v2.1.4",
"v2.1.5",
"v2.1.6",
"v2.1.7",
"v2.1.8",
"v2.1.9",
"v2.2.0",
"v2.2.1",
"v2.2.10",
"v2.2.11",
"v2.2.2",
"v2.2.3",
"v2.2.4",
"v2.2.5",
"v2.2.6",
"v2.2.7",
"v2.2.8",
"v2.2.9",
"v3.0.1",
"v3.0.2",
"v3.0.3",
"v3.0.4",
"v3.0.5",
"v3.0.6",
"v3.0.7"
]
}From 4.0.0 before 4.0.7
Fixed in 4.0.7
7 explicitly affected versions
4.0.0, 4.0.1, 4.0.2, 4.0.3, 4.0.4, 4.0.5, 4.0.6
View affected-version evidence
{
"ranges": [
{
"events": [
{
"introduced": "4.0.0"
},
{
"fixed": "4.0.7"
}
],
"type": "ECOSYSTEM"
}
],
"versions": [
"4.0.0",
"4.0.1",
"4.0.2",
"4.0.3",
"4.0.4",
"4.0.5",
"4.0.6"
]
}From 4.1.0 before 4.1.7
Fixed in 4.1.7
7 explicitly affected versions
4.1.0, 4.1.1, 4.1.2, 4.1.3, 4.1.4, 4.1.5, 4.1.6
View affected-version evidence
{
"ranges": [
{
"events": [
{
"introduced": "4.1.0"
},
{
"fixed": "4.1.7"
}
],
"type": "ECOSYSTEM"
}
],
"versions": [
"4.1.0",
"4.1.1",
"4.1.2",
"4.1.3",
"4.1.4",
"4.1.5",
"4.1.6"
]
}