Skip to content

[GHSA-jc38-x7x8-2xc8] PHP JWT Framework: JWSVerifier uses algorithm from unprotected header, enabling algorithm confusion attacks #8484

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Open
samuelwei wants to merge 1 commit into
base: samuelwei/advisory-improvement-8484
Choose a base branch
from
Open

[GHSA-jc38-x7x8-2xc8] PHP JWT Framework: JWSVerifier uses algorithm from unprotected header, enabling algorithm confusion attacks #8484

Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
160 changes: 156 additions & 4 deletions advisories/github-reviewed/2026/06/GHSA-jc38-x7x8-2xc8/GHSA-jc38-x7x8-2xc8.json
View file
Open in desktop
Original file line number Diff line number Diff line change
@@ -1,15 +1,15 @@
{
"schema_version": "1.4.0",
"id": "GHSA-jc38-x7x8-2xc8",
"modified": "2026-06-18T21:09:18Z",
"modified": "2026-06-18T21:09:20Z",
"published": "2026-06-18T21:09:17Z",
"aliases": [],
"summary": "PHP JWT Framework: JWSVerifier uses algorithm from unprotected header, enabling algorithm confusion attacks",
"details": "## Summary\n\n`JWSVerifier::getAlgorithm()` in `src/Library/Signature/JWSVerifier.php` (line 144) merges protected and unprotected headers using PHP's spread operator:\n\n```php\n$completeHeader = [...$signature->getProtectedHeader(), ...$signature->getHeader()];\n```\n\nIn PHP, when spreading arrays with duplicate string keys, the **last array's values take precedence**. Since the unprotected header (`getHeader()`) is spread second, an attacker can override the integrity-protected `alg` parameter by placing a different value in the unprotected header.\n\nThis creates a Time-of-Check/Time-of-Use (TOCTOU) vulnerability:\n1. `HeaderCheckerManager` validates `alg` from the **protected** header\n2. `JWSVerifier` uses `alg` from the **unprotected** header for actual verification\n\nThe same issue exists in `JWEDecrypter.php` (lines 120-124) where `array_merge()` exhibits the same last-wins behavior for `alg` and `enc`.\n\n## Affected Code\n\n**JWSVerifier.php line 144** — Spread operator merge order allows unprotected header to override `alg`:\n```php\n$completeHeader = [...$signature->getProtectedHeader(), ...$signature->getHeader()];\n```\n\n**JWEDecrypter.php lines 120-124** — `array_merge()` with same last-wins behavior:\n```php\n$completeHeader = array_merge(\n $jwe->getSharedProtectedHeader(),\n $jwe->getSharedHeader(),\n $recipient->getHeader()\n);\n```\n\n## Attack Vectors\n\n### Vector A — Mixed key sets (HIGH probability)\nIf the application uses a JWKSet containing keys of different types (common in multi-tenant or federation scenarios), the JWSVerifier iterates all keys (line 86). An attacker can force a different algorithm that matches a different key in the set.\n\n### Vector B — alg ONLY in unprotected header (HIGH probability)\nIf `alg` is placed EXCLUSIVELY in the unprotected header (not in the protected header at all), `HeaderCheckerManager::checkDuplicatedHeaderParameters()` does NOT trigger. The JSON Flattened/General serializers allow tokens with no protected header or a protected header without `alg`. RFC 7515 Section 4.1.1 states `alg` MUST be integrity-protected, but the library does not enforce this.\n\n### Vector C — Direct JWSVerifier usage (HIGH probability)\n`JWSLoader` takes `?HeaderCheckerManager` (nullable). If developers use `JWSVerifier` directly or create `JWSLoader` without a `HeaderCheckerManager`, the duplicate header check never runs.\n\n## Contrast with JWSBuilder (safe)\n\n`JWSBuilder::findSignatureAlgorithm()` (line 196) uses `[...$header, ...$protectedHeader]` where protected wins. It also has `checkDuplicatedHeaderParameters()` (line 218). The JWSVerifier has **neither** safeguard.\n\n## Proof of Concept\n\n```php\n<?php\n// Demonstrate algorithm override via unprotected header\n$protected = [\"alg\" => \"RS256\", \"typ\" => \"JWT\"];\n$unprotected = [\"alg\" => \"HS256\"];\n$merged = [...$protected, ...$unprotected];\n// $merged[\"alg\"] === \"HS256\" — unprotected wins!\n\n// JSON Flattened JWS with algorithm override:\n$maliciousJws = json_encode([\n 'payload' => base64url_encode($payload),\n 'protected' => base64url_encode('{\"alg\":\"RS256\"}'),\n 'header' => ['alg' => 'HS256'], // OVERRIDE\n 'signature' => base64url_encode($sig),\n]);\n// HeaderCheckerManager validates RS256 from protected header -> PASS\n// JWSVerifier uses HS256 from unprotected header -> attacker's algorithm choice\n```\n\nA full working PoC demonstrating HS512-to-HS256 downgrade with mixed keysets is available upon request.\n\n## Suggested Fix\n\nIn `JWSVerifier::getAlgorithm()`, read `alg` exclusively from the protected header:\n\n```php\nprivate function getAlgorithm(Signature $signature): Algorithm\n{\n $protectedHeader = $signature->getProtectedHeader();\n if (! isset($protectedHeader['alg'])) {\n throw new InvalidArgumentException('The \"alg\" parameter must be in the protected header.');\n }\n return $this->signatureAlgorithmManager->get($protectedHeader['alg']);\n}\n```\n\nFor `JWEDecrypter`, reverse the merge order so protected header wins, or extract `alg`/`enc` exclusively from the protected header.\n\n## Résolution\n\nUn 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).\n\n**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.\n\n**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`.",
"severity": [
{
"type": "CVSS_V4",
"score": "CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N/E:P"
"score": "CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N"
}
],
"affected": [
Expand All @@ -26,7 +26,159 @@
"introduced": "0"
},
{
"last_affected": "4.2.99"
"fixed": "3.4.10"
}
]
}
]
},
{
"package": {
"ecosystem": "Packagist",
"name": "web-token/jwt-framework"
},
"ranges": [
{
"type": "ECOSYSTEM",
"events": [
{
"introduced": "4.0.0"
},
{
"fixed": "4.0.7"
}
]
}
]
},
{
"package": {
"ecosystem": "Packagist",
"name": "web-token/jwt-framework"
},
"ranges": [
{
"type": "ECOSYSTEM",
"events": [
{
"introduced": "4.1.0"
},
{
"fixed": "4.1.7"
}
]
}
]
},
{
"package": {
"ecosystem": "Packagist",
"name": "web-token/jwt-bundle"
},
"ranges": [
{
"type": "ECOSYSTEM",
"events": [
{
"introduced": "0"
},
{
"fixed": "3.4.10"
}
]
}
]
},
{
"package": {
"ecosystem": "Packagist",
"name": "web-token/jwt-bundle"
},
"ranges": [
{
"type": "ECOSYSTEM",
"events": [
{
"introduced": "4.0.0"
},
{
"fixed": "4.0.7"
}
]
}
]
},
{
"package": {
"ecosystem": "Packagist",
"name": "web-token/jwt-bundle"
},
"ranges": [
{
"type": "ECOSYSTEM",
"events": [
{
"introduced": "4.1.0"
},
{
"fixed": "4.1.7"
}
]
}
]
},
{
"package": {
"ecosystem": "Packagist",
"name": "web-token/jwt-experimental"
},
"ranges": [
{
"type": "ECOSYSTEM",
"events": [
{
"introduced": "0"
},
{
"fixed": "3.4.10"
}
]
}
]
},
{
"package": {
"ecosystem": "Packagist",
"name": "web-token/jwt-experimental"
},
"ranges": [
{
"type": "ECOSYSTEM",
"events": [
{
"introduced": "4.0.0"
},
{
"fixed": "4.0.7"
}
]
}
]
},
{
"package": {
"ecosystem": "Packagist",
"name": "web-token/jwt-experimental"
},
"ranges": [
{
"type": "ECOSYSTEM",
"events": [
{
"introduced": "4.1.0"
},
{
"fixed": "4.1.7"
}
]
}
Expand Down Expand Up @@ -108,7 +260,7 @@
"cwe_ids": [
"CWE-345"
],
"severity": "HIGH",
"severity": "CRITICAL",
"github_reviewed": true,
"github_reviewed_at": "2026-06-18T21:09:17Z",
"nvd_published_at": null
Expand Down