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

> Attackers can bypass JWT signature verification by overriding the algorithm in unprotected headers, allowing token forgery.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 18 Jun 2026

## What is GHSA-jc38-x7x8-2xc8?

GHSA-jc38-x7x8-2xc8 is a vulnerability in web-token/jwt-bundle, published on 18 Jun 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

## Summary

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

```php
$completeHeader = [...$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:
1. `HeaderCheckerManager` validates `alg` from the **protected** header
2. `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`:
```php
$completeHeader = [...$signature->getProtectedHeader(), ...$signature->getHeader()];
```

**JWEDecrypter.php lines 120-124** — `array_merge()` with same last-wins behavior:
```php
$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
<?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:

```php
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`.

## Which versions are affected by GHSA-jc38-x7x8-2xc8?

24 open source packages ship an affected version, including web-token/jwt-bundle, web-token/jwt-experimental and web-token/jwt-framework.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `web-token/jwt-bundle` | Packagist | All versions | 3.4.10 |
| `web-token/jwt-bundle` | Packagist | All versions | 3.4.10 |
| `web-token/jwt-bundle` | Packagist | From 4.0.0 | 4.0.7 |
| `web-token/jwt-bundle` | Packagist | From 4.0.0 | 4.0.7 |
| `web-token/jwt-bundle` | Packagist | From 4.1.0 | 4.1.7 |
| `web-token/jwt-bundle` | Packagist | From 4.1.0 | 4.1.7 |
| `web-token/jwt-experimental` | Packagist | All versions | 3.4.10 |
| `web-token/jwt-experimental` | Packagist | All versions | 3.4.10 |
| `web-token/jwt-experimental` | Packagist | From 4.0.0 | 4.0.7 |
| `web-token/jwt-experimental` | Packagist | From 4.0.0 | 4.0.7 |
| `web-token/jwt-experimental` | Packagist | From 4.1.0 | 4.1.7 |
| `web-token/jwt-experimental` | Packagist | From 4.1.0 | 4.1.7 |
| `web-token/jwt-framework` | Packagist | All versions | 3.4.10 |
| `web-token/jwt-framework` | Packagist | All versions | 3.4.10 |
| `web-token/jwt-framework` | Packagist | From 4.0.0 | 4.0.7 |
| `web-token/jwt-framework` | Packagist | From 4.0.0 | 4.0.7 |
| `web-token/jwt-framework` | Packagist | From 4.1.0 | 4.1.7 |
| `web-token/jwt-framework` | Packagist | From 4.1.0 | 4.1.7 |
| `web-token/jwt-library` | Packagist | All versions | 3.4.10 |
| `web-token/jwt-library` | Packagist | All versions | 3.4.10 |
| `web-token/jwt-library` | Packagist | From 4.0.0 | 4.0.7 |
| `web-token/jwt-library` | Packagist | From 4.0.0 | 4.0.7 |
| `web-token/jwt-library` | Packagist | From 4.1.0 | 4.1.7 |
| `web-token/jwt-library` | Packagist | From 4.1.0 | 4.1.7 |

## How do I fix GHSA-jc38-x7x8-2xc8?

Upgrade web-token/jwt-bundle to a fixed version for your release line: 3.4.10, 4.0.7 or 4.1.7.

## Is GHSA-jc38-x7x8-2xc8 being exploited?

There is no known exploitation of GHSA-jc38-x7x8-2xc8 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for GHSA-jc38-x7x8-2xc8 and fix it if it is affected.

What it is: GHSA-jc38-x7x8-2xc8 is a vulnerability in web-token/jwt-bundle, published on 18 Jun 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of GHSA-jc38-x7x8-2xc8 in the wild and no public exploit code.

Affected packages and fixed versions:
- web-token/jwt-bundle (Packagist): affected all versions; fixed in 3.4.10
- web-token/jwt-bundle (Packagist): affected all versions; fixed in 3.4.10
- web-token/jwt-bundle (Packagist): affected from 4.0.0; fixed in 4.0.7
- web-token/jwt-bundle (Packagist): affected from 4.0.0; fixed in 4.0.7
- web-token/jwt-bundle (Packagist): affected from 4.1.0; fixed in 4.1.7
- web-token/jwt-bundle (Packagist): affected from 4.1.0; fixed in 4.1.7
- web-token/jwt-experimental (Packagist): affected all versions; fixed in 3.4.10
- web-token/jwt-experimental (Packagist): affected all versions; fixed in 3.4.10
- web-token/jwt-experimental (Packagist): affected from 4.0.0; fixed in 4.0.7
- web-token/jwt-experimental (Packagist): affected from 4.0.0; fixed in 4.0.7
- web-token/jwt-experimental (Packagist): affected from 4.1.0; fixed in 4.1.7
- web-token/jwt-experimental (Packagist): affected from 4.1.0; fixed in 4.1.7

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/GHSA-jc38-x7x8-2xc8/ (Markdown: https://insights.sbomify.com/GHSA-jc38-x7x8-2xc8/index.md)
```

## Sources

- OSV: https://osv.dev/vulnerability/GHSA-jc38-x7x8-2xc8

---
From sbomify Insights: https://insights.sbomify.com/GHSA-jc38-x7x8-2xc8/
Summaries are generated and may contain mistakes; the official description above is the record.