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The core of this vulnerability lies in the `Symfony\Component\Mailer\Bridge\Mailomat\Webhook\MailomatRequestParser::validateSignature()` method. To authenticate incoming webhooks, this parser reads the `X-MOM-Webhook-Signature` HTTP header. The expected format of this header is algo=signature, for example, sha256=abc123.... It then extracts the `algo` part and passes it, along with the signature itself, directly to PHP’s `hash_hmac()` function. The `hash_hmac()` function is designed to be flexible, accepting any algorithm that the system’s OpenSSL or Hash extension supports, as long as it is HMAC-compatible. This includes a wide range of algorithms, from modern standards like SHA-256 to legacy and cryptographically broken ones like MD4, MD5, and RIPEMD-128.
Critically, this design allows the remote sender of the webhook to dictate which HMAC algorithm is used for signature verification. While the webhook secret is a shared, server-side secret, an attacker could send a webhook with a header instructing the parser to use a weak algorithm, such as md5=<forged_signature>. If the server’s environment has not been hardened to remove these weak algorithms (and most default PHP installations have them enabled), `hash_hmac()` will happily attempt to verify using MD5. This creates a classic “algorithm confusion” vulnerability.
The security implication is profound. An attacker in a position to observe a single legitimate signed webhook could compute a valid HMAC-MD5 for a forged payload. Even without a known practical attack for MD5 today, the vulnerability is future-facing: any new cryptanalytic weakness discovered in any HMAC-compatible algorithm in the future would immediately become an exploit vector against all unpatched Symfony applications. This is analogous to the well-known `alg=none` attack against poorly implemented JWT libraries. The vendor documentation explicitly states that webhooks should be secured with SHA-256, yet the code failed to enforce this. The fix, therefore, is to reject any algorithm other than `sha256` and perform the comparison in constant time to prevent timing attacks.
DailyCVE Form:
Platform: Symfony Mailer Bridge
Version: <7.4.13, <8.0.13
Vulnerability: Algorithm confusion
Severity: Medium
date: 2026-05-27
Prediction: 2026-05-27
What Undercode Say:
To check if your Symfony application is vulnerable, you can search the source code for the vulnerable method. Alternatively, a dynamic test can be performed by sending a crafted webhook request.
Check for the vulnerable code directly in the vendor directory
grep -r "MailomatRequestParser" vendor/symfony/mailer/
A conceptual test to send a forged webhook using a weak algorithm
This command is a conceptual example and requires a valid webhook URL and signature
curl -X POST https://your-app.com/webhook/mailomat \
-H "X-MOM-Webhook-Signature: md5=fake_signature" \
-H "Content-Type: application/json" \
-d '{"event": "forged_payload"}'
Exploit:
An exploit would require the attacker to know the shared webhook secret. However, the attack is a downgrade attack:
1. The attacker intercepts a legitimate, signed webhook request.
2. They replace the `X-MOM-Webhook-Signature` header from `sha256=md4=<valid_sig>.
3. The server-side parser uses the attacker-supplied ‘md4’ algorithm to recompute the HMAC with the shared secret. If a collision or forgery attack against HMAC-MD4 exists, the attacker could forge a valid signature for a malicious payload.
Protection:
- Immediate Update: Upgrade the `symfony/mailer` package to version `7.4.13` or `8.0.13` or later.
- Patch: If an immediate upgrade is not possible, apply the patch that replaces the dynamic `hash_hmac()` call with hardcoded `hash_hmac(‘sha256’, $payload, $secret, true)` and a constant-time comparison.
- Configuration (if patched): Ensure your webhook secret is a strong, randomly generated string.
Impact:
A remote, unauthenticated attacker could potentially bypass webhook signature verification. This could allow them to:
1. Inject forged webhook events, leading to unauthorized actions within the application (e.g., marking an unpaid invoice as paid, granting premium features, or triggering unintended workflows).
2. Exploit future cryptographic weaknesses in legacy HMAC algorithms to achieve signature forgery.
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Sources:
Reported By: github.com
Extra Source Hub:
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