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How CVE-2026-45755 Works
Symfony is a widely-used PHP framework that provides a set of reusable components, including a Mailtrap mailer bridge. This bridge ships a webhook request parser (MailtrapRequestParser) which is designed to authenticate and decode event callbacks that Mailtrap POSTs to an application’s webhook endpoint.
The vulnerability exists in the `MailtrapRequestParser::doParse(Request $request, [\SensitiveParameter] string $secret)` method. This method receives the configured webhook secret as a parameter, but critically, it never reads or uses this secret to verify incoming requests. Instead, it decodes and returns the payload unconditionally, completely ignoring the `X-Mt-Signature` HMAC header that Mailtrap sends with each legitimate webhook request.
Under normal, secure operation, when a webhook secret is configured, the application should compute an HMAC-SHA256 of the raw request body using the secret and compare it against the `X-Mt-Signature` header using a constant-time comparison to prevent timing attacks. However, in vulnerable versions, this verification step is entirely absent. The `doParse()` method simply parses the request body and returns the event data without any cryptographic signature validation.
This means that any application that wires up the Mailtrap webhook endpoint will accept any POST request sent to that URL, regardless of whether it originates from Mailtrap or not. An attacker who discovers or guesses the webhook endpoint URL can submit forged event payloads, impersonating Mailtrap and injecting arbitrary delivery, bounce, open, click, or spam events into the application.
The impact is severe because these forged events can corrupt suppression lists (marking legitimate emails as bounced or spam), skew delivery metrics, trigger automated workflows based on false events, and potentially lead to denial of service or business logic abuse. The vulnerability affects all Symfony applications using the Mailtrap mailer bridge with a configured webhook secret prior to patched versions. The fix ensures that `doParse()` now requires and verifies the `X-Mt-Signature` header using HMAC-SHA256 before decoding the payload, and performs the comparison in constant time. If no secret is configured, the behavior remains unchanged (signature verification is opt-in), but once a secret is set, it is now actually enforced.
DailyCVE Form:
Platform: Symfony/PHP
Version: <7.4.12, <8.0.12
Vulnerability: Missing Auth
Severity: Moderate (CVSS 5.3)
date: 2026-07-14
Prediction: 2026-05-28 (patch)
What Undercode Say: Analytics
Check installed Symfony Mailtrap Mailer version
composer show symfony/mailtrap-mailer
Check symfony/symfony full-stack framework version
composer show symfony/symfony
Verify if webhook secret is configured (should be in .env)
grep MAILTRAP_WEBHOOK_SECRET .env
Test for vulnerability by sending an unsigned POST request
curl -X POST https://your-app.com/webhook/mailtrap \
-H "Content-Type: application/json" \
-d '{"event":"delivery","email":"[email protected]"}'
Monitor webhook access logs for unauthorized POSTs
grep "POST /webhook/mailtrap" /var/log/nginx/access.log
Affected Packages & Versions (Composer):
symfony/mailtrap-mailer: >=7.2.0, <7.4.12 | >=8.0.0, <8.0.12symfony/symfony: >=7.2.0, <7.4.12 | >=8.0.0, <8.0.12
CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N (Score: 5.3 Medium)
CWE: CWE-306 (Missing Authentication for Critical Function), CWE-347 (Improper Verification of Cryptographic Signature)
EPSS: 0.03% (probability of exploitation in the wild)
Exploit:
An attacker can exploit this vulnerability by sending a crafted HTTP POST request to the application’s Mailtrap webhook endpoint. Since the `doParse()` method ignores the `X-Mt-Signature` header, no cryptographic proof of authenticity is required. The attacker only needs to know or guess the webhook URL (often predictable, e.g., /webhook/mailtrap, /mailtrap/webhook, or as configured in routing).
Example exploitation steps:
- Identify the target webhook endpoint (e.g., through directory enumeration, source code leaks, or common patterns).
- Craft a JSON payload mimicking Mailtrap’s event structure. For example, a forged “bounce” event:
{ "event": "bounce", "email": "[email protected]", "bounce_type": "hard", "reason": "Mailbox unavailable" } - Send an unsigned POST request to the endpoint:
curl -X POST https://target.com/webhook/mailtrap \ -H "Content-Type: application/json" \ -d '{"event":"bounce","email":"[email protected]"}' - The application accepts the forged event, potentially marking the victim’s email as bounced, removing them from mailing lists, or triggering other automated actions.
The lack of signature verification means anyone on the internet can inject events, leading to data corruption and operational disruption.
Protection:
- Immediate: Upgrade to Symfony version 7.4.12 or 8.0.12 (or later) immediately.
composer require symfony/mailtrap-mailer:^7.4.12 or for full framework composer require symfony/symfony:^7.4.12
- If upgrading is not possible, apply the patch manually: modify `MailtrapRequestParser::doParse()` to verify the `X-Mt-Signature` header using HMAC-SHA256 with the configured secret, using a constant-time comparison function like
hash_equals().$signature = $request->headers->get('X-Mt-Signature'); $expected = hash_hmac('sha256', $request->getContent(), $secret); if (!hash_equals($expected, $signature)) { throw new AccessDeniedException('Invalid webhook signature.'); } - As a temporary workaround, use a web application firewall (WAF) or API gateway to filter requests to the webhook endpoint by source IP (allow only Mailtrap’s IP ranges) or by requiring an additional shared secret in a custom header.
- Review your application’s webhook handling logic to ensure that event data is not blindly trusted; implement additional business-level validation (e.g., verify that the email address exists in your system before processing a bounce).
- Monitor and audit webhook endpoints for unusual POST activity.
Impact:
- Suppression List Corruption: Attackers can forge bounce or spam events, causing legitimate email addresses to be removed from mailing lists or added to suppression lists, resulting in loss of customer reach and revenue.
- Delivery Metrics Fraud: Fake delivery, open, and click events can inflate metrics, misleading marketing teams and damaging analytics-based decision-making.
- Business Logic Abuse: If webhook events trigger automated workflows (e.g., updating order status, sending follow-up emails, or provisioning services), forged events can cause unintended actions, leading to financial loss or reputational damage.
- Data Integrity Compromise: The application’s internal state becomes untrustworthy, as event data can be arbitrarily injected without authentication.
- Compliance Violations: In regulated industries, processing falsified event data may violate data protection or anti-fraud compliance requirements.
- Wide Attack Surface: The vulnerability affects any Symfony application using the Mailtrap mailer bridge with a configured webhook secret, which is a recommended setup, making a large number of applications potentially vulnerable.
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Sources:
Reported By: nvd.nist.gov
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