Symfony: Mailomat Mailer Webhook Parser Reads the HMAC Algorithm from the Request: Signature Algorithm Downgrade

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

not scored

EPSS

• CVE-2026-48747: 0.25% estimated exploitation activity in the next 30 days; percentile 16.09 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Description Symfony\Component\Mailer\Bridge\Mailomat\Webhook\MailomatRequestParser::validateSignature() parses the X-MOM-Webhook-Signature request header as algo=signature and passes the wire-supplied $algo directly to hashhmac() when verifying the request against the configured webhook secret. The request therefore selects the HMAC primitive used to authenticate it. PHP's hashhmac() enforces only that the chosen algorithm is HMAC-compatible. That set still includes primitives with known cryptanalysis (md4, md5, ripemd128, tiger128,3, … — e.g. existential forgery of HMAC-MD4, Contini & Yin, ASIACRYPT 2006). This is the canonical algorithm-confusion shape, analogous to JWT alg=none / alg=HS256 downgrades: any future cryptographic weakness in any HMAC primitive PHP exposes becomes immediately exploitable against a Mailomat webhook receiver, the moment an attacker is in a position to compute a signature for that primitive, without a code change on the Symfony side. Mailomat's documented…

Published by the source

15 Jun 2026, 17:32 UTC

Last modified by the source

15 Jun 2026, 17:32 UTC

Source last checked

26 Oct 2026, 14:49 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-rrj9-5q2j-4gvr

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# Patch Next advisory brief: CVE-2026-48747

Advisory: Symfony: Mailomat Mailer Webhook Parser Reads the HMAC Algorithm from the Request: Signature Algorithm Downgrade
IDs: CVE-2026-48747, GHSA-rrj9-5q2j-4gvr
Record: https://patchnext.com/advisories/cve-2026-48747 (revision 1, source checked 26 Oct 2026, 14:49 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Unrated
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: symfony/symfony 8.0.13, 7.4.13; symfony/mailomat-mailer 8.0.13, 7.4.13. A vendor fix does not mean a deployment is fixed.

## Affected components
- Packagist symfony/symfony: affected >= 7.2.0, < 7.4.13; >= 8.0.0, < 8.0.13; fixed in 8.0.13, 7.4.13
- Packagist symfony/mailomat-mailer: affected >= 7.2.0, < 7.4.13; >= 8.0.0, < 8.0.13; fixed in 8.0.13, 7.4.13

## Suggested inspection (read-only)
- Packagist symfony/symfony: check the installed version with your package manager.
- Packagist symfony/mailomat-mailer: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on symfony/symfony, symfony/mailomat-mailer.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-rrj9-5q2j-4gvr (GitHub-reviewed advisory GHSA-rrj9-5q2j-4gvr lists affected ranges: Packagist symfony/mailomat-mailer >=7.2.0 =8.0.0 =7.2.0 =8.0.0 <8.0.13)

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