{"id":"GHSA-rrj9-5q2j-4gvr","aliases":["CVE-2026-48747"],"title":"Symfony: Mailomat Mailer Webhook Parser Reads the HMAC Algorithm from the Request: Signature Algorithm Downgrade","description":"### Description\n\n`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 `hash_hmac()` when verifying the request against the configured webhook secret. The request therefore selects the HMAC primitive used to authenticate it.\n\nPHP's `hash_hmac()` 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.\n\nMailomat's [documented webhook security](https://api.mailomat.swiss/docs/#tag/webhook-security) pins SHA-256; the parser did not.\n\n### Resolution\n\n`MailomatRequestParser::validateSignature()` now requires the signature header to be of the form `sha256=<hex>` and verifies the signature with HMAC-SHA256 keyed by the configured secret using a constant-time comparison. Any other algorithm declared on the wire (including the HMAC primitives PHP would otherwise accept) is rejected.\n\nThe patch for this issue is available [here](https://github.com/symfony/symfony/commit/bdfe9fe0d94d33dfaca0bc2fe0b00b54767b0c88) for branch 7.4 (and forward-ported to 8.0 and 8.1).\n\n### Credits\n\nSymfony would like to thank Omar Alshammari, Essam Alanazi and Alwaleed Alshammari for reporting the issue and Nicolas Grekas for providing the fix.","severity":"medium","score":null,"published":"2026-06-15T17:32:28.000Z","modified":"2026-09-10T03:50:50.617Z","withdrawn":false,"knownExploited":false,"affected":[{"ecosystem":"Packagist","name":"symfony/mailomat-mailer","ranges":[{"events":[{"introduced":"7.2.0"},{"fixed":"7.4.13"}],"type":"ECOSYSTEM"}],"versions":["v7.2.0","v7.3.0","v7.3.0-BETA1","v7.3.0-RC1","v7.4.0","v7.4.0-BETA1","v7.4.0-RC1"],"fixed":["7.4.13"]},{"ecosystem":"Packagist","name":"symfony/mailomat-mailer","ranges":[{"events":[{"introduced":"8.0.0"},{"fixed":"8.0.13"}],"type":"ECOSYSTEM"}],"versions":["v8.0.0"],"fixed":["8.0.13"]},{"ecosystem":"Packagist","name":"symfony/symfony","ranges":[{"events":[{"introduced":"7.2.0"},{"fixed":"7.4.13"}],"type":"ECOSYSTEM"}],"versions":["v7.2.0","v7.2.1","v7.2.2","v7.2.3","v7.2.4","v7.2.5","v7.2.6","v7.2.7","v7.2.8","v7.2.9","v7.3.0","v7.3.0-BETA1","v7.3.0-BETA2","v7.3.0-RC1","v7.3.1","v7.3.10","v7.3.11","v7.3.2","v7.3.3","v7.3.4","v7.3.5","v7.3.6","v7.3.7","v7.3.8","v7.3.9","v7.4.0","v7.4.0-BETA1","v7.4.0-BETA2","v7.4.0-RC1","v7.4.0-RC2","v7.4.0-RC3","v7.4.1","v7.4.10","v7.4.11","v7.4.12","v7.4.2","v7.4.3","v7.4.4","v7.4.5","v7.4.6","v7.4.7","v7.4.8","v7.4.9"],"fixed":["7.4.13"]},{"ecosystem":"Packagist","name":"symfony/symfony","ranges":[{"events":[{"introduced":"8.0.0"},{"fixed":"8.0.13"}],"type":"ECOSYSTEM"}],"versions":["v8.0.0","v8.0.1","v8.0.10","v8.0.11","v8.0.12","v8.0.2","v8.0.3","v8.0.4","v8.0.5","v8.0.6","v8.0.7","v8.0.8","v8.0.9"],"fixed":["8.0.13"]}],"references":["https://github.com/symfony/symfony/security/advisories/GHSA-rrj9-5q2j-4gvr","https://github.com/symfony/symfony/commit/bdfe9fe0d94d33dfaca0bc2fe0b00b54767b0c88","https://github.com/FriendsOfPHP/security-advisories/blob/master/symfony/mailomat-mailer/CVE-2026-48747.yaml","https://github.com/FriendsOfPHP/security-advisories/blob/master/symfony/symfony/CVE-2026-48747.yaml","https://github.com/symfony/symfony","https://symfony.com/cve-2026-48747"],"sources":[{"name":"OSV","id":"GHSA-rrj9-5q2j-4gvr","modified":"2026-09-10T03:50:50.617414342Z","url":"https://osv.dev/vulnerability/GHSA-rrj9-5q2j-4gvr"}],"assessments":[]}