{"id":"CVE-2026-48747","aliases":["GHSA-rrj9-5q2j-4gvr"],"title":"Symfony: Mailomat Mailer Webhook Parser Reads the HMAC Algorithm from the Request: Signature Algorithm Downgrade","summary":"Symfony: Mailomat Mailer Webhook Parser Reads the HMAC Algorithm from the Request: Signature Algorithm Downgrade","severity":"medium","cwe":["CWE-347","CWE-757"],"vendor":"symfony","product":"symfony/mailomat-mailer","ecosystem":"composer","affected":["symfony/mailomat-mailer >= 7.2.0, < 7.4.13","symfony/mailomat-mailer >= 8.0.0, < 8.0.13","symfony/symfony >= 7.2.0, < 7.4.13","symfony/symfony >= 8.0.0, < 8.0.13"],"patched":["symfony/mailomat-mailer 7.4.13","symfony/mailomat-mailer 8.0.13","symfony/symfony 7.4.13","symfony/symfony 8.0.13"],"published":"2026-06-15","updated":"2026-06-15","source":"GHSA","sourceUrl":"https://github.com/advisories/GHSA-rrj9-5q2j-4gvr","references":[{"url":"https://github.com/symfony/symfony/security/advisories/GHSA-rrj9-5q2j-4gvr"},{"url":"https://github.com/symfony/symfony/commit/bdfe9fe0d94d33dfaca0bc2fe0b00b54767b0c88"},{"url":"https://github.com/FriendsOfPHP/security-advisories/blob/master/symfony/mailomat-mailer/CVE-2026-48747.yaml"},{"url":"https://github.com/FriendsOfPHP/security-advisories/blob/master/symfony/symfony/CVE-2026-48747.yaml"},{"url":"https://symfony.com/cve-2026-48747"},{"url":"https://github.com/advisories/GHSA-rrj9-5q2j-4gvr"}],"tags":["ghsa","composer"],"ingestedAt":"2026-07-07T15:41:58.732Z","epss":0.00247,"epssPercentile":0.1629,"slug":"CVE-2026-48747","body":"## Overview\n\n### 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.\n\n## Affected packages\n\n- `symfony/mailomat-mailer >= 7.2.0, < 7.4.13`\n- `symfony/mailomat-mailer >= 8.0.0, < 8.0.13`\n- `symfony/symfony >= 7.2.0, < 7.4.13`\n- `symfony/symfony >= 8.0.0, < 8.0.13`\n\n## Remediation\n\nUpgrade to a patched release:\n\n- `symfony/mailomat-mailer 7.4.13`\n- `symfony/mailomat-mailer 8.0.13`\n- `symfony/symfony 7.4.13`\n- `symfony/symfony 8.0.13`","depth":"sunlit","depthScore":28,"depthScoreParts":{"impact":27.5,"likelihood":0,"exploitation":0,"ransomware":0},"changes":[]}