CVE-2026-121971 Affecting symfony package, versions <6.4.50+dfsg-0+deb13u1

Note: Versions mentioned in the description apply only to the upstream symfony package and not the symfony package as distributed by Echo. See How to fix? for Echo:latest relevant fixed versions and status.

The component ScopingHttpClient in Symfony HttpClient merges the options of its default scope (such as auth_bearer or auth_basic) into a request before resolving a relative URL against the base_uri, allowing a network-path reference such as "//evil.example/steal" to be sent to another origin together with the default scope’s credentials, and resulting in potential disclosure of those credentials to an attacker-controlled server. This affects Symfony in versions 5.x prior to 5.4.56, 6.x prior to 6.4.50, 7.x prior to 7.4.23, and 8.x prior to 8.1.11.

The resolved URL falls outside the origin of the base URI, yet it keeps the merged options, whereas the absolute spelling "https://evil.example/steal&#34; receives none. The same happens with a different port or user-info part on the same host.

This is reachable whenever an attacker controls the beginning of a URL passed to a client built with a base_uri and credentials, including those created with HttpClient::createForBaseUri() or configured under framework.http_client.scoped_clients.

To solve the issue, after resolving a relative URL against the base URI of the default scope, ScopingHttpClient now compares the scheme and authority of the result with those of the base URI, parsed the way the request is sent.

• https://advisory.echohq.com/cve/CVE-2026-121971
• https://github.com/symfony/symfony/commit/64c34e85bd03b3f8a735d8e1af393659dda91d7c
• https://github.com/symfony/symfony/security/advisories/GHSA-gq53-v8w2-gjhj