This indicates an attack attempt to exploit a Security Bypass Vulnerability in the Symfony.
The vulnerability is due to an error in the vulnerable application when handling a maliciously crafted request. A remote attacker may be able to exploit this to bypass the security mechanisms on the vulnerable application.
Symfony 2.7.0 to 2.7.48
Symfony 2.8.0 to 2.8.43
Symfony 3.3.0 to 3.3.17
Symfony 3.4.0 to 3.4.13
Symfony 4.0.0 to 4.0.13
Symfony 4.1.0 to 4.1.2
Security Bypass: Remote attackers can bypass security mechanism on vulnerable systems
Apply the most recent upgrade or patch from the vendor