CWE-757 (在会话协商时选择低安全性的算法(算法降级)) — Vulnerability Class 20

CWE-757 represents a critical configuration weakness where a protocol or its implementation fails to select the strongest available cryptographic algorithm during negotiation between multiple actors. This vulnerability typically arises when the system allows parties to agree on a protection mechanism, such as encryption or authentication, but defaults to or accepts a weaker option than what both sides support. Attackers exploit this by intercepting the negotiation process, forcing a downgrade to less secure algorithms like RC4 or MD5, thereby enabling traffic decryption or integrity forgery. To mitigate this risk, developers must enforce strict policy configurations that mandate the use of the highest-strength algorithms supported by all participants. Implementing default-deny rules for weak ciphers and validating negotiated parameters against a whitelist of approved strong algorithms ensures that security is never compromised by inferior choices during the handshake phase.