It was discovered that the Invariance Weakness of the RC4 stream cipher could be used to recover plaintext from a TLS connection, when RC4 encryption is used.

"The Invariance Weakness is an L-shape key pattern in RC4 keys, which once it exists in an RC4 key, preserves part of the state permutation intact throughout the initialization process. This intact part includes the least significant bits of the permutation, when processed by the PRGA algorithm, determines the least significant bits of the allegedly pseudo-random output stream along a long prefix of the stream."

This can lead to significant leakage of plaintext bytes from the ciphertext.

External Reference:

http://www.imperva.com/docs/HIIAttackingSSLwhenusingRC4.pdf

Huawei OceanStor 5300 V3, 5500 V3, 5600 V3, 5800 V3, 6800 V3, 18800 V3, and 18500 V3 before V300R003C10 sends the plaintext session token in the HTTP header, which allows remote attackers to conduct replay attacks and obtain sensitive information by sniffing the network.