Crossing the Streams: SSH Plaintext Recovery via a Common Compression Context in Multiplexed Channels

2026-09-07

AI Summary

研究揭示了SSH多路复用通道中的压缩侧信道问题，通过注入部分选择明文并观察密文长度来恢复秘密信息，评估了不同应用场景下的攻击效果。

Abstract

SSH is the standard protocol for secure remote administration of servers. At the transport layer, SSH uses the Binary Packet Protocol (BPP) for encrypted and authenticated communication. Above this, the SSH Connection Protocol multiplexes one or more logical channels over a single connection, supporting interactive shells, port forwarding, and related functionality. We show that SSH channel multiplexing creates a previously unrecognized compression side channel: all channels on a connection share the same compression context. When compression is enabled, an attacker can inject partially chosen plaintext into a channel and observe the length of the resulting ciphertext on the network. This enables an adaptive chosen-plaintext attack that recovers secrets from one channel by interacting with another. While related attacks such as CRIME and BREACH have been studied extensively for HTTP over TLS, this is, to our knowledge, the first compression side-channel attack on SSH and the first SSH analysis to consider a combined passive eavesdropper and web attacker threat model. We further demonstrate the attack in three different application scenarios and evaluate its effectiveness under varying levels of protocol noise. We find that, in the lowest-noise scenario, an 8-character secret over a 26-letter alphabet can be recovered using at most 276 guesses. Finally, we analyze the SSH ecosystem for compression support and other implementation characteristics that influence the practical efficacy of the attack.

Problem

• SSH
• compression side channel
• adaptive chosen-plaintext attack
• channel multiplexing
• secure remote administration

Innovation

• compression side channel
• multiplexed channels
• adaptive chosen-plaintext attack
• SSH security