Reduce Key Space
Where this stands
No Sigma rule can address this — the telemetry MITRE names has no Sigma logsource behind it.
None of MITRE's analytics for this technique name a log source that maps to a Sigma logsource — they describe abstract data ("Process::None"), API-hook telemetry, or prose channels with no reliable field names behind them.
What the technique is
Adversaries may reduce the level of effort required to decrypt data transmitted over the network by reducing the cipher strength of encrypted communications. Adversaries can weaken the encryption software on a compromised network device by reducing the key size used by the software to convert plaintext to ciphertext (e.g., from hundreds or thousands of bytes to just a couple of bytes). As a result, adversaries dramatically reduce the amount of effort needed to decrypt the protected information without the key. Adversaries may modify the key size used and other encryption parameters using specialized commands in a Network Device CLI introduced to the system through Modify System Image to change the configuration of the device.
Read it on attack.mitre.orgWhat MITRE says you would watch
- AN0681
Defenders may observe attempts to alter cryptographic settings on network devices that reduce key strength or allowable cipher suites. Suspicious indicators include configuration changes that downgrade encryption algorithms, key length parameters, or the disabling of strong encryption in favor of legacy ciphers. These activities often appear as CLI commands modifying crypto policies, firmware changes affecting crypto libraries, or unexpected updates to key management files. Correlation across device config logs and traffic analysis showing weaker ciphers provides higher confidence of malicious key space reduction.
Technique names and descriptions © MITRE ATT&CK®, CC BY 4.0. Not endorsed by MITRE.