Description

rsync 3.2.0 through 3.2.3 (openssl mode) and rsync-ssl through 3.4.4 (stunnel mode) contain a TLS certificate validation vulnerability that allows on-path attackers to intercept encrypted sessions by presenting self-signed or otherwise invalid certificates. Attackers can exploit the failure to validate server TLS certificates against a trusted CA or verify certificate hostname matching to decrypt or tamper with rsync session content without detection by the client.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "8.0",
  "pubDate": "2026-08-13T15:19:59.047Z",
  "pubdate": "2026-08-13T15:19:59.047Z",
  "executiveSummary": "A TLS certificate validation vulnerability exists in rsync versions 3.2.0 through 3.2.3 when operating in openssl mode, and in rsync-ssl versions through 3.4.4 when operating in stunnel mode.\nThe vulnerability involves a failure to properly validate server TLS certificates against a trusted Certificate Authority and a failure to verify certificate hostname matching.\nThis security flaw allows on-path attackers to intercept encrypted rsync sessions by presenting self-signed or otherwise invalid certificates.\nThe impact of successful exploitation includes the complete decryption, modification, or tampering of rsync session content without detection by the client.\nAttackers require an on-path network position to intercept the communication channel between the rsync client and the server.\nThis vulnerability poses a significant risk to data confidentiality and integrity during remote file synchronization operations over TLS or stunnel wrappers.",
  "technicalDetails": "The root cause of the vulnerability is the improper implementation or complete absence of strict TLS certificate validation checks within the affected rsync and rsync-ssl execution modes.\nSpecifically, the affected software fails to validate server TLS certificates against a trusted CA store and omits necessary checks for valid certificate hostname matching during the TLS handshake.\nThe vulnerable components include rsync in openssl mode for versions 3.2.0 through 3.2.3 and rsync-ssl in stunnel mode for versions through 3.4.4.\nExploitation occurs across the network layer where an on-path attacker can position themselves between the rsync client and the destination server.\nDuring the session establishment phase, the attacker intercepts the TLS handshake and presents a malicious, self-signed, or otherwise invalid certificate to the rsync client.\nBecause the client application fails to validate the presented certificate against trusted CAs and does not perform hostname verification, the client blindly accepts the fraudulent certificate.\nOnce the encrypted session is established under the attacker's control, the attacker can decrypt the traffic, inspect sensitive synchronized data, tamper with file contents in transit, and forward or modify requests without generating any error or detection alerts on the client side.\nNo authentication or special privileges are required by the attacker beyond achieving an on-path network position capable of intercepting traffic between the endpoints."
}