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The Libreswan Project was notified of an issue when it receives an invalidly formatted PKCS#1.5 RSA signature payload that authenticates the IKE exchange. The vulnerability is similar to CVE-2018-16151.
Use of RSA signatures over certificates during X.509 certificate verifications of the remote IKE peer are not affected by this vulnerability.

When the RSA exponent is weak (eg e=3), Bleichenbacher-style signature forgeries are possible, resulting in an authentication bypass. Note that most cryptographic library versions and libreswan raw RSA key generation have not allowed weak exponents for at least a decade, so valid RSA keys with weak exponents should be very rare.

Additionally, the invalid RSA IKE authentication payload can trigger an assertion, resulting in libreswan aborting and restarting. Continued sending of such packets can result in a denial of service.

Severity : Medium
Vulnerable versions : all version up to and including 5.3
Not vulnerable : 5.3.1 or later