RSA signature validation vulnerability on maleable encoded message in jsrsasign

Impact

Vulnerable jsrsasign will accept RSA signature with improper PKCS#1.5 padding.
Decoded RSA signature value consists following form:
01(ff...(8 or more ffs)...ff)00[ASN.1 OF DigestInfo]
Its byte length must be the same as RSA key length, however such checking was not sufficient.

To make crafted message for practical attack is very hard.

Patches

Users validating RSA signature should upgrade to 10.2.0 or later.

Workarounds

There is no workaround. Not to use RSA signature validation in jsrsasign.

ACKNOWLEDGEMENT

Thanks to Daniel Yahyazadeh @yahyazadeh for reporting and analyzing this vulnerability.