Lecture 3.1: Handling Remote Access: RADIUS Motivation

Attacking the password of a user
FIRST STEP: as previous case, but with valid user ID: RADIUS server
Victim User-ID
NAS
Access-Request
XOR (password)
Arbitrary Password (< 16 bytes)
Giuseppe Bianchi
User-Password attribute (16 bytes)
MD5(secret, RequestAuth)
SECOND STEP: Attacker now able to “encrypt” the user password!! May exploit:
1) lack of upper limit on authentication rate at server-side (limits imposed on clients
are by-passed)
2) RADIUS servers typically do not check for authenticator reuse
Works only with 16 or less byte passwords (most cases)
Giuseppe Bianchi
Spoofed Access-Request, with:
New-passwords XOR MD5(secret, RequestAuth)
poor PRNG implementations
Replay Attacks
Security of radius depends on the uniqueness and
non-predictable generation of the Request
Authenticator
Some implementations exploit poor Pseudo-Random
Number Generators (PRNGs)
Short cycles, predictable
Immediate exploitation: replay attack:
authenticate/authorize an illegal user with no valid
password
Valid users NAS
Access-Request (Request authenticator)
Access-Accept (Response authenticator)
Dictionary of ReqAuth/RespAuth
Access-Request (ReqAuth in dictionary)
Access-Accept (corresponding RespAuth)
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