passwd_recovery.lua on the TP-Link Archer C9(UN)_V2_160517 allows an attacker to reset the admin password by leveraging a predictable random number generator seed. This is fixed in C9(UN)_V2_170511.
The product uses a Pseudo-Random Number Generator (PRNG) but does not correctly manage seeds.
| Name | Vendor | Start Version | End Version |
|---|---|---|---|
| Archer_c9_(2.0)_firmware | Tp-link | 160517 (including) | 160517 (including) |
PRNGs are deterministic and, while their output appears
random, they cannot actually create entropy. They rely on
cryptographically secure and unique seeds for entropy so
proper seeding is critical to the secure operation of the
PRNG.
Management of seeds could be broken down into two main areas:
PRNGs require a seed as input to generate a stream of
numbers that are functionally indistinguishable from
random numbers. While the output is, in many cases,
sufficient for cryptographic uses, the output of any
PRNG is directly determined by the seed provided as
input. If the seed can be ascertained by a third party,
the entire output of the PRNG can be made known to
them. As such, the seed should be kept secret and
should ideally not be able to be guessed. For example,
the current time may be a poor seed. Knowing the
approximate time the PRNG was seeded greatly reduces
the possible key space.
Seeds do not necessarily need to be unique, but reusing seeds may open up attacks if the seed is discovered.