Re: [pbs-devel] [PATCH proxmox v2 04/12] auth-api: use constant time comparison for csrf tokens

[Max Carrara <m.carrara@proxmox.com>]

On 3/6/24 13:36, Stefan Sterz wrote:
> by using openssl's `memcmp::eq()` we can avoid potential side-channel
> attack on the csrf token comparison. this comparison's runtime only
> depends on the length of the two byte vectors, not their contents.
> 
> Signed-off-by: Stefan Sterz <s.sterz@proxmox.com>
> ---
>  proxmox-auth-api/src/api/access.rs | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/proxmox-auth-api/src/api/access.rs b/proxmox-auth-api/src/api/access.rs
> index 428d22a..e22eea2 100644
> --- a/proxmox-auth-api/src/api/access.rs
> +++ b/proxmox-auth-api/src/api/access.rs
> @@ -286,14 +286,15 @@ fn verify_csrf_prevention_token_do(
>      }
>  
>      let timestamp = parts.pop_front().unwrap();
> -    let sig = parts.pop_front().unwrap();
> +    let sig = parts.pop_front().unwrap().as_bytes();

Thought these `unwrap()`s here seemed a bit spicy at first, but we do
check for the length of `parts` beforehand, so this is fine.

>  
>      let ttime = i64::from_str_radix(timestamp, 16)
>          .map_err(|err| format_err!("timestamp format error - {}", err))?;
>  
>      let digest = compute_csrf_secret_digest(ttime, secret, userid);
> +    let digest = digest.as_bytes();
>  
> -    if digest != sig {
> +    if digest.len() != sig.len() || !openssl::memcmp::eq(digest, sig) {
>          bail!("invalid signature.");
>      }
>