free5GC is an open-source implementation of the 5G core network. Prior to 1.4.5, the AUSF component performs cryptographic authentication comparisons in internal/sbi/processor/ue_authentication.go with ordinary equality helpers. Auth5gAkaComfirmRequestProcedure compares RES* and XRES* with strings.EqualFold and logs the expected XRES* value at INFO level before comparison. EapAuthComfirmRequestProcedure compares AT_MAC and XMAC with bytes.Equal and evaluates XRES == RES with ordinary string equality. These comparisons can return at mismatch-dependent times, although testing did not demonstrate a practical remote timing oracle because of HTTP/SBI timing noise. The INFO log exposes authentication material to operators, log collectors, sidecars, or processes able to read AUSF logs. This issue is fixed in version 1.4.5.

Affected: Go: github.com/free5gc/ausf

### Summary

The AUSF component of free5GC compares authentication response values with normal Go equality helpers instead of constant-time cryptographic comparison functions.

Two authentication flows are affected in `internal/sbi/processor/ue_authentication.go`:

1. 5G-AKA confirmation compares `RES*` and `XRES*` with `strings.EqualFold()`. 2. EAP-AKA' confirmation compares `AT_MAC` with `bytes.Equal()` and compares `XRES` and `RES` with `==`.

These functions are not designed to be constant-time cryptographic comparators and may return earlier depending on the location of the first mismatch.

Additionally, the 5G-AKA confirmation path logs both the received `res*` and the expected `Xres*` at INFO level immediately before comparing them. The `XRES*` value is authentication material and should not be written to application logs.

The timing side channel was confirmed as a code issue, but practical exploitation over HTTP was not demonstrated in the lab because the comparator-level signal is much smaller than HTTP/SBI noise. The `XRES*` logging issue is directly observable in AUSF logs.

Confirmed on `github.com/free5gc/ausf` v1.4.4 and current main as of the May 2026 analysis.

### Details

#### 5G-AKA: `RES*` / `XRES*`

In `Auth5gAkaComfirmRequestProcedure()`, the AUSF logs both values and then compares them with `strings.EqualFold()`:

```go // internal/sbi/processor/ue_authentication.go logger.Auth5gAkaLog.Infof("res*: %x\nXres*: %x\n", updateConfirmationData.ResStar,…