Utils.readChallengeTx does not verify the server account signature

Vulnerability Description

js-stellar-sdk is a Javascript library for communicating with a Stellar Horizon server. The `Utils.readChallengeTx` function used in SEP-10 Stellar Web Authentication states in its function documentation that it reads and validates the challenge transaction including verifying that the `serverAccountID` has signed the transaction. In js-stellar-sdk before version 8.2.3, the function does not verify that the server has signed the transaction. Applications that also used `Utils.verifyChallengeTxThreshold` or `Utils.verifyChallengeTxSigners` to verify the signatures including the server signature on the challenge transaction are unaffected as those functions verify the server signed the transaction. Applications calling `Utils.readChallengeTx` should update to version 8.2.3, the first version with a patch for this vulnerability, to ensure that the challenge transaction is completely valid and signed by the server creating the challenge transaction.

References

• https://github.com/stellar/js-stellar-sdk/security/advisories/GHSA-6cgh-hjpw-q3gq
• https://github.com/stellar/js-stellar-sdk/releases/tag/v8.2.3