Prometheus through version 3.15.1 contains a flaw that allows an attacker to capture Azure AD or Google IAM bearer tokens by redirecting remote_write requests to a malicious host. An attacker who can influence the remote-write endpoint or control a redirecting server can issue a 3xx response that points to their own host; the bearer token sent with the original request is then replayed by the attacker, enabling unauthorized writes or other privileged actions. This vulnerability undermines the confidentiality of authentication tokens used by the Prometheus cluster and can lead to data exfiltration or service tampering.

Vulnerable Prometheus deployments include all releases up to and including 3.15.1. The issue is present in the core Prometheus server code and affects any configuration that exposes a remote_write endpoint to untrusted actors.

The CVSS score of 6.3 indicates moderate severity with potential impact on confidentiality. The EPSS score is not available, and the vulnerability is not listed in CISA's KEV catalog. The likely attack vector involves an adversary gaining control over the remote_write endpoint or a redirecting server; this could occur during a compromised client, an attacker with write access to the Prometheus configuration, or a malicious network relay. The exploitation requires the attacker to send a 3xx redirect and capture the token, which can then be replayed to perform unauthorized writes to the victim's backend.