The vulnerability allows a remote attacker to bypass rate limiting by altering the X-Forwarded-For header. Echo's context delivers a client IP that comes from the header when no IP extractor is configured, and the default rate limiter uses that IP as a bucket identifier. By rotating arbitrary forwarded-address values, an attacker can generate additional buckets and slip past IP-based throttling intended for brute‑force or request‑volume protection. The weakness is a misuse of client‑supplied data for authentication purposes, classified as CWE‑290.

Affected deployments are those running labstack Echo version 4.0.0 through 5.0.x. The production applications that rely on the default configuration for IP extraction and rate limiting are impacted, while versions before 4.0.0 or after 5.1.0 have the fix applied.

The CVSS score of 5.3 places the issue in the medium severity range. Exploitation requires only the ability to send HTTP requests with a custom X-Forwarded-For header, which is possible from any remote client; the EPSS score is not available, and the vulnerability is not yet listed in CISA's KEV catalog. The attack would grant the attacker access to unthrottled request rates, enabling brute‑force attempts against authentication endpoints or denial‑of‑service through traffic amplification. While no privileged access or code execution is required, the ability to bypass security controls represents a tangible risk.