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CVE-2026-42554: Fiber: XSS in AutoFormat Content Negotiation

• Severity: medium (CVSS 6.1 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.2% probability of exploitation in the next 30 days
• Public exploit or detection template: yes
• Published: 2026-05-11 · updated 2026-10-26
• Weakness: CWE-79

Description

Fiber is a web framework for Go. Prior to 2.52.12 and 3.1.0, Cross-Site Scripting vulnerability in Go Fiber allows a remote attacker to inject arbitrary HTML/JavaScript by supplying Accept: text/html on any request whose handler passes attacker-influenced data to the AutoFormat() feature. The developer opts into content negotiation by calling AutoFormat(), but does not opt into raw HTML emission for a particular request; Fiber chooses that branch from attacker-controlled Accept. The html branch is the sole outlier in a method whose name (AutoFormat) and symmetrical structure actively telegraph "safe, format-agnostic reply." This vulnerability is fixed in 2.52.12 and 3.1.0.

Affected products

• gofiber fiber: before 2.52.12; from 3.0.0 before 3.1.0 — fixed in 2.52.12, 3.1.0

How to fix

Upgrade to a fixed version: gofiber fiber 2.52.12, 3.1.0. Check the vendor advisory for mitigations.

References

• https://github.com/gofiber/fiber/security/advisories/GHSA-qjv7-627w-8qjv
• https://github.com/gofiber/fiber/security/advisories/GHSA-qjv7-627w-8qjv

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