CVE-2025-66565: Fiber Utils UUIDv4 and UUID Silent Fallback to Predictable Values

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• Not in CISA KEV, but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
• EPSS 0.5%, percentile 39: FIRST’s model estimates a 0.5% chance of exploitation activity in the next 30 days, higher than 39% of all scored CVEs (low). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Only applies if your dependency tree includes github.com/gofiber/utils/v2 (Go) < 2.0.0-rc.3.0.20251205210924-6c6cf047032b, fixed in 2.0.0-rc.4; github.com/gofiber/utils (Go) <= 1.1.0, fixed in 1.2.0. Check your lockfile, including transitive dependencies.

What it is

Fiber Utils is a collection of common functions created for Fiber. In versions 2.0.0-rc.3 and below, when the system's cryptographic random number generator (crypto/rand) fails, both functions silently fall back to returning predictable UUID values, including the zero UUID "00000000-0000-0000-0000-000000000000". The vulnerability occurs through two related but distinct failure paths, both ultimately caused by crypto/rand.Read() failures, compromising the security of all Fiber applications using these functions for security-critical operations. This issue is fixed in version 2.0.0-rc.4.

From the GitHub advisory

Summary

Critical security vulnerabilities exist in both the UUIDv4() and UUID() functions of the github.com/gofiber/utils package. When the system's cryptographic random number generator (crypto/rand) fails, both functions silently fall back to returning predictable UUID values, the zero UUID "00000000-0000-0000-0000-000000000000". This compromises the security of all Fiber applications using these functions for security-critical operations on Go versions prior to 1.24.

Both functions are vulnerable to the same root cause (crypto/rand failure):

UUIDv4(): Indirect vulnerability through uuid.NewRandom() → crypto/rand.Read() → fallback to UUID() UUID(): Direct vulnerability through crypto/rand.Read(uuidSeed[:]) → silent zero UUID return

> Note: Go 1.24 and later panics on crypto/rand Read() failures, mitigating this vulnerability. Applications running on Go 1.24+ are not affected by the silent fallback behavior.

Weakness type: CWE-252, CWE-331, CWE-338.

Product: github.com/gofiber/utils/v2 (Go) · Assigned by: GitHub_M · Published: 2025-12-09

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Attack requirements: none
• Privileges required: none (no login)
• User interaction: none

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation poc, automatable yes, technical impact total.
• FIRST EPSS (model of 2026-10-21): 0.47% probability of exploitation activity in the next 30 days; percentile 39.

Timeline

• 2025-12-08 — GitHub-reviewed advisory GHSA-m98w-cqp3-qcqr published (critical)
• 2025-12-09 — CVE record last updated
• 2025-12-09 — CVE record published by GitHub_M

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-m98w-cqp3-qcqr github.com
2. CVE.org record CVE-2025-66565 cve.org
3. NVD entry CVE-2025-66565 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com

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