CVE-2026-44332: Fiber BasicAuth Bypass Vulnerability

CVE-2026-44332 Overview

CVE-2026-44332 is a username enumeration vulnerability in the Fiber web framework for Go. The flaw resides in the default Authorizer function of the BasicAuth middleware, located in middleware/basicauth/config.go. The implementation uses short-circuit evaluation that skips password hash comparison when the supplied username does not exist. Attackers can measure response timing differences to reliably distinguish valid usernames from invalid ones over the network. The issue is classified under [CWE-203: Observable Discrepancy] and affects Fiber versions prior to 3.3.0.

Critical Impact
Remote, unauthenticated attackers can enumerate valid usernames on any endpoint protected by Fiber's default BasicAuth middleware by measuring authentication response timing.

Affected Products

Fiber web framework (github.com/gofiber/fiber) prior to version 3.3.0
Any Go application using the default BasicAuth middleware Authorizer from middleware/basicauth/config.go
Services exposing HTTP endpoints protected by unmodified Fiber BasicAuth configuration

Discovery Timeline

2026-07-08 - CVE-2026-44332 published to NVD
2026-07-08 - Last updated in NVD database
Fiber v3.3.0 - Patch released via GitHub Pull Request #4245 and GitHub Release v3.3.0

Technical Details for CVE-2026-44332

Vulnerability Analysis

The vulnerability is a classic timing side channel [CWE-203]. When a client submits HTTP Basic authentication credentials, Fiber's default Authorizer first checks whether the supplied username exists in the configured user map. If the username is not present, the function returns immediately without invoking the password hash comparison routine.

Password hash verification against algorithms such as bcrypt is intentionally slow. Skipping this step for unknown users creates a measurable timing gap between requests with valid usernames and requests with invalid usernames. Attackers can send probe requests with candidate usernames and arbitrary passwords, then compare response latencies to determine which usernames are registered on the target service. This reduces the effective search space for follow-on password brute-force or credential-stuffing attacks.

Root Cause

The root cause is short-circuit evaluation in the authorization logic. The original code returns early when the username lookup fails, bypassing the constant-time work performed for valid usernames. The fix in Fiber 3.3.0 ensures the code path performs equivalent work regardless of whether the username exists, removing the observable timing discrepancy.

Attack Vector

Exploitation requires only network access to a Fiber HTTP endpoint protected by the default BasicAuth middleware. No prior authentication or user interaction is required. An attacker issues repeated HTTP requests with varying Authorization: Basic headers containing candidate usernames and dummy passwords, then statistically analyzes response times to identify valid accounts.

// Patch context from middleware/basicauth/config.go
// Source: https://github.com/gofiber/fiber/commit/c7ac00edd19f9669b1aebbec6e229658baaa059e
 	"encoding/hex"
 	"errors"
 	"fmt"
+	"sort"
 	"strconv"
 	"strings"

The patch introduces changes to the default Authorizer so that password comparison work is performed even when the username is absent, eliminating the timing signal. See the GitHub Security Advisory GHSA-g5vh-55hw-rxm8 for full technical details.

Detection Methods for CVE-2026-44332

Indicators of Compromise

High volumes of HTTP requests with Authorization: Basic headers targeting a single endpoint from one or a small set of source IPs
Repeated 401 Unauthorized responses correlated with sequential or dictionary-derived usernames
Request patterns that iterate usernames while reusing identical or randomized password values

Detection Strategies

Instrument BasicAuth endpoints to log both the username attempted and server-side processing latency, then alert on anomalous username diversity from single sources
Deploy WAF or reverse-proxy rules that rate-limit failed BasicAuth attempts per source IP and per target route
Inspect application dependency manifests (go.mod, go.sum) to identify services running Fiber releases earlier than 3.3.0

Monitoring Recommendations

Baseline normal authentication failure rates per endpoint and alert on statistically significant deviations
Aggregate authentication telemetry into a central SIEM or data lake to correlate enumeration attempts across services
Track 4xx response distribution over time on any route using BasicAuth to identify credential-probing campaigns

How to Mitigate CVE-2026-44332

Immediate Actions Required

Upgrade the Fiber dependency to version 3.3.0 or later in every affected Go service
Audit all routes and middleware chains using basicauth.New with default configuration and confirm they are covered by the upgrade
Rotate any credentials for accounts whose usernames may have been exposed through enumeration prior to patching

Patch Information

The fix is available in Fiber v3.3.0. The changes were merged through GitHub Pull Request #4245 and applied in commit c7ac00e. Full release notes are published at the GitHub Release v3.3.0 page.

Workarounds

Replace the default Authorizer with a custom implementation that always performs a bcrypt or equivalent comparison against a fixed dummy hash when the username is absent
Enforce per-IP rate limiting on all BasicAuth-protected routes to raise the cost of enumeration
Front BasicAuth endpoints with an authentication gateway or WAF that normalizes response timing and blocks rapid credential-probing patterns

bash

# Update Fiber dependency to the patched release
go get github.com/gofiber/fiber/v3@v3.3.0
go mod tidy

# Verify the resolved version
go list -m github.com/gofiber/fiber/v3