CVE-2026-42882: S3-Proxy has Security Issues in its Resource Path Matching Implementation

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 0.7%, percentile 53: FIRST’s model estimates a 0.7% chance of exploitation activity in the next 30 days, higher than 53% of all scored CVEs (moderate). 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/oxyno-zeta/s3-proxy (Go) < 0.0.0-20260424211602-1320e4abd46a, fixed in 0.0.0-20260424211602-1320e4abd46a. Check your lockfile, including transitive dependencies.

What it is

oxyno-zeta/s3-proxy is an aws s3 proxy written in go. Prior to 5.0.0, s3-proxy contains an authentication bypass caused by inconsistent URL path interpretation between the authentication middleware and the bucket handler. The authentication middleware evaluates resource path patterns against the percent-encoded request URI (r.URL.RequestURI()), while the bucket handler constructs S3 object keys from the decoded path (r.URL.Path). This mismatch, combined with the glob library being invoked without a path separator (causing to match across / boundaries), allows unauthenticated attackers to write to, read from, or delete objects in protected S3 namespaces. Exploitation is possible via three techniques: (1) using patterns that match across path separators to reach protected routes via path traversal (e.g., /open/foo/drafts/../restricted/), (2) using percent-encoded slashes (%2F) to collapse multiple path segments into a single token at the auth layer while the decoded form resolves to a protected namespace at the storage layer, and (3) using dot-dot segments (../) under prefix patterns, where the raw path matches an open route while Go's URL parser resolves the traversal to a protected path before the bucket handler runs. An unauthenticated attacker with network access can perform unauthorized PUT, GET, or DELETE operations on objects in authentication-protected S3 namespaces. This vulnerability is fixed in 5.0.0.

From the GitHub advisory

Background

The original concern is functional: a resource pattern should treat a percent-encoded segment like some%2Fvalue as a single opaque token rather than splitting it into two path segments at the decoded /. Investigation into why %2F was being decoded and how routes matched against the result surfaced three related security issues, documented below.

Rather than landing a fix directly, the problem space warrants discussion first. Different fixes carry different compliance and compatibility tradeoffs, and every viable option is a breaking change in some form. Aligning on a direction before committing to an implementation is the safer path.

Root cause: two different path representations

Go's net/http decodes percent-encoded characters when it parses an incoming URL: %2F becomes / in r.URL.Path, while the original encoded form is preserved in r.URL.RawPath. Two different parts of s3-proxy use different fields:

Weakness type: CWE-22, CWE-863.

Product: github.com/oxyno-zeta/s3-proxy (Go) · Assigned by: GitHub_M · Published: 2026-05-11

Affected and fixed versions

Severity scores

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

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): 0.72% probability of exploitation activity in the next 30 days; percentile 53.

Timeline

• 2026-05-05 — GitHub-reviewed advisory GHSA-rfgq-wgg8-662p published (critical)
• 2026-05-11 — CVE record published by GitHub_M
• 2026-05-13 — CVE record last updated

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-rfgq-wgg8-662p github.com
2. CVE.org record CVE-2026-42882 cve.org
3. NVD entry CVE-2026-42882 nvd.nist.gov

Other references from the CVE record and advisory

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

Critical Go advisories · Today’s Security Digest