Arbitrary file write via unchecked route parameters
Published Apr 8, 2026 · Updated Apr 8, 2026
Path traversal in Hono 4.0.0 through 4.12.11 allows attackers to write files outside the static-site output directory. toSSG() passes dynamic ssgParams values into generateFilePath, whose joined path was written without verifying containment beneath outDir. Exploitation requires influence over ssgParams during build-time static generation; request-time routing is unaffected, while build artifacts or other writable files can be overwritten.
Summary
What happened
Path traversal in Hono 4.0.0 through 4.12.11 allows attackers to write files outside the static-site output directory. toSSG() passes dynamic ssgParams values into generateFilePath, whose joined path was written without verifying containment beneath outDir. Exploitation requires influence over ssgParams during build-time static generation; request-time routing is unaffected, while build artifacts or other writable files can be overwritten.
The record
- CVE
- CVE-2026-39408
- Published
- Apr 8, 2026
- Updated
- Apr 8, 2026
- Vendor
- Hono
- Product
- Hono
- Classifications
- CWE-22, T1565.001
- Attack vector
- local
- Privileges
- unauthenticated
Timeline
How it unfolded
- Apr 8, 2026CVE publishedPublication date reported by the CVE source.
- Apr 8, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=< 4.12.12
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
- GitHub advisory ssgParams traversal exampleproof of concept · demonstrated
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Technologies
Your stack
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Your stack
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