CVE-2026-27699 is a critical-severity (CVSS 9.1) Path Traversal vulnerability in basic-ftp. A fix is available for basic-ftp — see the affected versions and patch details below.
Basic FTP has Path Traversal Vulnerability in its downloadToDir() method
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- A successful exploit gives an attacker total control of the affected component, not partial access.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-27699.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2026-27699 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 377,636 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
basic-ftpnpmDescription
The basic-ftp library contains a path traversal vulnerability in the downloadToDir() method. A malicious FTP server can send directory listings with filenames containing path traversal sequences (../) that cause files to be written outside the intended download directory.
Source-to-Sink Flow
1. SOURCE: FTP server sends LIST response
└─> "-rw-r--r-- 1 user group 1024 Jan 20 12:00 ../../../etc/passwd"
2. PARSER: parseListUnix.ts:100 extracts filename
└─> file.name = "../../../etc/passwd"
3. VALIDATION: parseListUnix.ts:101 checks
└─> if (name === "." || name === "..") ❌ (only filters exact matches)
└─> "../../../etc/passwd" !== "." && !== ".." ✅ PASSES
4. SINK: Client.ts:707 uses filename directly
└─> const localPath = join(localDirPath, file.name)
└─> join("/safe/download", "../../../etc/passwd")
└─> Result: "/safe/download/../../../etc/passwd" → resolves to "/etc/passwd"
5. FILE WRITE: Client.ts:512 opens file
└─> fsOpen(localPath, "w") → writes to /etc/passwd (outside intended directory)
Vulnerable Code
File: src/Client.ts:707
protected async _downloadFromWorkingDir(localDirPath: string): Promise<void> {
await ensureLocalDirectory(localDirPath)
for (const file of await this.list()) {
const localPath = join(localDirPath, file.name) // ⚠️ VULNERABLE
// file.name comes from untrusted FTP server, no sanitization
await this.downloadTo(localPath, file.name)
}
}
Root Cause:
- Parser validation (parseListUnix.ts:101) only filters exact . or .. entries
- No sanitization of ../ sequences in filenames
path.join()doesn't prevent traversal,fs.open()resolves paths
Impact
A malicious FTP server can: - Write files to arbitrary locations on the client filesystem - Overwrite critical system files (if user has write access) - Potentially achieve remote code execution
Affected Versions
- Tested: v5.1.0
- Likely: All versions (code pattern exists since initial implementation)
Mitigation
Workaround: Do not use downloadToDir() with untrusted FTP servers.
Fix: Sanitize filenames before use:
import { basename } from 'path'
// In _downloadFromWorkingDir:
const sanitizedName = basename(file.name) // Strip path components
const localPath = join(localDirPath, sanitizedName)
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | basic-ftp | all versions | 5.2.0npm install basic-ftp@5.2.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for basic-ftp, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update basic-ftp to 5.2.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-27699 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-27699 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-27699. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-27699 in your dependencies?
O3 Security finds CVE-2026-27699 across npm dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.