CVE-2026-39377

Arbitrary File Write via Path Traversal in Cell Attachment Filenames

nbconvert allows arbitrary file writes to locations outside the intended output directory when processing notebooks containing crafted cell attachment filenames. The ExtractAttachmentsPreprocessor passes attachment filenames directly to the filesystem without sanitization, enabling path traversal attacks. This vulnerability provides complete control over both the destination path and file extension.

Workarounds

disable ExtractAttachmentsPreprocessor by setting:

c. ExtractAttachmentsPreprocessor.enabled = False

This vulnerability allows writing files with arbitrary content to arbitrary filesystem locations, limited only by the permissions of the process running nbconvert. The attacker controls:

• Full destination path (via ../ traversal)
• Filename
• File extension
• File content

Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files. Typical impact: unauthorized file read or write outside the intended directory.

CVE-2026-39377 has a CVSS score of 6.5 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (7.17.1); upgrading removes the vulnerable code path.

Kodem intelligence

• upgrade to nbconvert v7.17.1

1. What is CVE-2026-39377? CVE-2026-39377 is a medium-severity path traversal vulnerability in nbconvert (pip), affecting versions >= 6.5.0, < 7.17.1. It is fixed in 7.17.1. Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files.
2. How severe is CVE-2026-39377? CVE-2026-39377 has a CVSS score of 6.5 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of nbconvert are affected by CVE-2026-39377? nbconvert (pip) versions >= 6.5.0, < 7.17.1 is affected.
4. Is there a fix for CVE-2026-39377? Yes. CVE-2026-39377 is fixed in 7.17.1. Upgrade to this version or later.
5. Is CVE-2026-39377 exploitable, and should I be worried? Whether CVE-2026-39377 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-39377 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-39377? Upgrade nbconvert to 7.17.1 or later.