⚠ Reference material — not professional advice. Test in staging, back up first, verify against your specific version. Use your own judgment for your environment.
● High · CVSS 8.6

How to Fix CVE-2026-25635: Path Traversal in calibre

Other vulnerabilities in the same area that are worth patching alongside this one:

*By Sai Kiran Pandrala*

⚡ At a glance
SeverityCVSS 8.6 - High
Actively exploited?Not currently listed in CISA KEV
Affected< 9.2.0
Fixed in9.2.0.
Type (CWE)CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')

What is CVE-2026-25635?

CVE-2026-25635 is a path traversal flaw in calibre. The product fails to canonicalize or restrict file paths supplied by a remote caller, so .. sequences or absolute paths reach restricted parts of the filesystem. Vendor description: calibre is an e-book manager. Prior to 9.2.0, Calibre's CHM reader contains a path traversal vulnerability that allows arbitrary file writes anywhere the user has write permissions.

Why this CVE matters

Path traversal flaws look low-impact on paper but routinely chain into full compromise. An attacker who can read arbitrary files often pulls configuration secrets, session databases, or private keys, and many traversal bugs also allow writes that drop a webshell into the document root.

For deployments of calibre that have been exposed to the public internet during the disclosure window, the operating assumption should be that scanning has already happened. Even where exploitation has not been publicly observed, scanning for the vulnerable fingerprint is cheap and routine. Patching closes the door; log review and credential rotation close out the rest of the response.

Am I affected?

You are affected if your installation matches any of these version ranges:

Check your installed version against the list above. If you cannot determine the version, treat the system as affected and follow the upgrade path below.

Open calibre's About dialog or run the vendor-documented version-check command. Compare the result against the affected ranges in the advisory.

How to fix CVE-2026-25635

  1. Read the vendor advisory in full: https://github.com/kovidgoyal/calibre/security/advisories/GHSA-32vh-whvh-9fxr
  2. Upgrade calibre to the patched build listed in the vendor advisory.
  3. Back up the configuration (and database, where applicable) before upgrading.
  4. Apply the patch in a maintenance window. For HA pairs, upgrade the standby node first, fail over, then upgrade the former primary.
  5. Restart the affected service so the patched binary loads, then verify the new version (see verification section).

Patched-version commands

Vendor advisory: https://github.com/kovidgoyal/calibre/security/advisories/GHSA-32vh-whvh-9fxr

Affected: calibre: < 9.2.0

Patched in: 9.2.0


# Vendor advisory (read the KB number that ships the fix from here):
# https://github.com/kovidgoyal/calibre/security/advisories/GHSA-32vh-whvh-9fxr

# Confirm whether the KB is already installed (substitute the real KB ID).
Get-HotFix | Where-Object { $_.HotFixID -like 'KB*' } | Sort-Object InstalledOn -Desc | Select-Object -First 10

# Install the latest cumulative + security rollup that contains the fix.
Install-Module PSWindowsUpdate -Force -SkipPublisherCheck -Confirm:$false
Import-Module PSWindowsUpdate
Get-WindowsUpdate -AcceptAll -Install -AutoReboot

# After reboot, confirm the patch landed.
Get-HotFix | Sort-Object InstalledOn -Desc | Select-Object -First 5


# Fleet inventory of missing security updates via Ansible (winrm).
ansible windows -m win_updates -a "category_names=SecurityUpdates state=installed"

Verify the fix landed


# Vendor advisory: https://github.com/kovidgoyal/calibre/security/advisories/GHSA-32vh-whvh-9fxr
# Post-patch verification (replace <service> with the real service unit).
journalctl -u <service> --since "10 minutes ago"
dmesg --since "10 minutes ago"

# Re-scan with your vulnerability scanner (Nessus, Qualys, Tenable, OpenVAS).
# It should no longer flag CVE-2026-25635 on the patched target.

If you cannot patch immediately

Block requests containing ../, ..%2f, or absolute path prefixes at a reverse proxy. Restrict access to the affected endpoint to trusted networks. Apply the patched build as the real fix.

How to verify the fix worked

If your installation was internet-reachable during the disclosure window, treat log review as part of the remediation rather than an optional follow-up. Look for unusually long URI paths containing traversal sequences, unexpectedly large responses from the affected endpoint, and outbound requests from the application to internal addresses or cloud-metadata endpoints. Treat any sensitive file the bug could disclose as exposed.

Frequently asked questions

Is CVE-2026-25635 being exploited in the wild?

Public exploitation has not been confirmed by CISA at the time of writing. Treat the patch as time-sensitive anyway; reports often lag actual abuse.

Will a WAF or IDS rule fully mitigate CVE-2026-25635?

No. Network-layer filters can reduce noise and slow opportunistic scanners, but they will not stop a determined attacker. The vendor patch is the only durable fix.

How long should I plan for the upgrade?

Typical vendor-documented upgrade windows for calibre run from a few minutes to under an hour depending on cluster size. Test in a staging environment first and follow the vendor's documented HA upgrade order.

References

*This guide was assembled from the official vendor advisory, the NVD record, and the CISA KEV catalog entry on 2026-05-25. Always confirm against the vendor advisory before applying changes in production.*