⚠ 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 7.8

How to Fix CVE-2026-25731: Server-Side Template Injection in calibre

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

*By Sai Kiran Pandrala*

⚡ At a glance
SeverityCVSS 7.8 - High
Actively exploited?Not currently listed in CISA KEV
Affected< 9.2.0
Fixed in9.2.0.
Type (CWE)CWE-1336: Improper Neutralization of Special Elements Used in a Template Engine

What is CVE-2026-25731?

CVE-2026-25731 is a server-side template injection flaw in calibre. User input is rendered as part of a template expression, which the engine then evaluates, giving the attacker code execution. Vendor description: calibre is an e-book manager. Prior to 9.2.0, a Server-Side Template Injection (SSTI) vulnerability in Calibre's Templite templating engine allows arbitrary code execution when a user converts an ebook using a malicious custom template file via the --template-html or --template-html-index command-line options.

Why this CVE matters

Server-side template injection is one of the cleanest paths from web request to code execution. The vulnerable parameter is usually attacker-controlled by design, which means proof-of-concept payloads are short and reliable.

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-2026-25731

  1. Read the vendor advisory in full: https://github.com/kovidgoyal/calibre/security/advisories/GHSA-xrh9-w7qx-3gcc
  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-xrh9-w7qx-3gcc

Affected: calibre: < 9.2.0

Patched in: 9.2.0


# Vendor advisory: https://github.com/kovidgoyal/calibre/security/advisories/GHSA-xrh9-w7qx-3gcc

# npm / Yarn / pnpm.
npm install calibre@9.2.0
npm ls calibre

# Python / pip.
python -m pip install --upgrade "calibre>=9.2.0"
python -m pip show calibre

# Container image.
docker pull <your-registry>/calibre:9.2.0
docker stop calibre && docker rm calibre
docker run -d --name calibre <your-registry>/calibre:9.2.0


# Vendor advisory: https://github.com/kovidgoyal/calibre/security/advisories/GHSA-xrh9-w7qx-3gcc
# Same flow from a Windows admin workstation.
npm install calibre@9.2.0
docker pull <your-registry>/calibre:9.2.0

Verify the fix landed


# Vendor advisory: https://github.com/kovidgoyal/calibre/security/advisories/GHSA-xrh9-w7qx-3gcc
# 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-2026-25731 on the patched target.

If you cannot patch immediately

Block the affected endpoint or input from untrusted users. There is no configuration-only mitigation; patch is required.

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 unexpected administrator accounts in calibre, scheduled tasks or cron jobs you did not create, new files in web-accessible directories, and outbound connections to addresses not in your baseline. Suspicious requests to the vulnerable endpoint immediately followed by successful 200-class responses with unusually large bodies are a strong indicator of exploitation.

Frequently asked questions

Is CVE-2026-25731 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-2026-25731?

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.*