Summary
Remote Code Execution Through Image Uploads in BookStack
Workarounds
Depending on BookStack version, you could use the local_secure image storage option, or use s3 or a similar compatible service.
Preventing direct execution of any php files, apart from the public/index.php file, though web-server configuration would also prevent this.
References
BookStack Beta v0.25.3
BookStack Beta v0.25.4
BookStack Beta v0.25.5
For more information
If you have any questions or comments about this advisory:
- Open an issue in the BookStack GitHub repository.
- Ask on the BookStack Discord chat.
- Follow the BookStack Security advise to contact someone privately.
Impact
A user could upload PHP files through image upload functions, which would allow them to execute code on the host system remotely. They would then have the permissions of the PHP process.
This most impacts scenarios where non-trusted users are given permission to upload images in any area of the application.
CVE-2020-5256 has a CVSS score of 7.9 (High). The vector is network-reachable, low 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 (0.25.5); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
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. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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The issue was addressed in a series of patches: v0.25.3, v0.25.4 and v0.25.5.
Users should upgrade to at least v0.25.5 to avoid this patch but ideally the latest BookStack version as previous versions are un-supported.
Frequently Asked Questions
- What is CVE-2020-5256? CVE-2020-5256 is a high-severity security vulnerability in ssddanbrown/bookstack (composer), affecting versions < 0.25.3. It is fixed in 0.25.5.
- How severe is CVE-2020-5256? CVE-2020-5256 has a CVSS score of 7.9 (High). 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.
- Which versions of ssddanbrown/bookstack are affected by CVE-2020-5256? ssddanbrown/bookstack (composer) versions < 0.25.3 is affected.
- Is there a fix for CVE-2020-5256? Yes. CVE-2020-5256 is fixed in 0.25.5. Upgrade to this version or later.
- Is CVE-2020-5256 exploitable, and should I be worried? Whether CVE-2020-5256 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
- What actually determines whether CVE-2020-5256 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.
- How do I fix CVE-2020-5256? Upgrade
ssddanbrown/bookstackto 0.25.5 or later.