CVE-2026-21883

CVE-2026-21883 is a medium-severity security vulnerability in bokeh (pip), affecting versions < 3.8.2. It is fixed in 3.8.2.

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Summary

Bokeh server applications have Incomplete Origin Validation in WebSockets

This vulnerability allows for Cross-Site WebSocket Hijacking (CSWSH) of a deployed Bokeh server instance.

Scope

This vulnerability is only relevant to deployed Bokeh server instances. There is no impact on static HTML output, standalone embedded plots, or Jupyter notebook usage.

This vulnerability does not prevent any requirements for up-front authentication on Bokeh servers that have authentication hooks in place, and cannot be used to make Bokeh servers deployed on private, internal networks accessible outside those networks.

Workarounds

None

Technical description

The match_host function in src/bokeh/server/util.py contains a flaw in how it compares hostnames against the allowlist patterns. The function uses Python's zip() function to iterate over the parts of the hostname and the pattern simultaneously. However, zip() stops iteration when the shortest iterable is exhausted.

Because the code only checks if the pattern is longer than the host (lines 232-233), but fails to check if the host is longer than the pattern, a host that starts with the pattern (but has additional segments) will successfully match.

For example, if the allowlist is configured to ['[example.com](http://example.com/)'], the function will incorrectly validate [example.com.bad.com](http://example.com.evil.com/) as a match:

  1. host parts: ['example', 'com', 'bad', 'com']
  2. pattern parts: ['example', 'com']
  3. zip compares example==example (OK) and com==com (OK).
  4. Iteration stops, and the function returns True.

Impact

If a Bokeh server is configured with an allowlist (e.g., dashboard.corp), an attacker can register a domain like dashboard.corp.attacker.com (or use a subdomain if applicable) and lure a victim to visit it. The malicious site can then initiate a WebSocket connection to the vulnerable Bokeh server. Since the Origin header (e.g., http://dashboard.corp.attacker.com/) matches the allowlist according to the flawed logic, the connection is accepted.

Once connected, the attacker can interact with the Bokeh server on behalf of the victim, potentially accessing sensitive data, or modifying visualizations.

Affected versions

bokeh (< 3.8.2)

Security releases

bokeh → 3.8.2 (pip)

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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Remediation advice

Patched in versions 3.8.2 and later.

Frequently Asked Questions

  1. What is CVE-2026-21883? CVE-2026-21883 is a medium-severity security vulnerability in bokeh (pip), affecting versions < 3.8.2. It is fixed in 3.8.2.
  2. Which versions of bokeh are affected by CVE-2026-21883? bokeh (pip) versions < 3.8.2 is affected.
  3. Is there a fix for CVE-2026-21883? Yes. CVE-2026-21883 is fixed in 3.8.2. Upgrade to this version or later.
  4. Is CVE-2026-21883 exploitable, and should I be worried? Whether CVE-2026-21883 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
  5. What actually determines whether CVE-2026-21883 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.
  6. How do I fix CVE-2026-21883? Upgrade bokeh to 3.8.2 or later.

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