CVE-2026-21883 — bokeh
Fix: bokeh/bokeh@cedd113CVE-2026-21883 is a CWE-1385 vulnerability in bokeh. A fix is available for bokeh — see the affected versions and patch details below.
Bokeh server applications have Incomplete Origin Validation in WebSockets
Exploitation Status
No confirmed exploitation observed yet
- A successful exploit gives an attacker total control of the affected component, not partial access.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-21883.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
Real-World Exposure
bokehReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
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.
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.
Patches
Patched in versions 3.8.2 and later.
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:
hostparts:['example', 'com', 'bad', 'com']patternparts:['example', 'com']zipcomparesexample==example(OK) andcom==com(OK).- Iteration stops, and the function returns
True.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | bokeh | all versions | 3.8.2pip install --upgrade 'bokeh==3.8.2' |
Affected Products
bokehbokehDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for bokeh, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update bokeh to 3.8.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-21883 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
This vulnerability is rated Moderate for Red Hat OpenShift AI (RHOAI) as it affects Bokeh server applications with an allowlist configured. An attacker could exploit this by registering a malicious domain that appears to be a subdomain of an allowed origin, then luring a victim to visit it. This allows the attacker to…
Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.Source: Red Hat security advisory for CVE-2026-21883 (CC BY 4.0)
Frequently Asked Questions
Is CVE-2026-21883 in your dependencies?
Find it across PyPI, including transitive dependencies.