GHSA-6x64-9x62-f2gx CVE-2026-50159
Mermaid allows CSS injection applying to sibling elements of the diagram
- Severity
- 5.3 / 10
- Medium · CVSS 4.0
- Exploit chance (EPSS)
- 0.60%
- Chance of exploitation in the next 30 days. Higher than 48.85% of scored vulnerabilities.
- Known exploited
- No
- Not in CISA's catalog
- Weakness
- CWE-94 Improper Control of Generation of Code ('Code Injection')
- Fix
11.16.1- Upgrade mermaid to this version or later
Description
Summary
Mermaid does not fully restrict CSS to the rendered SVG subtree. Although selectors are prefixed with #mermaid-X, sibling (~ and +) combinators can still escape the Mermaid container and inject styles to DOM elements adjacent to the diagram <svg>.
Most users of mermaid would not be affected by this, as mermaid adds its <svg> as an only child of it's parent element. However, you may be affected if you manually insert the <svg> (or other elements) into the DOM yourself.
Details
Mermaid namespaces CSS through with a middleware intended to scope all rules to the diagram's SVG element. CSS nesting expands & ~ * { ... } to #svgId ~ *, which selects all sibling elements following the SVG in the DOM, outside the diagram boundary.
Impact
An attacker able to supply diagram source to a page (e.g., user-generated content rendered by Mermaid) could inject CSS rules affecting sibling elements to the diagram <svg> on the host page. This can be used for UI redressing, hiding content, conditional CSS-based probing, or phishing-style visual manipulation.
JavaScript execution is not possible via this vector.
Patches
This has been patched in https://github.com/mermaid-js/mermaid/commit/12d472c9ed43f94814b110da8d7a9ae6dd5266ed and released in Mermaid v11.16.1.
A backport has been made for the v10 branch in 7e83f1533318b307764d961906a73377266f4c5e and was released in Mermaid v10.9.8
Workarounds
If you are inserting the <svg> into the DOM yourself, you can wrap it in an element with no other children, e.g. <div><svg>...</svg></div> or element.innerHTML = svg. Alternatively, you can use mermaid.run() or mermaid.initialize() which will do this for you.
Setting "securityLevel": "sandbox" will also prevent this, or setting the secure config value in the mermaid config to avoid allowing diagrams to modify fontFamily, themeCSS, altFontFamily, and themeVariables.
To test, you can try using a themeCSS with & + * { /* my CSS here */} and see if it's applied outside of your mermaid <svg>.
---
config:
themeCSS: |-
& + * { background:red !important; width:100vw !important; height:100vh !important; position:fixed !important; inset:0 !important; }
---
info
References
- GHSA-87f9-hvmw-gh4p/CVE-2026-41159 (related vulnerability)
- https://github.com/mermaid-js/mermaid/commit/12d472c9ed43f94814b110da8d7a9ae6dd5266ed
- https://github.com/mermaid-js/mermaid/releases/tag/mermaid%4011.16.1
- https://github.com/mermaid-js/mermaid/commit/7e83f1533318b307764d961906a73377266f4c5e
- https://github.com/mermaid-js/mermaid/releases/tag/v10.9.8
Affected versions
npm
| Package | Vulnerable | Fixed in |
|---|---|---|
| mermaid | >= 11.0.0-alpha.1, < 11.16.1 |
11.16.1 |
| mermaid | < 10.9.8 |
10.9.8 |
| mermaid | >= 11.0.0-alpha.1, < 11.16.1 |
11.16.1 |
| mermaid | < 10.9.8 |
10.9.8 |
How to fix
Move to the fixed version or later with your package manager, update the lockfile, and run your tests. If the package only arrives through another dependency, upgrade that dependency first. If none of its releases pulls a fixed version yet, force the version with your package manager's override.
mermaid npm → 11.16.1
- npm
npm install mermaid@11.16.1- yarn
yarn add mermaid@11.16.1- pnpm
pnpm add mermaid@11.16.1
Severity breakdown
CVSS 4.0 5.3
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:L/VA:N/SC:L/SI:L/SA:L
- Attack vector
- Network
- Attack complexity
- Low
- Attack requirements
- None
- Privileges required
- None
- User interaction
- Passive
- Confidentiality (vulnerable system)
- None
- Integrity (vulnerable system)
- Low
- Availability (vulnerable system)
- None
- Confidentiality (subsequent systems)
- Low
- Integrity (subsequent systems)
- Low
- Availability (subsequent systems)
- Low
Weakness
CWE-94: Improper Control of Generation of Code ('Code Injection')
The product constructs all or part of a code segment using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the syntax or behavior of the intended code segment.
How to prevent it
- Architecture and Design: Refactor your program so that you do not have to dynamically generate code.
- Architecture and Design: Run your code in a "jail" or similar sandbox environment that enforces strict boundaries between the process and the operating system. This may effectively restrict which code can be executed by your product. Examples include the Unix chroot jail and AppArmor. In general, managed code may provide some protection. This may not be a feasible solution, and it only limits the impact to the operating system; the rest of your application may still be subject to compromise. Be careful to avoid CWE-243 and other weaknesses related to jails.
- Implementation: Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue." Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright. To reduce the likelihood of code injection, use stringent allowlists that limit which constructs are allowed. If you are dynamically constructing code that invokes a function, then verifying that the input is alphanumeric might be insufficient. An attacker might still be able to reference a dangerous function that you did not intend to allow, such as system(), exec(), or exit().
- Testing: Use dynamic tools and techniques that interact with the product using large test suites with many diverse inputs, such as fuzz testing (fuzzing), robustness testing, and fault injection. The product's operation may slow down, but it should not become unstable, crash, or generate incorrect results.
- Operation: Run the code in an environment that performs automatic taint propagation and prevents any command execution that uses tainted variables, such as Perl's "-T" switch. This will force the program to perform validation steps that remove the taint, although you must be careful to correctly validate your inputs so that you do not accidentally mark dangerous inputs as untainted (see CWE-183 and CWE-184).
- Operation: Run the code in an environment that performs automatic taint propagation and prevents any command execution that uses tainted variables, such as Perl's "-T" switch. This will force the program to perform validation steps that remove the taint, although you must be careful to correctly validate your inputs so that you do not accidentally mark dangerous inputs as untainted (see CWE-183 and CWE-184).
- Implementation: For Python programs, it is frequently encouraged to use the ast.literal_eval() function instead of eval, since it is intentionally designed to avoid executing code. However, an adversary could still cause excessive memory or stack consumption via deeply nested structures [REF-1372], so the python documentation discourages use of ast.literal_eval() on untrusted data [REF-1373].
References
Advisories
Fixes and patches
Packages
Other links
Timeline
- Published
- Last updated by the source
- Latest EPSS score
Related advisories
| Advisory | Severity | Why it's related |
|---|---|---|
| GHSA-rhh3-jpg6-66xh Mermaid radar diagrams are vulnerable to DoS | Medium | Also affects mermaid |
| GHSA-c4c3-pg64-4m4v Mermaid configuration APIs allow prototype pollution | Low | Also affects mermaid |
| GHSA-3rrr-jr9j-h3q3 Mermaid Architecture diagrams are vulnerable to prototype pollution | Medium | Also affects mermaid |
| GHSA-2v8p-3f2j-5mp7 Mermaid XY Charts are vulnerable to an infinite loop DoS | Medium | Also affects mermaid |
| GHSA-87f9-hvmw-gh4p Mermaid: Improper sanitization of configuration leads to CSS injection | Medium | Also affects mermaid |
| GHSA-6m6c-36f7-fhxh Mermaid Gantt Charts are vulnerable to an Infinite Loop DoS | Medium | Also affects mermaid |
| GHSA-ghcm-xqfw-q4vr Mermaid: Improper sanitization of `classDef` in state diagrams leads to HTML injection | Medium | Also affects mermaid |
| GHSA-xcj9-5m2h-648r Mermaid: Improper sanitization of `classDefs` in diagrams leads to CSS injection | Medium | Also affects mermaid |
Credits: aloisklink, h3ri0s
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