The npm classnames package is a small, dependency-free utility that conditionally joins CSS class names into a single string, and from a security standpoint it is about as low-risk as a JavaScript library gets, but "low-risk" is not the same as "no-risk," and the interesting exposure is in the supply chain rather than the code. With something on the order of 28 million weekly downloads, classnames npm is one of the most depended-upon packages in the front-end ecosystem, which is exactly why it is worth understanding what it does and does not do.
The library's job is trivial to describe. You pass it strings, objects, and arrays, and it returns a space-separated class string, dropping any falsy values. In React you write className={classNames('btn', { 'btn-active': isActive })} and get "btn" or "btn btn-active" depending on state. That is the entire feature set. The current version is 2.5.1, and the API has been stable for years.
Why a class-joining utility has a security profile at all
A function that concatenates strings does not sound like an attack surface, and directly, it is not. classnames does not evaluate code, touch the network, read the filesystem, or handle untrusted input in any dangerous way. It has no runtime dependencies of its own, which is the single most important security property a package can have, because a package with zero dependencies cannot pull a compromised transitive package into your tree. There is no meaningful history of code-execution vulnerabilities in classnames, and given what it does, there is little room for one.
The risk, such as it is, comes from three directions that apply to any popular micro-package.
Supply-chain risk: the real concern
The first and most serious risk is account or publish compromise. classnames is downloaded tens of millions of times a week, which makes its npm publishing account a high-value target. If an attacker were to gain publish rights and push a malicious version, that version would flow into an enormous number of builds within hours through automatic minor and patch upgrades. This is not hypothetical for the ecosystem, popular packages have been hijacked this way, and it is the reason the defense that matters most is not auditing classnames' code but controlling how versions enter your build.
Pin your dependencies with a committed lockfile so a build uses the exact resolved version you tested, not whatever the registry serves at build time. Review dependency updates rather than auto-merging them blindly, especially for packages this deep in your tree. And run software composition analysis in your pipeline so that if a known-bad version is published and flagged, an SCA scan catches it before it ships. The lockfile plus a scanning gate is what turns "a popular package could be hijacked" from a catastrophe into a caught incident.
Typosquatting and confusion
The second risk is installing the wrong package. Because classnames is so common, its name is a natural target for typosquatting: an attacker publishes a package with a name one keystroke away, hoping a developer or a build script fetches the malicious look-alike. The mitigation is boring but effective. Copy the exact package name from the official listing rather than typing it, review what your package.json actually references, and let your scanner flag packages that are suspiciously similar to popular names. The alternative library clsx is a legitimate, smaller drop-in with the same API, and it is worth knowing which one you actually intend to use so you can tell a real substitution from a malicious one.
The one real footgun: it does not sanitize
The third risk is a usage mistake, not a library flaw. classnames joins whatever you give it. If you pass user-controlled input as a class name, that string ends up in your rendered HTML's class attribute. React escapes attribute values, so this is not a direct XSS vector in normal React rendering, but the pattern is still wrong: user input should never determine CSS class names, because it can enable CSS-based data exfiltration or let a user trigger styles they should not have access to. Keep class names derived from your own application state, never from raw user input. This is a discipline issue, and no library can enforce it for you.
Keeping it current
Even a stable package deserves a place in your update routine. classnames rarely changes, but the value of keeping it current is not about fixing classnames bugs, it is about staying on versions your tooling recognizes and not letting your dependency tree ossify. A tree where every package is pinned and never touched is a tree where, when you finally must upgrade something for a real security reason, you face a wall of accumulated breaking changes. Small, regular updates with a lockfile and a scan are healthier than a frozen tree you are afraid to touch.
A sensible checklist
For a package this benign, the security work is proportionate and quick. Commit a lockfile so versions are reproducible. Run an SCA scan in CI so a hijacked or vulnerable version is flagged before it ships. Verify the package name matches the official one to avoid typosquats. Never feed user input into class names. And fold classnames into a normal, low-effort dependency-update cadence rather than freezing it forever. Do those five things and the npm classnames package is exactly what it looks like: a safe, tiny utility that does one job well.
FAQ
Does npm classnames have any known vulnerabilities?
There is no significant history of code-execution vulnerabilities in classnames, and its zero-dependency design means it cannot drag in a compromised transitive package. The relevant risk is supply-chain compromise of the package itself, which you mitigate with lockfiles and scanning rather than by auditing its code.
Is classnames safe to use with user input?
Do not pass raw user input as a class name. React escapes attribute values so it is not a direct XSS vector, but user-controlled class names can enable CSS-based abuse. Derive class names from your own application state.
classnames or clsx, which should I use?
Both are safe and share the same API. clsx is smaller and faster; classnames is more established with more weekly downloads. Pick one deliberately and pin it, so a substitution of one for the other is a conscious choice rather than a possible typosquat.
How do I protect against a hijacked classnames version?
Commit a lockfile so builds use the version you tested, review dependency bumps instead of auto-merging, and run software composition analysis in CI so a flagged malicious version is caught before it reaches production.