CVE-2025-66020

The EMOJI_REGEX used in the emoji action is vulnerable to a Regular Expression Denial of Service (ReDoS) attack. A short, maliciously crafted string (e.g., <100 characters) can cause the regex engine to consume excessive CPU time (minutes), leading to a Denial of Service (DoS) for the application.

Details

The ReDoS vulnerability stems from "catastrophic backtracking" in the EMOJI_REGEX. This is caused by ambiguity in the regex pattern due to overlapping character classes.

Specifically, the class \p{Emoji_Presentation} overlaps with more specific classes used in the same alternation, such as [\u{1F1E6}-\u{1F1FF}] (regional indicator symbols used for flags) and \p{Emoji_Modifier_Base}.

When the regex engine attempts to match a string that almost matches but ultimately fails (like the one in the PoC), this ambiguity forces it to explore an exponential number of possible paths. The matching time increases exponentially with the length of the crafted input, rather than linearly.

PoC

The following code demonstrates the vulnerability.

import * as v from 'valibot';

const schema = v.object({
  x: v.pipe(v.string(), v.emoji()),
});

const attackString = '\u{1F1E6}'.repeat(49) + '0';

console.log(`Input length: ${attackString.length}`);
console.log('Starting parse... (This will take a long time)');

// On my machine, a length of 99 takes approximately 2 minutes.
console.time();
try {
  v.parse(schema, {x: attackString });
} catch (e) {}
console.timeEnd();

Any project using Valibot's emoji validation on user-controllable input is vulnerable to a Denial of Service attack.

An attacker can block server resources (e.g., a web server's event loop) by submitting a short string to any endpoint that uses this validation. This is particularly dangerous because the attack string is short enough to bypass typical input length restrictions (e.g., maxLength(100)).

A regular expression with worst-case exponential or polynomial matching time is applied to untrusted input, causing excessive CPU use. Typical impact: denial of service when input is crafted to trigger backtracking.

CVE-2025-66020 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. 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 (1.2.0); upgrading removes the vulnerable code path.

Kodem intelligence

The root cause is the overlapping character classes. This can be resolved by making the alternatives mutually exclusive, typically by using negative lookaheads ((?!...)) to subtract the specific classes from the more general one.

The following modified EMOJI_REGEX applies this principle:

export const EMOJI_REGEX: RegExp =
  // eslint-disable-next-line redos-detector/no-unsafe-regex, regexp/no-dupe-disjunctions -- false positives
  /^(?:[\u{1F1E6}-\u{1F1FF}]{2}|\u{1F3F4}[\u{E0061}-\u{E007A}]{2}[\u{E0030}-\u{E0039}\u{E0061}-\u{E007A}]{1,3}\u{E007F}|(?:\p{Emoji}\uFE0F\u20E3?|\p{Emoji_Modifier_Base}\p{Emoji_Modifier}?|(?![\p{Emoji_Modifier_Base}\u{1F1E6}-\u{1F1FF}])\p{Emoji_Presentation})(?:\u200D(?:\p{Emoji}\uFE0F\u20E3?|\p{Emoji_Modifier_Base}\p{Emoji_Modifier}?|(?![\p{Emoji_Modifier_Base}\u{1F1E6}-\u{1F1FF}])\p{Emoji_Presentation}))*)+$/u;

1. What is CVE-2025-66020? CVE-2025-66020 is a high-severity inefficient regular expression (ReDoS) vulnerability in valibot (npm), affecting versions >= 0.31.0, < 1.2.0. It is fixed in 1.2.0. A regular expression with worst-case exponential or polynomial matching time is applied to untrusted input, causing excessive CPU use.
2. How severe is CVE-2025-66020? CVE-2025-66020 has a CVSS score of 7.5 (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.
3. Which versions of valibot are affected by CVE-2025-66020? valibot (npm) versions >= 0.31.0, < 1.2.0 is affected.
4. Is there a fix for CVE-2025-66020? Yes. CVE-2025-66020 is fixed in 1.2.0. Upgrade to this version or later.
5. Is CVE-2025-66020 exploitable, and should I be worried? Whether CVE-2025-66020 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
6. What actually determines whether CVE-2025-66020 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.
7. How do I fix CVE-2025-66020? Upgrade valibot to 1.2.0 or later.