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smol-toml is a small, fast, and correct TOML parser and serializer for JavaScript/TypeScript applications. Prior to version 1.7.1, the library’s `parse()` function contains a critical flaw that can be triggered by a specific malformed TOML document, leading to an infinite loop and a Denial of Service (DoS) condition.
The vulnerability is rooted in the `skipUntil()` function within the `src/util.ts` file. When the `parse()` function encounters a value inside an array or an inline table that is immediately followed by a comment with no trailing newline (i.e., the comment is the very last element of the document), the parser’s internal logic fails to correctly handle this edge case.
Specifically, the `skipUntil()` function calls `indexOfNewline()` to find the end of the current structure. When the input ends with a comment and lacks a trailing newline, this function returns -1, indicating that no newline character was found. Instead of correctly recognizing this as the end of the input and exiting the structure scan, the parser erroneously resets its cursor to the very beginning of the string.
This reset causes the parser to re-enter the same section of the document, beginning the scan anew. Because the malformed input condition persists, the parser becomes trapped in an infinite loop, repeatedly scanning the same content without ever making progress. This results in the thread consuming 100% CPU utilization indefinitely, effectively hanging the application process.
An attacker can exploit this vulnerability by sending a specially crafted TOML document to an application that uses the vulnerable library to parse untrusted input. For services that expose TOML parsing functionality over a network—such as web applications or APIs that accept user-provided configuration files—this poses a significant availability risk. A single malicious payload can render the service unresponsive, leading to a widespread outage for dependent systems.
The issue is addressed in version 1.7.1, where the parser’s logic has been corrected to properly break out of the loop and throw the expected `TomlError` when encountering this malformed input.
DailyCVE Form:
Platform: smol-toml
Version: <= 1.7.0
Vulnerability: Infinite Loop DoS
Severity: High (CVSS 8.2)
Date: 2026-09-04
Prediction: Patch already available (1.7.1)
What Undercode Say:
The vulnerability resides in the `skipUntil` function. The following analysis highlights the flawed logic:
Affected Code Path (`src/util.ts`):
// Simplified representation of the flawed logic
function skipUntil(str: string, start: number, delimiter: string): number {
let cursor = start;
while (true) {
// ... scanning logic ...
const newlineIndex = indexOfNewline(str, cursor);
if (newlineIndex === -1) {
// FLAW: Resets cursor to beginning instead of exiting
cursor = 0; // Should break/throw error
continue;
}
// ...
}
}
Triggering Payload:
a=[1 <-- No trailing newline; comment ends the document
Impact on Service:
- CPU pinned at 100%
- Process becomes unresponsive
- Denial of Service for dependent services
Dependency Check:
Check current version in package.json cat package.json | grep smol-toml Check installed version npm list smol-toml
Exploit: (Educational Purposes!)
The following proof-of-concept demonstrates how to trigger the vulnerability:
Node.js Exploit Script:
import { parse } from 'smol-toml';
// Malicious payload: array value followed by a comment with no trailing newline
const maliciousToml = 'a=[1 ';
try {
// This call will never return on vulnerable versions (<= 1.7.0)
const result = parse(maliciousToml);
console.log('Parsed successfully:', result);
} catch (error) {
console.error('Parsing error:', error);
}
cURL-based Attack (for web services):
Send malicious TOML to a vulnerable endpoint curl -X POST https://target-api.com/parse \ -H "Content-Type: application/toml" \ -d 'a=[1 '
Expected Outcome:
- The `parse()` function hangs indefinitely
- CPU usage spikes to 100%
- The application thread becomes unresponsive
Protection:
Immediate Action:
Upgrade to version 1.7.1 or later immediately:
npm install [email protected]
Verification:
Confirm the updated version npm list smol-toml Expected output: [email protected]
Defensive Measures:
- Implement request timeout mechanisms for TOML parsing operations
- Set CPU/memory resource limits on processes that parse untrusted data
- Audit codebases to identify all services using `smol-toml` for parsing external input
- Consider input validation or sanitization for TOML documents from untrusted sources
Workarounds:
None available that don’t involve significant refactors. Upgrading the library is the only effective solution.
Impact:
Severity:
– CVSS v4.0 Score: 8.2 (High)
– Attack Vector: Network
– Attack Complexity: Low
– Privileges Required: None
– User Interaction: None
– Availability Impact: High
Consequences:
- Applications parsing arbitrary TOML documents are vulnerable to major availability issues
- A single malicious payload can cause 100% CPU utilization and service unresponsiveness
- Particularly severe for web services, APIs, and any system processing user-provided TOML configuration
- Potential for widespread outages affecting dependent systems
Affected Versions:
- All versions prior to 1.7.0 are vulnerable
- Version 1.7.1 contains the fix
EPSS Score: 0.37% (probability of exploitation in the wild)
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Sources:
Reported By: github.com
Extra Source Hub:
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