CVE-2026-55254

NCalc: Denial of Service via Unbounded and Non-Terminating Factorial Evaluation

Workarounds

If upgrading is not immediately possible:

• Do not evaluate expressions originating from untrusted users.
• Validate or sanitize expressions before evaluation and reject factorial operations on large values.
• Implement execution time limits, request timeouts, or cancellation mechanisms around expression evaluation.

These mitigations may reduce exposure but do not fully address the underlying vulnerability.

A denial-of-service (DoS) vulnerability exists in the factorial operator implementation of NCalc. Specially crafted expressions containing extremely large factorial operands can trigger excessive CPU consumption or cause evaluation to enter a non-terminating loop due to integer overflow in the factorial calculation logic.

Applications that evaluate untrusted expressions using affected versions of NCalc may be vulnerable to resource exhaustion, potentially resulting in service disruption or application unresponsiveness.

This issue can be triggered with expressions such as:

99999999999999!
9223372036854775807!
1.5e16!

An arithmetic operation produces a value that exceeds the integer type's maximum, causing it to wrap to an unexpected small value. Typical impact: incorrect size calculations leading to heap overflows or logic errors.

CVE-2026-55254 has a CVSS score of 4.8 (Medium). The vector is reachable from an adjacent network, no privileges required, and user interaction required. 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 (6.1.1); upgrading removes the vulnerable code path.

Kodem intelligence

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The vulnerability has been fixed by adding bounds validation for factorial operands and rejecting unsupported values before evaluation.

Users should upgrade to the first release containing the fix from pull request #575. (v6.1.1+)

1. What is CVE-2026-55254? CVE-2026-55254 is a medium-severity integer overflow or wraparound vulnerability in NCalc.Core (nuget), affecting versions < 6.1.1. It is fixed in 6.1.1. An arithmetic operation produces a value that exceeds the integer type's maximum, causing it to wrap to an unexpected small value.
2. How severe is CVE-2026-55254? CVE-2026-55254 has a CVSS score of 4.8 (Medium). 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 packages are affected by CVE-2026-55254?
   • NCalc.Core (nuget) (versions < 6.1.1)
   • NCalcSync (nuget) (versions < 6.1.1)
4. Is there a fix for CVE-2026-55254? Yes. CVE-2026-55254 is fixed in 6.1.1. Upgrade to this version or later.
5. Is CVE-2026-55254 exploitable, and should I be worried? Whether CVE-2026-55254 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-2026-55254 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-2026-55254?
   • Upgrade NCalc.Core to 6.1.1 or later
   • Upgrade NCalcSync to 6.1.1 or later