Summary
Koa uses an evil regex to parse the X-Forwarded-Proto and X-Forwarded-Host HTTP headers. This can be exploited to carry out a Denial-of-Service attack.
PoC
Coming soon.
Impact
This is a Regex Denial-of-Service attack and causes memory exhaustion. The regex should be improved and empty values should not be allowed.
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.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
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. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Remediation advice
koa to 2.15.4 or later; koa to 3.0.0-alpha.3 or later; koa to 1.7.1 or later; koa to 0.21.2 or later
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
Frequently Asked Questions
- What is CVE-2025-25200? CVE-2025-25200 is a critical-severity inefficient regular expression (ReDoS) vulnerability in koa (npm), affecting versions >= 2.0.0, < 2.15.4. It is fixed in 2.15.4, 3.0.0-alpha.3, 1.7.1, 0.21.2. A regular expression with worst-case exponential or polynomial matching time is applied to untrusted input, causing excessive CPU use.
- Which versions of koa are affected by CVE-2025-25200? koa (npm) versions >= 2.0.0, < 2.15.4 is affected.
- Is there a fix for CVE-2025-25200? Yes. CVE-2025-25200 is fixed in 2.15.4, 3.0.0-alpha.3, 1.7.1, 0.21.2. Upgrade to this version or later.
- Is CVE-2025-25200 exploitable, and should I be worried? Whether CVE-2025-25200 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
- What actually determines whether CVE-2025-25200 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.
- How do I fix CVE-2025-25200?
- Upgrade
koato 2.15.4 or later - Upgrade
koato 3.0.0-alpha.3 or later - Upgrade
koato 1.7.1 or later - Upgrade
koato 0.21.2 or later
- Upgrade