Summary
Hono: ReDoS in CORS middleware via Access-Control-Request-Headers
The built-in CORS middleware (hono/cors) parses the attacker-controlled Access-Control-Request-Headers request header during a preflight (OPTIONS) request using a regular expression whose running time is quadratic in the input length. A single request carrying a long run of whitespace can consume seconds of CPU, and repeated requests can render the service unresponsive. This parsing runs under the default configuration.
Details
On a CORS preflight, when allowHeaders is not configured - the default - the middleware reflects and parses the Access-Control-Request-Headers value. The parser used a whitespace-tolerant regular expression whose backtracking makes the work grow quadratically (O(n²)) with the length of the value when it contains a long whitespace sequence without a delimiter.
Because the header value is bounded only by the deployment's maximum HTTP header size, a single preflight can block request processing for a noticeable amount of time; on runtimes that share one execution thread across requests, this stalls concurrent requests as well. No authentication, special origin, or user interaction is required.
This issue arises for any application using cors() with the default (or an empty) allowHeaders. Applications that set a non-empty allowHeaders do not reach the affected path.
Impact
An unauthenticated attacker can send preflight requests that each consume disproportionate CPU relative to their size, degrading or denying service. This is a denial-of-service issue only; it does not expose or modify data.
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-2026-69207 has a CVSS score of 5.3 (Medium). 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 (4.12.34); upgrading removes the vulnerable code path.
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.
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Frequently Asked Questions
- What is CVE-2026-69207? CVE-2026-69207 is a medium-severity inefficient regular expression (ReDoS) vulnerability in hono (npm), affecting versions < 4.12.34. It is fixed in 4.12.34. A regular expression with worst-case exponential or polynomial matching time is applied to untrusted input, causing excessive CPU use.
- How severe is CVE-2026-69207? CVE-2026-69207 has a CVSS score of 5.3 (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.
- Which versions of hono are affected by CVE-2026-69207? hono (npm) versions < 4.12.34 is affected.
- Is there a fix for CVE-2026-69207? Yes. CVE-2026-69207 is fixed in 4.12.34. Upgrade to this version or later.
- Is CVE-2026-69207 exploitable, and should I be worried? Whether CVE-2026-69207 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-2026-69207 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-2026-69207? Upgrade
honoto 4.12.34 or later.