Summary
Regular Expression Denial of Service in Deno.upgradeWebSocket API
Impact
Versions of the package deno before 1.31.0 are vulnerable to Regular Expression Denial of Service (ReDoS) due to the upgradeWebSocket function, which contains regexes in the form of /s*,s*/, used for splitting the Connection/Upgrade header. A specially crafted Connection/Upgrade header can be used to significantly slow down a web socket server.
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-2023-26103 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 (1.31.0); 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.
Remediation advice
It is recommended that users upgrade to Deno 1.31.0.
Frequently Asked Questions
- What is CVE-2023-26103? CVE-2023-26103 is a medium-severity inefficient regular expression (ReDoS) vulnerability in deno (rust), affecting versions >= 1.12.0, < 1.31.0. It is fixed in 1.31.0. A regular expression with worst-case exponential or polynomial matching time is applied to untrusted input, causing excessive CPU use.
- How severe is CVE-2023-26103? CVE-2023-26103 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 deno are affected by CVE-2023-26103? deno (rust) versions >= 1.12.0, < 1.31.0 is affected.
- Is there a fix for CVE-2023-26103? Yes. CVE-2023-26103 is fixed in 1.31.0. Upgrade to this version or later.
- Is CVE-2023-26103 exploitable, and should I be worried? Whether CVE-2023-26103 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-2023-26103 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-2023-26103? Upgrade
denoto 1.31.0 or later.