CVE-2026-59947 is a medium-severity (CVSS 4.7) CWE-532 vulnerability in composer/composer. A fix is available for composer/composer — see the affected versions and patch details below.
Composer: URL-embedded HTTP-Basic username leaks to verbose logs (GitHub PAT exposure)
Exploitation Status
No confirmed exploitation observed yet
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-59947.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2026-59947 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 394,803 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
composer/composer🐘composer/composerReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Packagist packages — download data is not available via public APIs for these ecosystems.
Description
Summary
When Composer is run with -vvv (debug verbosity), it could print a credential that was embedded directly in a repository or package URL, but not as a password, to its debug output. Composer already masked the password portion of such URLs, but the username portion was shown in clear text. Because GitHub and several other services support placing an access token in the username position of a URL (for example https://[token]@github.com/owner/repo), a token used that way could end up written to the verbose log in full.
This is an information disclosure issue. The credential is only ever exposed to whoever can already read Composer's debug output.
Am I affected?
You are potentially affected only if all of the following apply:
- A credential is embedded inside a URL that Composer handles, e.g. in a
repositoriesentry in composer.json, in a package dist/source URL - rather than being supplied through auth.json or the COMPOSER_AUTH environment variable. - The secret sits in the username slot of that URL (e.g.
https://TOKEN@host/…). A normalusername:password@hostpair where the username is an ordinary account name did not expose the password, that was already masked. - Composer is run with -vvv (debug verbosity), and that output is retained or shared somewhere others can read it: public CI build logs, output pasted into an issue or chat, archived terminal sessions, and so on.
If you keep credentials in auth.json or environment variables, or you never run Composer at debug verbosity, you were not exposed.
The most realistic exposure is the documented pattern of embedding a GitHub Personal Access Token in a URL's username position on a machine (often CI) that captures verbose output.
Patched versions
The username is now masked the same way the password already was, so an embedded token no longer appears in verbose output (a short, non-secret prefix may be shown to aid debugging, but never the full value).
Fixed in Composer 2.10.2 and the 2.2.29.
Workarounds
There is no configuration setting that prevents outputting credentials while keeping normal behavior.
If you cannot upgrade right away, you can reduce exposure by:
- Not running Composer with -vvv (debug verbosity) in environments where output is captured or shared.
- Moving credentials out of URLs and into auth.json or COMPOSER_AUTH.
- Scrubbing existing CI/build logs that may already contain a leaked token.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐘Packagist | composer/composer | ≥ 2.3.0&&< 2.10.2 | 2.10.2composer require composer/composer:^2.10.2 |
| 🐘Packagist | composer/composer | ≥ 1.0.0&&< 2.2.29 | 2.2.29composer require composer/composer:^2.2.29 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for composer/composer, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update composer/composer to 2.10.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-59947 is resolved across your whole dependency graph.
Workarounds
Assume what was exposed is already known: rotate any credential, token or key that the affected component could return, restrict the endpoint to callers that genuinely need it, and strip sensitive fields from responses and error output at the boundary rather than relying on the client not to read them.
Frequently Asked Questions
Is CVE-2026-59947 in your dependencies?
Find it across Packagist, including transitive dependencies.