# CVE-2026-6253: proxy credentials leak over redirect-to proxy

> An attacker can steal proxy credentials when curl follows redirects between different proxy configurations, exposing sensitive authentication data.

- **Severity:** Medium 5.9 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 13 May 2026
- **Updated:** 15 Sep 2026
- **Also known as:** ALPINE-CVE-2026-6253, AZL-86666, AZL-86676, BELL-CVE-2026-6253, CURL-CVE-2026-6253, DEBIAN-CVE-2026-6253, ECHO-e31e-7e52-e694, GHSA-6qjr-r96v-3wcx, JLSEC-2026-1206, MINI-6888-qchf-6vhq, MINI-fg45-x759-w8pp, ROOT-OS-DEBIAN-11-CVE-2026-6253, ROOT-OS-DEBIAN-12-CVE-2026-6253, UBUNTU-CVE-2026-6253

## What is CVE-2026-6253?

CVE-2026-6253 is a medium-severity vulnerability in Haxx Curl, published on 13 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

curl might erroneously pass on credentials for a first proxy to a second
proxy.

This can happen when the following conditions are true:

1. curl is setup to use specific different proxies for different URL schemes
2. the first proxy needs credentials
3. the second proxy uses no credentials
4. while using the first proxy (using say `http://`), curl is asked to follow
   a redirect to a URL using another scheme (say `https://`), accessed using a
   second, different, proxy

Weakness: CWE-522 (Insufficiently Protected Credentials)

## Which versions are affected by CVE-2026-6253?

It affects 1 product.


Products: Haxx Curl.

## How do I fix CVE-2026-6253?

Upgrade to a fixed version.

## Is CVE-2026-6253 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-6253 exists.

## Other vulnerabilities in Haxx Curl

- [CVE-2026-19931](https://insights.sbomify.com/CVE-2026-19931/): Negotiate ambient user conn reuse (critical 9.8)
- [CVE-2026-10536](https://insights.sbomify.com/CVE-2026-10536/): HTTP/2 stream-dependency tree UAF (critical 9.8)
- [CVE-2026-11856](https://insights.sbomify.com/CVE-2026-11856/): cross-origin Digest auth state leak (critical 9.8)
- [CVE-2026-8925](https://insights.sbomify.com/CVE-2026-8925/): SASL double-free (critical 9.8)
- [CVE-2026-9079](https://insights.sbomify.com/CVE-2026-9079/): stale proxy password leak (critical 9.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-6253 and fix it if it is affected.

What it is: CVE-2026-6253 is a medium-severity vulnerability in Haxx Curl, published on 13 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-6253 exists.

Affected product: Haxx Curl. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-6253/ (Markdown: https://insights.sbomify.com/CVE-2026-6253/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-6253
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-6253
- OSV: https://osv.dev/vulnerability/CVE-2026-6253

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-6253/
Summaries are generated and may contain mistakes; the official description above is the record.