CVE-2026-8927: env-set cross-proxy Digest auth state leak

Severity: critical (CVSS 9.1 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.5% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-07-03 · updated 2026-09-15
Weakness: CWE-294

Description

When reusing a libcurl handle for sequential transfers driven by environment-variable proxy configuration, libcurl fails to clear the proxy authentication state between requests. Specifically, if the initial transfer authenticates against `proxyA` using Digest auth, a subsequent transfer routed through `proxyB` erroneously leaks the `Proxy-Authorization:` header intended solely for `proxyA`.

Affected products

curl: from 7.12.0 before 8.21.0 — fixed in 8.21.0

How to fix

Upgrade to a fixed version: curl 8.21.0. Check the vendor advisory for mitigations.

References

https://curl.se/docs/CVE-2026-8927.html
https://curl.se/docs/CVE-2026-8927.json
https://hackerone.com/reports/3744543
https://hackerone.com/reports/3744543

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