CVE-2026-66339: Libsoup: libsoup: proxy credentials leak to destination server via proxy-authorization header in connect tunnels

mediumCVSS 6.5CVE-2026-66339

CSIRTS triage

vendor: GNOME product: libsoup Information disclosure

What: Proxy credentials are leaked to the destination server via the proxy-authorization header in CONNECT tunnels.
Who is affected: All users of libsoup that route traffic through HTTP proxies with authentication.
Urgency: Medium priority; not currently exploited but credentials exposure poses a security risk.
Action: Update libsoup to a patched version when available.

AI-assisted analysis generated from the source advisory — verify against the original.

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Details

Source: Microsoft Security Response Center (INTL · vendor-psirt · site)
Severity: medium — CVSS 6.5
Published: 2026-08-06
Exploitation: Not in CISA KEV at last sync

Original advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-66339

Exploitation outlook

EPSS (FIRST.org) estimates each CVE’s probability of exploitation in the next 30 days — here is the CSIRTS.com read on those numbers.

Low exploitation riskCVE-2026-663390.23% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 14% of all EPSS-scored CVEs.

Referenced CVEs

CVE	CSIRTS overview	External
CVE-2026-66339	coverage & exploitation status	NVD · CVE.org

Same CVEs, other sources

How other CERTs, PSIRTs and databases cover the vulnerabilities in this advisory.

mediumCVE-2026-66339: A flaw was found in libsoup. After a CONNECT tunnel is established through an HTTP proxy, libs…nvd

Recent advisories for Libsoup

A cluster of recent advisories against the same product widens the attack surface — attackers routinely chain freshly published CVEs on one product, so review these together.

lowCVE-2026-3632: Libsoup: libsoup: http smuggling and server-side request forgery via malformed hostnamesmsrc · 2026-08-06
mediumCVE-2026-66338: Libsoup: libsoup: http request smuggling via permissive chunk-size parsing in soup_body_input_…msrc · 2026-08-06
mediumCVE-2026-66337: Libsoup: libsoup: heap buffer over-read via integer underflow in soup_filter_input_stream_read…msrc · 2026-08-06
lowCVE-2026-3633: Libsoup: libsoup: header and http request injection via crlf injectionmsrc · 2026-08-06
mediumCVE-2025-9901: Libsoup: improper handling of http vary header in libsoup cachingmsrc · 2026-08-06
mediumCVE-2026-3099: Libsoup: libsoup: authentication bypass via digest authentication replay attackmsrc · 2026-08-06