CVE-2025-9901: Libsoup: improper handling of http vary header in libsoup caching

mediumCVSS 5.9CVE-2025-9901

CSIRTS triage

vendor: GNOME product: libsoupMisconfiguration

What: Improper handling of the HTTP Vary header in libsoup caching allows cache confusion or bypass.
Who is affected: Applications using libsoup for HTTP caching with malformed or malicious Vary headers.
Urgency: Medium priority with a CVSS score of 5.9; not yet exploited.
Action: Update libsoup to the patched version.

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 5.9
Published: 2026-08-06
Exploitation: Not in CISA KEV at last sync

Original advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2025-9901

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-2025-99010.46% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 37% of all EPSS-scored CVEs.

Referenced CVEs

CVE	CSIRTS overview	External
CVE-2025-9901	coverage & exploitation status	NVD · CVE.org

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
lowCVE-2026-3633: Libsoup: libsoup: header and http request injection via crlf injectionmsrc · 2026-08-06
lowCVE-2026-3634: Libsoup: libsoup: http header injection and response splitting via crlf injection in content-ty…msrc · 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-2026-66339: Libsoup: libsoup: proxy credentials leak to destination server via proxy-authorization header …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