Libsoup: libsoup: http/2 client crash in on_data_source_read_callback when settings initial_window_size shrinks during deferred body read
Published Sep 4, 2026
5.9
MEDIUMCVSS 3.1
EPSS 0.32%
Description
A flaw was found in libsoup. When a client sends an HTTP/2 request body from a non-pollable input stream, the library can buffer more data than the current flow-control window later allows. A malicious HTTP/2 server can shrink SETTINGS_INITIAL_WINDOW_SIZE while that buffered read is still in progress. The client then copies the full buffer into a smaller DATA callback without a runtime bounds check, which can abort the process or fail the HTTP/2 session.
Affected products
Default status is the baseline for the product, each version can override it (e.g. patched versions marked unaffected).
| Vendor | Product | Default status | Versions | |||
|---|---|---|---|---|---|---|
| Red Hat | Red Hat Enterprise Linux 10 | affected |
|
No data.
No data.
Red Hat Enterprise Linux 10
libsoup3
Fix deferred
Red Hat Enterprise Linux 6
libsoup
Not affected
Red Hat Enterprise Linux 7
libsoup
Not affected
Red Hat Enterprise Linux 8
libsoup
Not affected
Red Hat Enterprise Linux 9
libsoup
Not affected
| Product | Package | State | Advisory |
|---|---|---|---|
| Red Hat Enterprise Linux 10 | libsoup3 | Fix deferred | n/a |
| Red Hat Enterprise Linux 6 | libsoup | Not affected | n/a |
| Red Hat Enterprise Linux 7 | libsoup | Not affected | n/a |
| Red Hat Enterprise Linux 8 | libsoup | Not affected | n/a |
| Red Hat Enterprise Linux 9 | libsoup | Not affected | n/a |
No package ranges for this CVE.
Remediation
Vendor solution
Do not HTTP/2-upload from non-pollable streams (e.g. GFileInputStream) to untrusted servers. soup_message_set_request_body_from_bytes() is the pollable path. Assertions abort instead of returning an oversized length; that is not a fix.
Red Hat statement
A libsoup 3 HTTP/2 client can abort or drop the session when it uploads a request body from a non-pollable stream (for example GFileInputStream) to a malicious server that shrinks SETTINGS_INITIAL_WINDOW_SIZE during the deferred read. Red Hat builds keep GLib assertions enabled, so the usual result is a client process abort rather than memory corruption. Exploitation requires the client to connect to an attacker-controlled HTTP/2 endpoint and use that request-body path. Version affected: libsoup >= 3.0.0
Red Hat mitigation
Do not HTTP/2-upload from non-pollable streams (e.g. GFileInputStream) to untrusted servers. soup_message_set_request_body_from_bytes() is the pollable path. Assertions abort instead of returning an oversized length; that is not a fix.
Metrics
No CVSS v4.0 score for this CVE.
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
No CVSS v3.0 score for this CVE.
No CVSS v2.0 score for this CVE.
This CVE is not in the KEV list.
CISA SSVC (Vulnrichment)
Stakeholder-Specific Vulnerability Categorization from CISA ADP.
Exploitation
NoneAutomatable
NoTechnical Impact
PartialDecision
n/aAssessed Sep 4, 2026 · SSVC 2.0.3
Estimated probability of exploitation in the wild in the next 30 days (FIRST EPSS). As of Oct 4, 2026.
Score over time
Sep-Oct 2026- EPSS v5
Percentile over time
- EPSS v5
Table of values (2 key points)
Flat stretches are collapsed. Showing up to 120 newest points.
| Date | Score | Percentile | Model |
|---|---|---|---|
| Oct 4, 2026 | 0.32% (0.00315) | 22.25th | v5 (v2026.06.15) |
| Sep 5, 2026 | 0.32% (0.00315) | 23.99th | v5 (v2026.06.15) |
References (6)
- https://access.redhat.com/security/cve/CVE-2026-85534 vdb-entryx_refsource_REDHATVendor Advisory
- https://bugzilla.redhat.com/show_bug.cgi?id=2528440 issue-trackingx_refsource_REDHATIssue Tracking
- https://gitlab.gnome.org/GNOME/libsoup/-/blob/master/libsoup/http2/soup-client-message-io-http2.c
- https://gitlab.gnome.org/GNOME/libsoup/-/work_items/551
- https://nvd.nist.gov/vuln/detail/CVE-2026-85534
- https://www.cve.org/CVERecord?id=CVE-2026-85534
| Link | Providers | Tags |
|---|---|---|
| https://access.redhat.com/security/cve/CVE-2026-85534 | vdb-entryx_refsource_REDHATVendor Advisory | |
| https://bugzilla.redhat.com/show_bug.cgi?id=2528440 | issue-trackingx_refsource_REDHATIssue Tracking | |
| https://gitlab.gnome.org/GNOME/libsoup/-/blob/master/libsoup/http2/soup-client-message-io-http2.c | ||
| https://gitlab.gnome.org/GNOME/libsoup/-/work_items/551 | ||
| https://nvd.nist.gov/vuln/detail/CVE-2026-85534 | ||
| https://www.cve.org/CVERecord?id=CVE-2026-85534 |
Change history (0)
No recorded changes yet.