[committed] Document CVE-2026-18374 and new CVSS tag

The security team has been scoring issues recently, so also add the
score to the advisory file as the single source of truth.  Document the
new tag in README.

Signed-off-by: Siddhesh Poyarekar <[email protected]>
Reviewed-by: Carlos O'Donell <[email protected]>
---
 advisories/GLIBC-SA-2026-0015 | 16 ++++++++++++++++
 advisories/README             |  3 +++
 2 files changed, 19 insertions(+)
 create mode 100644 advisories/GLIBC-SA-2026-0015

diff --git a/advisories/GLIBC-SA-2026-0015 b/advisories/GLIBC-SA-2026-0015
new file mode 100644
index 0000000000..3ea6602399
--- /dev/null
+++ b/advisories/GLIBC-SA-2026-0015
@@ -0,0 +1,16 @@
+Buffer overflow in fopen mode argument processing
+
+Passing an effectively empty string to the `,ccs=` syntax extension of
+the mode argument in the `fopen` function in the GNU C Library version
+2.45 or earlier may result in a heap buffer overflow when the mode
+string input to the function is attacker controlled.
+
+This usage pattern is not seen in applications in common GNU or Linux
+distributions and applications that process user-supplied values for
+`ccs` should not pass them through without validation.
+
+CVE-Id: CVE-2026-18374
+Public-Date: 2026-08-27
+Vulnerable-Commit: 129d706d77587e4d6627cc1ebef9be0f7cbc65f0
+Reported-by: AISLE in partnership with Red Hat
+CVSS: CVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:L - 4.9
diff --git a/advisories/README b/advisories/README
index 330a31dff3..3adad91339 100644
--- a/advisories/README
+++ b/advisories/README
@@ -45,6 +45,9 @@ Reported-By:
 The entity that reported this issue. There could be multiple entries, one for
 each reporter.
 
+CVSS:
+CVSS score for the issue.
+
 Adding an Advisory
 ------------------
 
-- 
2.55.0