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Fencer Vulns
high 7.5

CVE-2026-5928 json

Potential buffer under-read in ungetwc

published 2026-04-20 6 mo ago modified 2026-10-15 25 d ago also EUVD-2026-23980

cvss
7.5
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.4%
EPSS 0.4% chance of exploitation in 30 days, percentile 31%
public exploits
0
none known

Summary

Calling the ungetwc function on a FILE stream with wide characters encoded in a character set that has overlaps between its single byte and multi-byte character encodings, in the GNU C Library version 2.43 or earlier, may result in an attempt to read bytes before an allocated buffer, potentially resulting in unintentional disclosure of neighboring data in the heap, or a program crash.

A bug in the wide character pushback implementation (_IO_wdefault_pbackfail in libio/wgenops.c) causes ungetwc() to operate on the regular character buffer (fp->_IO_read_ptr) instead of the actual wide-stream read pointer (fp->_wide_data->_IO_read_ptr). The program crash may happen in cases where fp->_IO_read_ptr is not initialized and hence points to NULL. The buffer under-read requires a special situation where the input character encoding is such that there are overlaps between single byte representations and multibyte representations in that encoding, resulting in spurious matches. The spurious match case is not possible in the standard Unicode character sets.

Patch / fix

  • Siemens SIMATIC S7-1500 CPU 1518-4 PN/DP MFP V3.1.6
  • Siemens SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP V3.1.6
  • Siemens SIPLUS S7-1500 CPU 1518-4 PN/DP MFP V3.1.6
  • glibc Debian 13 2.41-12+deb13u4
  • glibc Debian 14 2.42-17

debian advisories DEBIAN-CVE-2026-5928

  • glibc Ubuntu 22.04 LTS 2.35-0ubuntu3.14
  • glibc Ubuntu 24.04 LTS 2.39-0ubuntu8.8
  • glibc Ubuntu 26.04 LTS 2.43-2ubuntu2.3

ubuntu advisories UBUNTU-CVE-2026-5928

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • glibc RHEL 9 2.34-274.el9_8
  • nscd RHEL 9 2.34-274.el9_8
  • libnsl RHEL 9 2.34-274.el9_8
  • nss_db RHEL 9 2.34-274.el9_8
  • nss_hesiod RHEL 9 2.34-274.el9_8
  • glibc-utils RHEL 9 2.34-274.el9_8
  • glibc-common RHEL 9 2.34-274.el9_8
  • glibc-benchtests RHEL 9 2.34-274.el9_8

red hat advisories RHSA-2026:42952RHSA-2026:42733RHSA-2026:42694RHSA-2026:12740

  • glibc SLES for SAP 15 SP5, SLES for SAP 15 SP4, SLES 15 SP5-LTSS +8 more 2.31-150300.101.1
  • glibc Linux Micro Extras 6.2 2.40-160000.5.1
  • glibc SLES LTSS Extended Security 12 SP5, SLES 12 SP5-LTSS 2.22-114.46.1

suse advisories SUSE-SU-2026:2440-1SUSE-SU-2026:2333-1SUSE-SU-2026:21780-1

  • glibc openSUSE Tumbleweed 2.43-3.1

opensuse advisories openSUSE-SU-2026:10770-1

  • glibc AlmaLinux 10 2.39-128.el10_2.alma.1
  • libnsl AlmaLinux 10 2.39-128.el10_2.alma.1
  • nss_db AlmaLinux 10 2.39-128.el10_2.alma.1
  • nss_hesiod AlmaLinux 10 2.39-128.el10_2.alma.1
  • glibc-utils AlmaLinux 10 2.39-128.el10_2.alma.1
  • glibc-common AlmaLinux 10 2.39-128.el10_2.alma.1
  • glibc-benchtests AlmaLinux 10 2.39-128.el10_2.alma.1
  • glibc-gconv-extra AlmaLinux 10 2.39-128.el10_2.alma.1

almalinux advisories ALSA-2026:42694ALSA-2026:42733ALSA-2026:42952

  • glibc Rocky Linux 9 2.34-274.el9_8
  • glibc Rocky Linux 10 2.39-128.el10_2
  • glibc Rocky Linux 8 2.28-251.el8_10.40

rocky linux advisories RLSA-2026:42952RLSA-2026:42694RLSA-2026:42733

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-5928 updated cvss 5.0

glibc: glibc: Information disclosure or denial of service via ungetwc function with specific wide character encodings

  • Red Hat Enterprise Linux 6 nscd, glibc, glibc-utils, compat-glibc, glibc-common Red Hat Enterprise Linux 7 nscd, glibc, glibc-utils, compat-glibc, glibc-common Red Hat OpenShift Container Platform 4 openshift/ose-rhel-coreos-8, openshift/ose-rhel-coreos-9
    affected: workaround, no fix available “Fix deferred”
  • Red Hat Cert Manager support for Red Hat OpenShift release 1.17 cert-manager/cert-manager-operator-rhel9 1791788287, cert-manager/jetstack-cert-manager-rhel9 1791371069, cert-manager/cert-manager-istio-csr-rhel9 1791378395, cert-manager/cert-manager-operator-bundle 1791970484, cert-manager/jetstack-cert-manager-acmesolver-rhel9 1791371080
    fixed: vendor fix RHSA-2026:82336
  • Red Hat Cert Manager support for Red Hat OpenShift release 1.18 cert-manager/cert-manager-operator-rhel9 1789029076, cert-manager/jetstack-cert-manager-rhel9 1791459711, cert-manager/cert-manager-istio-csr-rhel9 1791459759, cert-manager/cert-manager-operator-bundle 1791966259, cert-manager/jetstack-cert-manager-acmesolver-rhel9 1791459707
    fixed: vendor fix RHSA-2026:82354
  • Red Hat Cert Manager support for Red Hat OpenShift release 1.20 cert-manager/cert-manager-operator-rhel9 1790272426, cert-manager/cert-manager-operator-rhel9 1790589855, cert-manager/jetstack-cert-manager-rhel9 1790223279, cert-manager/jetstack-cert-manager-rhel9 1790589912, cert-manager/cert-manager-istio-csr-rhel9 1790223719, cert-manager/cert-manager-istio-csr-rhel9 1790589914, +2 more
    fixed: vendor fix RHSA-2026:72475
  • Red Hat Cost Management 4 costmanagement/costmanagement-metrics-rhel9-operator 1790844333
    fixed: vendor fix RHSA-2026:81201
  • Red Hat Discovery 2 discovery/discovery-ui-rhel9 1784821750, discovery/discovery-server-rhel9 1784821670
    fixed: vendor fix RHSA-2026:46836
  • Red Hat Enterprise Linux AppStream (v. 8) glibc-utils 2.28-251.el8_10.40, compat-libpthread-nonshared 2.28-251.el8_10.40
    fixed: vendor fix RHSA-2026:42733
  • Red Hat Enterprise Linux AppStream (v. 9) glibc-utils 2.34-274.el9_8, glibc-locale-source 2.34-274.el9_8
    fixed: vendor fix RHSA-2026:42952
show 12 more
  • Red Hat Enterprise Linux AppStream (v. 10) glibc-utils 2.39-128.el10_2, glibc-locale-source 2.39-128.el10_2
    fixed: vendor fix RHSA-2026:42694
  • Red Hat Enterprise Linux BaseOS (v. 8) nscd 2.28-251.el8_10.40, glibc 2.28-251.el8_10.40, libnsl 2.28-251.el8_10.40, nss_db 2.28-251.el8_10.40, glibc-common 2.28-251.el8_10.40, glibc-gconv-extra 2.28-251.el8_10.40, +3 more
    fixed: vendor fix RHSA-2026:42733
  • Red Hat Enterprise Linux BaseOS (v. 9) nscd 2.34-274.el9_8, glibc 2.34-274.el9_8, libnsl 2.34-274.el9_8, glibc-common 2.34-274.el9_8, glibc-gconv-extra 2.34-274.el9_8, glibc-all-langpacks 2.34-274.el9_8, +1 more
    fixed: vendor fix RHSA-2026:42952
  • Red Hat Enterprise Linux BaseOS (v. 10) glibc 2.39-128.el10_2, libnsl 2.39-128.el10_2, glibc-common 2.39-128.el10_2, glibc-gconv-extra 2.39-128.el10_2, glibc-all-langpacks 2.39-128.el10_2, glibc-minimal-langpack 2.39-128.el10_2
    fixed: vendor fix RHSA-2026:42694
  • Red Hat Enterprise Linux CodeReady Linux Builder (v. 9) nss_db 2.34-274.el9_8, nss_hesiod 2.34-274.el9_8, glibc-benchtests 2.34-274.el9_8
    fixed: vendor fix RHSA-2026:42952
  • Red Hat Enterprise Linux CodeReady Linux Builder (v. 10) nss_db 2.39-128.el10_2, nss_hesiod 2.39-128.el10_2, glibc-benchtests 2.39-128.el10_2
    fixed: vendor fix RHSA-2026:42694
  • Red Hat Enterprise Linux CRB (v. 8) nss_hesiod 2.28-251.el8_10.40, glibc-benchtests 2.28-251.el8_10.40
    fixed: vendor fix RHSA-2026:42733
  • Red Hat Hardened Images glibc 2.42-12.hum1, glibc32 2.42-12.hum1, glibc-utils 2.42-12.hum1, glibc-common 2.42-12.hum1, glibc-benchtests 2.42-12.hum1, glibc-gconv-extra 2.42-12.hum1, +5 more
    fixed: vendor fix RHSA-2026:12740
  • Red Hat Insights proxy 1.5 insights-proxy/insights-proxy-container-rhel9 1786433656
    fixed: vendor fix RHSA-2026:53371
  • Red Hat OpenShift distributed tracing 3.10.1 rhosdt/tempo-rhel9 1784775768, rhosdt/tempo-query-rhel9 1784775793, rhosdt/tempo-gateway-rhel9 1784775770, rhosdt/tempo-rhel9-operator 1784775782, rhosdt/tempo-operator-bundle 1784777166, rhosdt/tempo-gateway-opa-rhel9 1784775772, +3 more
    fixed: vendor fix RHSA-2026:50205
  • Red Hat Update Infrastructure 5 rhui5/cds-rhel9 1784794778, rhui5/rhua-rhel9 1784795076, rhui5/haproxy-rhel9 1784795112, rhui5/rhua-tp-rhel9 1787241260, rhui5/installer-rhel9 1784794289, rhui5/installer-tp-rhel9 1787135742, +2 more
    fixed: vendor fix RHSA-2026:44481
  • Red Hat Cert Manager support for Red Hat OpenShift release 1.20 cert-manager/jetstack-cert-manager-acmesolver-rhel9 1790223630 Red Hat Cost Management 4 costmanagement/costmanagement-metrics-operator-bundle 1790851152 Red Hat Hardened Images libnsl 2.42-12.hum1, nss_db 2.42-12.hum1, nss_hesiod 2.42-12.hum1, compat-libpthread-nonshared 2.42-12.hum1 Red Hat OpenShift AI (RHOAI) rhoai/odh-llama-stack-k8s-operator-rhel9 Red Hat OpenShift distributed tracing 3.10.1 rhosdt/opentelemetry-operator-bundle 1785744179, rhosdt/opentelemetry-target-allocator-rhel9 1785704551 Red Hat Update Infrastructure 5 rhui5/cds-tp-rhel9 1787241206
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
Proof of concept is public
Automatable
yes
Technical impact
partial
Assessed
6 mo ago

Exploit prediction

0.37% chance of exploitation in the next 30 days, higher than 31% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • sourceware.org https://sourceware.org/bugzilla/show_bug.cgi?id=33998 referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
CISA ADP 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
CNA 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Affected products

  • The GNU C Library glibc
  • Siemens SIMATIC S7-1500 CPU 1518-4 PN/DP MFP
  • Siemens SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP
  • Siemens SIPLUS S7-1500 CPU 1518-4 PN/DP MFP

Classification

Weakness

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.