CVE-2026-5928 (GCVE-0-2026-5928)

Vulnerability from cvelistv5 – Published: 2026-04-20 20:37 – Updated: 2026-10-15 01:12
VLAI
Title
Potential buffer under-read in ungetwc
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.
Severity
7.5 (High)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
SSVC
Exploitation: poc Automatable: yes Technical Impact: partial
CISA Coordinator · CISA-ADP (v2.0.3)
Decision recorded 2026-04-21 16:10 UTC
CWE
Assigner
glibc CNA under the redhat root
CNA scorecard B 79/100 over 16 records in the last 180 days details
References
Impacted products
Vendor Product Version CPE status
The GNU C Library glibc Affected: 2.1.1-89 , ≤ 2.43 (custom)
guessed Create a notification for this product.
Siemens SIMATIC S7-1500 CPU 1518-4 PN/DP MFP Affected: V3.1.5 , < V3.1.6 (custom)
guessed Create a notification for this product.
Siemens SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP Affected: V3.1.5 , < V3.1.6 (custom)
guessed Create a notification for this product.
Siemens SIPLUS S7-1500 CPU 1518-4 PN/DP MFP Affected: V3.1.5 , < V3.1.6 (custom)
guessed Create a notification for this product.
Credits
Rocket Ma
Show details on NVD website
To clipboard 

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                    "role": "CISA Coordinator",
                    "timestamp": "2026-04-21T16:10:57.731635Z",
                    "version": "2.0.3"
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              "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
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                "tags": [
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                "url": "https://sourceware.org/bugzilla/show_bug.cgi?id=33998"
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            "title": "CISA ADP Vulnrichment"
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              "product": "glibc",
              "vendor": "The GNU C Library",
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              "type": "finder",
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                  "value": "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.\u003cbr\u003e\u003cbr\u003eA bug in the wide character pushback implementation (_IO_wdefault_pbackfail in libio/wgenops.c) causes ungetwc() to operate on the regular character buffer (fp-\u0026gt;_IO_read_ptr) instead of the actual wide-stream read pointer (fp-\u0026gt;_wide_data-\u0026gt;_IO_read_ptr). The program crash may happen in cases where fp-\u0026gt;_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."
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          "providerMetadata": {
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            "orgId": "3ff69d7a-14f2-4f67-a097-88dee7810d18",
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          "references": [
            {
              "url": "https://sourceware.org/bugzilla/show_bug.cgi?id=33998"
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          "source": {
            "discovery": "UNKNOWN"
          },
          "title": "Potential buffer under-read in ungetwc",
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        "assignerShortName": "glibc",
        "cveId": "CVE-2026-5928",
        "datePublished": "2026-04-20T20:37:31.743Z",
        "dateReserved": "2026-04-08T22:47:29.814Z",
        "dateUpdated": "2026-10-15T01:12:14.900Z",
        "state": "PUBLISHED"
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      "dataType": "CVE_RECORD",
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    }
  }
}



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Sightings

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Nomenclature

  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
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  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
  • Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
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Vulnerability timeline

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How durations are calculated

Following the VTEM framework (Black Hat USA 2015), time to patch starts when the vendor or a coordinator was informed, total time to patch at discovery, and disclosure to patch at public disclosure; vendor response time runs from the report to its acknowledgement. Time to exploit and time unpatched start at the earliest discovery or report, falling back to public disclosure. Unpatched time ends at the first recorded patch, or the calculation time when no patch is recorded. Exploitation to patch shows the interval between those two events; a published proof of concept is a public exploit, not exploitation.

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    Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.