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Unbounded memory buffering in SelectorSocketTransport.writelines()

Type: advisorySources: OSV

Starting in Python 3.12.0, the asyncio._SelectorSocketTransport.writelines() method would not "pause" writing and signal to the Protocol to drain the buffer to the wire once the write buffer reached the "high-water mark". Because of this, Protocols would not periodically drain the write buffer potentially leading to memory exhaustion. This vulnerability likely impacts a small number of users, you must be using Python 3.12.0 or later, on macOS or Linux, using the asyncio module with protocols, and using .writelines() method which had new zero-copy-on-write behavior in Python 3.12.0 and later. If not all of these factors are true then your usage of Python is unaffected.

Products
Not specified
Affected versions
Not specified
Fixed versions
Not specified
CVEs
CVE-2024-12254
Confirmed fixed CVEs
Not specified
Updated in this feed
4 Nov 2026, 18:16 UTC · Revision 1

Source evidence

Unbounded memory buffering in SelectorSocketTransport.writelines()

core-osv · Original source

Published 6 Dec 2024, 15:19 UTC

Starting in Python 3.12.0, the asyncio._SelectorSocketTransport.writelines() method would not "pause" writing and signal to the Protocol to drain the buffer to the wire once the write buffer reached the "high-water mark". Because of this, Protocols would not periodically drain the write buffer potentially leading to memory exhaustion. This vulnerability likely impacts a small number of users, you must be using Python 3.12.0 or later, on macOS or Linux, using the asyncio module with protocols, and using .writelines() method which had new zero-copy-on-write behavior in Python 3.12.0 and later. If not all of these factors are true then your usage of Python is unaffected.

osv:CVE-2024-12254
Complete source record
{
  "id": "f3de6b26ee4dda13ff8dfcf51b4123bd791d2af07a320cb4369e8f94329ad23a",
  "url": "https://osv.dev/vulnerability/CVE-2024-12254",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "Unbounded memory buffering in SelectorSocketTransport.writelines()",
    "summary": "Starting in Python 3.12.0, the asyncio._SelectorSocketTransport.writelines() method would not \"pause\" writing and signal to the Protocol to drain the buffer to the wire once the write buffer reached the \"high-water mark\". Because of this, Protocols would not periodically drain the write buffer potentially leading to memory exhaustion. This vulnerability likely impacts a small number of users, you must be using Python 3.12.0 or later, on macOS or Linux, using the asyncio module with protocols, and using .writelines() method which had new zero-copy-on-write behavior in Python 3.12.0 and later. If not all of these factors are true then your usage of Python is unaffected.",
    "evidence": "osv:CVE-2024-12254",
    "products": [],
    "severity": "high",
    "structured": {
      "id": "CVE-2024-12254",
      "aliases": [
        "BIT-libpython-2024-12254",
        "BIT-python-2024-12254",
        "BIT-python-min-2024-12254",
        "CVE-2024-12254",
        "PSF-2024-14"
      ],
      "metrics": [
        {
          "type": "CVSS_V4",
          "score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N"
        }
      ],
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                {
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          "database_specific": {
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          }
        }
      ],
      "modified": "2026-10-28T02:30:38.497249616Z",
      "provider": "osv",
      "references": [
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2024/12xxx/CVE-2024-12254.json",
        "https://mail.python.org/archives/list/security-announce@python.org/thread/H4O3UBAOAQQXGT4RE3E4XQYR5XLROORB/",
        "https://nvd.nist.gov/vuln/detail/CVE-2024-12254",
        "https://security.netapp.com/advisory/ntap-20250404-0010/",
        "https://github.com/python/cpython/issues/127655",
        "https://github.com/python/cpython/commit/71e8429ac8e2adc10084ab5ec29a62f4b6671a82",
        "https://github.com/python/cpython/commit/9aa0deb2eef2655a1029ba228527b152353135b5",
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        "https://github.com/python/cpython/pull/127656",
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      "provider_metadata": {
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        "cna_assigner": "PSF",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2024/12xxx/CVE-2024-12254.json"
      }
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    "related_cves": [
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        "value": "CVE-2024-12254",
        "evidence": "/id"
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    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
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  "source_id": "core-osv",
  "document_id": 505635,
  "published_at": "2024-12-06T15:19:41.576+00:00"
}