CVE-2026-97689: urllib3 HTTPResponse.stream()/read_chunked() Buffers An Unbounded Chunk-size Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-vxq7-64xx-v4gw",
  "modified" : "2026-09-30T14:46:18Z",
  "published" : "2026-09-30T14:46:18Z",
  "aliases" : [ "CVE-2026-97689" ],
  "summary" : "urllib3: HTTPResponse.stream()/read_chunked() buffers an unbounded chunk-size line into memory",
  "details" : "## Impact\n\nurllib3's [streaming API](https://urllib3.readthedocs.io/en/2.7.0/advanced-usage.html#streaming-and-i-o) is designed for efficiently handling large HTTP responses by reading the content in chunks, rather than loading the entire response body into memory at once. When decoding a [chunked-transfer-encoded response](https://httpwg.org/specs/rfc9112.html#chunked.encoding), this API reads each chunk's size field by buffering until it sees `\\n` or EOF.\n\nA malicious HTTP server can return `Transfer-Encoding: chunked` and then send a very long run of bytes without any newline, causing the streaming client to buffer that entire run before urllib3 can reject the chunk size as invalid and allocate more memory than intended.\n\nTo fix the issue, we'll reject chunk-size fields larger than 65536 bytes, as already done in the non-streaming case, which is currently handled by the Python standard library. Note that this only applies to reading the chunk-size field, not the chunk data, which is already handled correctly. Thus, there shouldn't be any impact for non-malicious servers.\n\n## Affected usages\n\nApplications and libraries using urllib3 versions earlier than 2.8.0 may be affected when streaming a chunked response from untrusted sources. Specifically, this affects the [read_chunked()](https://urllib3.readthedocs.io/en/stable/reference/urllib3.response.html#urllib3.response.BaseHTTPResponse.read_chunked) and [stream()](https://urllib3.readthedocs.io/en/stable/reference/urllib3.response.html#urllib3.response.BaseHTTPResponse.stream) methods of the HTTPResponse object.\n\nThis also affects requests streaming API, which uses urllib3 under the hood.\n\n## Remediation\n\nUpgrade to urllib3 version 2.8.0 or later, where chunk-size fields larger than 65536 will be rejected. If upgrading is not immediately possible, consider reading the response at once using the [read()](https://urllib3.readthedocs.io/en/stable/reference/urllib3.response.html#urllib3.response.BaseHTTPResponse.read) method.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "urllib3"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "1.10.3"
      }, {
        "fixed" : "2.8.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/urllib3/urllib3/security/advisories/GHSA-vxq7-64xx-v4gw"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-97689"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/urllib3/urllib3/commit/cd770b059b543be29298ea5c52afb0b1b090f5ed"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/urllib3/urllib3"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/urllib3/urllib3/releases/tag/2.8.0"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-770" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-30T14:46:18Z",
    "nvd_published_at" : "2026-09-29T16:17:18Z"
  }
}