BREW-VS-PREVIEW-CVE-2026-97688 (GHSA-GH4C-6FX4-QH6G)
Vulnerability from osv_homebrew – Published: 2026-09-30 21:45 – Updated: 2026-09-30 21:45 – Source websiteImpact
urllib3's streaming API is designed for the efficient handling of large HTTP responses by reading content in chunks instead of loading the entire response body into memory at once.
urllib3 can decompress response bodies according to the HTTP Content-Encoding header. When streaming a compressed, chunked response, urllib3 first consumes data already buffered by the decoder before reading the next HTTP chunk.
However, urllib3 versions from 2.6.2 through 2.7.0 could enter an infinite loop when a chunked response contained bytes after the end of the Deflate stream. If the decompressed body exceeded the requested streaming chunk size, Python's zlib implementation could retain the trailing bytes as unconsumed input after reaching the end of the compressed stream. urllib3 would repeatedly attempt to decode those same bytes without making progress or reading more data from the network.
A malicious server could exploit this behavior to cause excessive CPU usage and prevent the affected request from completing on the client. Network read timeouts would not interrupt the loop because no further socket operation was required.
Affected usages
Applications and libraries using urllib3 versions 2.6.2 through 2.7.0 may be affected when all of the following conditions are met:
- Compressed responses from an untrusted source are streamed using
HTTPResponse.stream(amt=N)orHTTPResponse.read_chunked(amt=N)with a positive, finite chunk size. - Content decoding is enabled.
- The response uses both
Transfer-Encoding: chunkedandContent-Encoding: deflate. - The decoded body exceeds the requested chunk size and the encoded body contains bytes after the end of the Deflate stream.
HTTPResponse.stream() uses a finite chunk size by default and is therefore affected when the other conditions are met.
Remediation
Upgrade to urllib3 2.8.0, in which the Deflate decoder stops accepting input after reaching the end of the compressed stream and no longer reports trailing bytes as data that can produce more decoded output.
If upgrading is not immediately possible, disable automatic content decoding for responses from untrusted sources by setting decode_content=False, or reject streamed responses using the Deflate content encoding. Applications that disable automatic decoding must handle the compressed response safely at another layer.
{
"affected": [
{
"ecosystem_specific": {
"fix": null,
"range_state": "affected",
"resource": "urllib3",
"resource_purl": "pkg:pypi/urllib3@2.6.3",
"upstream_fixed_in": "2.8.0"
},
"package": {
"ecosystem": "Homebrew",
"name": "vs-preview",
"purl": "pkg:brew/vs-preview"
},
"ranges": [
{
"events": [
{
"introduced": "0.20.1"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"database_specific": {
"confidence": "high",
"source": "matched",
"strategy": "registry",
"upstream_evidence": [
{
"ecosystem": "PyPI",
"key": "pkg:pypi/urllib3@2.6.3",
"name": "urllib3",
"resource": "urllib3",
"strategy": "registry",
"subject_version": "2.6.3"
}
]
},
"details": "### Impact\n\nurllib3\u0027s streaming API is designed for the efficient handling of large HTTP responses by reading content in chunks instead of loading the entire response body into memory at once.\n\nurllib3 can decompress response bodies according to the HTTP `Content-Encoding` header. When streaming a compressed, chunked response, urllib3 first consumes data already buffered by the decoder before reading the next HTTP chunk.\n\nHowever, urllib3 versions from 2.6.2 through 2.7.0 could enter an infinite loop when a chunked response contained bytes after the end of the Deflate stream. If the decompressed body exceeded the requested streaming chunk size, Python\u0027s zlib implementation could retain the trailing bytes as unconsumed input after reaching the end of the compressed stream. urllib3 would repeatedly attempt to decode those same bytes without making progress or reading more data from the network.\n\nA malicious server could exploit this behavior to cause excessive CPU usage and prevent the affected request from completing on the client. Network read timeouts would not interrupt the loop because no further socket operation was required.\n\n### Affected usages\n\nApplications and libraries using urllib3 versions 2.6.2 through 2.7.0 may be affected when all of the following conditions are met:\n\n1. Compressed responses from an untrusted source are streamed using `HTTPResponse.stream(amt=N)` or `HTTPResponse.read_chunked(amt=N)` with a positive, finite chunk size.\n2. Content decoding is enabled.\n3. The response uses both `Transfer-Encoding: chunked` and `Content-Encoding: deflate`.\n4. The decoded body exceeds the requested chunk size and the encoded body contains bytes after the end of the Deflate stream.\n\n`HTTPResponse.stream()` uses a finite chunk size by default and is therefore affected when the other conditions are met.\n\n### Remediation\n\nUpgrade to urllib3 2.8.0, in which the Deflate decoder stops accepting input after reaching the end of the compressed stream and no longer reports trailing bytes as data that can produce more decoded output.\n\nIf upgrading is not immediately possible, disable automatic content decoding for responses from untrusted sources by setting `decode_content=False`, or reject streamed responses using the Deflate content encoding. Applications that disable automatic decoding must handle the compressed response safely at another layer.",
"id": "BREW-vs-preview-CVE-2026-97688",
"modified": "2026-09-30T21:45:02Z",
"published": "2026-09-30T21:45:02Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/urllib3/urllib3/security/advisories/GHSA-gh4c-6fx4-qh6g"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-97688"
},
{
"type": "WEB",
"url": "https://github.com/urllib3/urllib3/commit/ea2ad7b21a80da3632f80016526a18864586077f"
},
{
"type": "PACKAGE",
"url": "https://github.com/urllib3/urllib3"
},
{
"type": "WEB",
"url": "https://github.com/urllib3/urllib3/releases/tag/2.8.0"
}
],
"schema_version": "1.7.3",
"severity": [
{
"score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N",
"type": "CVSS_V4"
}
],
"summary": "urllib3: Chunked Deflate streaming can enter an infinite loop",
"upstream": [
"GHSA-gh4c-6fx4-qh6g",
"CVE-2026-97688"
]
}
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or observed by the user.
- Confirmed: The vulnerability has been validated from an analyst's perspective.
- 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.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.
Related by attack behaviour
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.