ghsa-gm62-xv2j-4w53
Vulnerability from github
Impact
urllib3 supports chained HTTP encoding algorithms for response content according to RFC 9110 (e.g., Content-Encoding: gzip, zstd).
However, the number of links in the decompression chain was unbounded allowing a malicious server to insert a virtually unlimited number of compression steps leading to high CPU usage and massive memory allocation for the decompressed data.
Affected usages
Applications and libraries using urllib3 version 2.5.0 and earlier for HTTP requests to untrusted sources unless they disable content decoding explicitly.
Remediation
Upgrade to at least urllib3 v2.6.0 in which the library limits the number of links to 5.
If upgrading is not immediately possible, use preload_content=False and ensure that resp.headers["content-encoding"] contains a safe number of encodings before reading the response content.
{
"affected": [
{
"package": {
"ecosystem": "PyPI",
"name": "urllib3"
},
"ranges": [
{
"events": [
{
"introduced": "1.24"
},
{
"fixed": "2.6.0"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2025-66418"
],
"database_specific": {
"cwe_ids": [
"CWE-770"
],
"github_reviewed": true,
"github_reviewed_at": "2025-12-05T18:15:19Z",
"nvd_published_at": "2025-12-05T16:15:51Z",
"severity": "HIGH"
},
"details": "## Impact\n\nurllib3 supports chained HTTP encoding algorithms for response content according to RFC 9110 (e.g., `Content-Encoding: gzip, zstd`).\n\nHowever, the number of links in the decompression chain was unbounded allowing a malicious server to insert a virtually unlimited number of compression steps leading to high CPU usage and massive memory allocation for the decompressed data.\n\n\n## Affected usages\n\nApplications and libraries using urllib3 version 2.5.0 and earlier for HTTP requests to untrusted sources unless they disable content decoding explicitly.\n\n\n## Remediation\n\nUpgrade to at least urllib3 v2.6.0 in which the library limits the number of links to 5.\n\nIf upgrading is not immediately possible, use [`preload_content=False`](https://urllib3.readthedocs.io/en/2.5.0/advanced-usage.html#streaming-and-i-o) and ensure that `resp.headers[\"content-encoding\"]` contains a safe number of encodings before reading the response content.",
"id": "GHSA-gm62-xv2j-4w53",
"modified": "2025-12-05T18:32:59Z",
"published": "2025-12-05T18:15:19Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/urllib3/urllib3/security/advisories/GHSA-gm62-xv2j-4w53"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2025-66418"
},
{
"type": "WEB",
"url": "https://github.com/urllib3/urllib3/commit/24d7b67eac89f94e11003424bcf0d8f7b72222a8"
},
{
"type": "PACKAGE",
"url": "https://github.com/urllib3/urllib3"
}
],
"schema_version": "1.4.0",
"severity": [
{
"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",
"type": "CVSS_V4"
}
],
"summary": "urllib3 allows an unbounded number of links in the decompression chain"
}
Sightings
| Author | Source | Type | Date |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or seen somewhere by the user.
- Confirmed: The vulnerability is confirmed from an analyst perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: This vulnerability was exploited and seen by the user reporting the sighting.
- Patched: This vulnerability was successfully patched by the user reporting the sighting.
- Not exploited: This vulnerability was not exploited or seen by the user reporting the sighting.
- Not confirmed: The user expresses doubt about the veracity of the vulnerability.
- Not patched: This vulnerability was not successfully patched by the user reporting the sighting.