GHSA-GM62-XV2J-4W53
Vulnerability from github – Published: 2025-12-05 18:15 – Updated: 2025-12-05 18:32Impact
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 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.