{
    "id": "CVE-2021-27850",
    "published": "2021-04-15 08:15:14",
    "last_modified": "2026-06-17 03:45:30",
    "cvss_score": "9.8",
    "cvss_severity": "CRITICAL",
    "cvss_version": "3.1",
    "cvss_vector": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H",
    "cwe": "CWE-200",
    "description": "A critical unauthenticated remote code execution vulnerability was found all recent versions of Apache Tapestry. The affected versions include 5.4.5, 5.5.0, 5.6.2 and 5.7.0. The vulnerability I have found is a bypass of the fix for CVE-2019-0195. Recap: Before the fix of CVE-2019-0195 it was possible to download arbitrary class files from the classpath by providing a crafted asset file URL. An attacker was able to download the file `AppModule.class` by requesting the URL `http://localhost:8080/assets/something/services/AppModule.class` which contains a HMAC secret key. The fix for that bug was a blacklist filter that checks if the URL ends with `.class`, `.properties` or `.xml`. Bypass: Unfortunately, the blacklist solution can simply be bypassed by appending a `/` at the end of the URL: `http://localhost:8080/assets/something/services/AppModule.class/` The slash is stripped after the blacklist check and the file `AppModule.class` is loaded into the response. This class usually contains the HMAC secret key which is used to sign serialized Java objects. With the knowledge of that key an attacker can sign a Java gadget chain that leads to RCE (e.g. CommonsBeanUtils1 from ysoserial). Solution for this vulnerability: * For Apache Tapestry 5.4.0 to 5.6.1, upgrade to 5.6.2 or later. * For Apache Tapestry 5.7.0, upgrade to 5.7.1 or later.",
    "epss_score": "0.93471",
    "epss_percentile": "0.99838",
    "kev": 0,
    "kev_due": null,
    "has_exploit": 0,
    "updated_at": "2026-09-26 18:48:29",
    "priority": {
        "rank": 2,
        "label": "Patch early",
        "why": "EPSS 93.5% — above the 10% action threshold."
    },
    "products": [
        {
            "vendor": "apache",
            "product": "tapestry"
        }
    ],
    "kev_detail": null,
    "exploits": [],
    "refs_list": [
        "http://www.openwall.com/lists/oss-security/2021/04/15/1",
        "https://lists.apache.org/thread.html/r237ff7f286bda31682c254550c1ebf92b0ec61329b32fbeb2d1c8751%40%3Cusers.tapestry.apache.org%3E",
        "https://security.netapp.com/advisory/ntap-20210528-0002/",
        "http://www.openwall.com/lists/oss-security/2021/04/15/1",
        "https://lists.apache.org/thread.html/r237ff7f286bda31682c254550c1ebf92b0ec61329b32fbeb2d1c8751%40%3Cusers.tapestry.apache.org%3E",
        "https://security.netapp.com/advisory/ntap-20210528-0002/"
    ]
}