{"id":"CVE-2021-27850","aliases":["GHSA-mj8x-cpr8-x39h"],"url":"https://o3.security/vulnerability/CVE-2021-27850","summary":"Remote code execution in Apache Tapestry","details":"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.","published":"2021-04-15T08:15:14.823Z","modified":"2026-07-08T21:26:23.585831Z","cvss":{"score":9.8,"severity":"CRITICAL","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H"},"epss":{"score":0.93802,"percentile":0.9984,"asOf":"2026-08-23"},"cisaKev":null,"exploitsKnown":9,"affectedPackages":[{"ecosystem":"Maven","name":"org.apache.tapestry:tapestry-core","fixedVersion":"5.6.3"},{"ecosystem":"Maven","name":"org.apache.tapestry:tapestry-core","fixedVersion":"5.7.1"}],"fix":null,"references":[{"type":"ADVISORY","url":"https://lists.apache.org/thread.html/r237ff7f286bda31682c254550c1ebf92b0ec61329b32fbeb2d1c8751%40%3Cusers.tapestry.apache.org%3E"},{"type":"ADVISORY","url":"https://security.netapp.com/advisory/ntap-20210528-0002/"},{"type":"EVIDENCE","url":"http://www.openwall.com/lists/oss-security/2021/04/15/1"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-07-08T21:26:23.585831Z"}}