{"cve":"CVE-2021-27850","nvd":{"published":"2021-04-15T08:15:14.823","modified":"2026-06-17T03:45:30.777","status":"Modified","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.","cvss_v4":null,"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H","score":9.8},"cwes":["CWE-502","CWE-200"],"references":["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"],"products":["apache:tapestry"],"source":"nvd","synced_at":"2026-10-19T21:17:01.373894544+00:00"},"epss":{"cve":"CVE-2021-27850","score":0.9356,"percentile":0.99841,"date":"2026-10-31"},"kev":null,"osv":[{"id":"GHSA-mj8x-cpr8-x39h","url":"https://osv.dev/vulnerability/GHSA-mj8x-cpr8-x39h","summary":"Remote code execution in Apache Tapestry","modified":"2024-12-02T05:47:35.826691Z","packages":[{"ecosystem":"Maven","name":"org.apache.tapestry:tapestry-core","purl":"pkg:maven/org.apache.tapestry/tapestry-core","ranges":[">=5.4.0 <5.6.3"],"fixed":["5.6.3"]},{"ecosystem":"Maven","name":"org.apache.tapestry:tapestry-core","purl":"pkg:maven/org.apache.tapestry/tapestry-core","ranges":[">=5.7.0 <5.7.1"],"fixed":["5.7.1"]}]}],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}