{"cve_id":"CVE-2021-27850","published":"2021-04-15T08:15:14.823000+00:00","last_modified":"2026-06-17T03:45:30.777000+00:00","vuln_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_version":"3.1","cvss_vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H","base_score":9.8,"base_severity":"CRITICAL","attack_vector":"NETWORK","privileges_req":"NONE","user_interaction":"NONE","scope":"UNCHANGED","impact_c":"HIGH","impact_i":"HIGH","impact_a":"HIGH","cwe_ids":["CWE-200","CWE-502"],"in_kev":false,"kev_date_added":null,"kev_ransomware":false,"epss_score":0.9356,"epss_percentile":0.99841,"cap_rule_version":"0.1.1","stage":"INITIAL_ACCESS","req_access":"NONE","req_vector":"NETWORK","req_user_interaction":false,"grant_access":"USER","caps":["CODE_EXEC","INFO_DISCLOSURE"],"terminal":false,"cap_confidence":"high","ingested_at":"2026-09-18T09:37:49.285398+00:00","first_seen":"2021-04-15T08:15:14.823000+00:00","kev_first_seen":null,"novelty_score":0.0,"novelty_reasons":[],"novelty_at":null,"vendors":["apache"],"products":["tapestry"],"affected":[{"vendor":"apache","product":"tapestry","ranges":["≥ 5.4.0 and < 5.6.2","≥ 5.7.0 and < 5.7.1"]}]}