CriticalPublic exploit

Unauthenticated RCE in Apache Tapestry asset handling

IdentifiersCVE-2021-27850CWE-22

CVE-2021-27850 is a critical unauthenticated remote code execution vulnerability in recent Apache Tapestry versions caused by an incomplete fix for CVE-2019-0195. Tapestry's asset handling attempted to block retrieval of sensitive classpath resources using a blacklist that rejected paths ending in .class, .properties, or .xml. This protection could be bypassed by appending a trailing slash to the requested asset path, for example requesting an asset URL ending in AppModule.class/. After the blacklist check, the trailing slash was stripped and the underlying class file was still served. This allowed unauthenticated attackers to download arbitrary sensitive class files from the classpath, notably AppModule.class, which commonly contains the application's HMAC secret key. With that secret, an attacker could forge signed serialized Java objects and deliver a gadget chain such as CommonsBeanUtils1 to achieve remote code execution.

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ANALYST BRIEF

Impact, mitigation & remediation

What it means. What to do now. Patch path, mitigations, and the assume-compromise checklist.

Impact

What an attacker gets, and what they’ve been doing with it.

Successful exploitation allows an unauthenticated remote attacker to retrieve sensitive application class files from the classpath, recover the HMAC signing secret used by the application, and then craft valid signed serialized payloads that the application will trust. In practice this can lead to full remote code execution in the context of the Tapestry application, with corresponding risks of complete application compromise, data theft or modification, deployment of additional malware or tooling, and potential lateral movement depending on the privileges of the application process.

Mitigation

If you can’t patch tonight, do this now.

If immediate patching is not possible, restrict or disable external access to Tapestry asset endpoints where feasible, place the application behind filtering or WAF rules that block suspicious asset requests targeting .class, .properties, or .xml resources including variants with trailing slashes or path normalization tricks, and rotate any exposed HMAC signing secrets. Because exploitation depends on recovering the signing key and then abusing trusted deserialization, review whether serialized object handling can be disabled or constrained and remove vulnerable gadget libraries where possible. These are temporary measures and do not replace upgrading.

Remediation

Patch, then assume compromise.

Upgrade Apache Tapestry to a fixed release. Based on the provided content, affected 5.4.x through 5.6.x installations should be upgraded to 5.6.3 or later, and 5.7.0 should be upgraded to 5.7.1 or later. More generally, deploy a version that fully fixes the CVE-2019-0195 bypass in asset path validation and prevents exposure of classpath resources such as .class files regardless of path normalization tricks like trailing slashes.

PUBLIC EXPLOITS

Exploits

No valid public exploits. Mallory filtered out 1 candidate as fakes, detection scripts, or README-only repos.

VALID 0 / 1 TOTALView more in app

All candidate exploits were filtered out by Mallory's validation.

EXPOSURE SURFACE

Affected products & vendors

Products and vendors Mallory has correlated with this vulnerability. Open in Mallory to drill down to specific CPE configurations and version ranges.

VendorProductType

Apache Software FoundationTapestryapplication

Apache Software FoundationTapestry-Coreapplication

Vendor-confirmed product mapping. Mallory continuously reconciles this list against your asset inventory.

ACTIVITY FEED

Recent activity

No public activity tracked yet. Mallory keeps watching.

0 SOURCESView more in app

No public activity observed for this vulnerability.

What this page doesn’t show

The version that knows your environment.

This page is what’s public. Mallory adds the parts that aren’t: which of your assets are affected, which adversaries are exploiting it right now, which detections to deploy, and what to do tonight.

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Exposure mapping

Query your assets running an affected version, and investigate the blast radius.

Threat actor evidence

Every observed campaign linking this CVE to a named adversary.

Associated malware

Malware families riding this exploit, with evidence and IOCs.

Detection signatures1

YARA, Sigma, Snort, and vendor rules, auto-deployed to your SIEM.

Vendor-by-vendor mapping

Cross-references every affected SKU, including bundled OEM variants.

Social activity

Community discussion across Reddit, Mastodon, and other social sources.

EXTERNAL REFERENCES

MITRE CVE

cve.org · CVE-2021-27850

NVD

nvd.nist.gov/vuln/detail/CVE-2021-27850

MITRE CWE · CWE-22

cwe.mitre.org

Vendor Advisory

lists.apache.org/thread.html/r237ff7f286bda31682c254550c1ebf92b0ec61329b32fbeb2d1c8751%40%3Cusers.tapestry.apache.org%3E

Third Party Advisory

openwall.com/lists/oss-security/2021/04/15/1

Third Party Advisory

security.netapp.com/advisory/ntap-20210528-0002

Unauthenticated RCE in Apache Tapestry asset handling

IdentifiersCVE-2021-27850CWE-22