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CVE-2021-42321 is a post-authentication remote code execution vulnerability in Microsoft Exchange Server caused by unsafe deserialization of attacker-controlled data. The provided analysis describes the vulnerable path as involving Exchange deserialization logic in Microsoft.Exchange.Compliance.Serialization.Formatters.TypedBinaryFormatter, where the binder was not correctly passed to ExchangeBinaryFormatterFactory.CreateBinaryFormatter(). As described, this resulted in use of a ChainedSerializationBinder in non-strict mode, enabling insufficient type validation during BinaryFormatter deserialization. The write-up further states that deserialization of System.Security.Claims.ClaimsPrincipal triggered a secondary BinaryFormatter.Deserialize() call without effective filtering, creating the exploitable sink. The attacker-controlled serialized stream could be supplied through Exchange Web Services user configuration data, specifically the BinaryData field in a CreateUserConfiguration request, and later triggered via the GetClientAccessToken EWS operation. Successful exploitation allows execution of a gadget chain such as ClaimsPrincipal or TypeConfusedDelegate, leading to code execution in the Exchange worker process context.
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1 valid exploit after Mallory filtered fakes, detection scripts, and README-only repos.
This repository contains a proof-of-concept exploit for CVE-2021-42321, a remote code execution vulnerability in Microsoft Exchange Server. The main file, 'CVE-2021-42321_poc.py', is a Python script that crafts and sends a malicious serialized .NET payload (gadgetData) to the Exchange Web Services (EWS) SOAP API endpoint '/ews/exchange.asmx' on a target Exchange server. The script authenticates using NTLM credentials and performs a sequence of SOAP requests to first retrieve the inbox folder ID, delete any existing user configuration, and then create a new user configuration containing the malicious payload. The payload is designed to trigger code execution on the server (e.g., launching calc or mspaint). The exploit requires valid credentials and a vulnerable Exchange Server. The repository also includes a brief README referencing the CVE and the official Microsoft patch. No hardcoded endpoints are present; the user must specify the target Exchange server URL and credentials.
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A Microsoft Exchange vulnerability (CVE-2021-42321) referenced in the context of a threat actor claiming to sell an 'implementation' (i.e., exploit capability) for it to gain access to victim networks.
A vulnerability listed as one of several publicly available exploits leveraged by Forest Blizzard.
A post-auth remote code execution deserialization vulnerability in Microsoft Exchange Server. The content describes technical analysis of the deserialization sink and source, and shows how authenticated attackers can achieve RCE via EWS user configuration data and GetClientAccessToken.
A specific vulnerability in Microsoft Exchange that, per the content, was demonstrated at Tianfu Cup 2021 and then rapidly exploited in the wild shortly thereafter—suggesting fast operationalization by threat actors with early access.
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