CVE-2026-71888

A flaw was found in Bouncy Castle. In its Cryptographic Message Syntax (CMS) streaming parser, messages with mismatched attribute and algorithm fields are improperly handled, allowing attributes to be returned as valid even though they were not protected by the Message Authentication Code (MAC). A remote attacker capable of intercepting and modifying network traffic can exploit this flaw by inserting unauthorized attributes into an otherwise valid message. This can allow an attacker to manipulate application behavior and bypass security checks if the application relies on those attributes to make authorization or routing decisions.

A flaw was found in Bouncy Castle's Cryptographic Message Syntax (CMS) streaming parser (CMSAuthenticatedDataParser). When parsing an AuthenticatedData structure where the digestAlgorithm field is absent but authenticated attributes (authAttrs) are present, the parser validates the Message Authentication Code (MAC) over content alone while returning authAttrs as authenticated. An attacker capable of modifying messages in transit can inject arbitrary attributes, such as security classification labels, into valid messages without holding MAC keys. This allows attackers to manipulate application authorization or routing decisions reliant on these attributes.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

Avoid using CMSAuthenticatedDataParser to stream untrusted CMS AuthenticatedData structures, or validate that messages containing authenticated attributes explicitly define a digestAlgorithm before trusting attribute values. Alternatively, process messages using non-streaming CMS parsers.