CVE-2026-89011 in isomorphic-git
Summary
by MITRE • 09/10/2026
isomorphic-git before 1.42.0 contains a prototype pollution vulnerability in the getRemoteInfo function that allows a malicious Git server operator to pollute Object.prototype by advertising crafted ref names containing '__proto__' path segments during ref negotiation. Attackers controlling a Git server can advertise a specially crafted ref such as '__proto__/corsProxy' to reroute all subsequent network operations through an attacker-controlled proxy, causing isomorphic-git to invoke the victim's onAuth callback and transmit credentials to the attacker when the victim calls getRemoteInfo with an attacker-supplied URL.
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Analysis
by VulDB Data Team • 09/10/2026
The vulnerability identified in versions of isomorphic-git prior to 1.42.0 represents a critical prototype pollution flaw within the getRemoteInfo function, exploiting how JavaScript handles object property assignment and inheritance chains. In standard JavaScript environments, objects inherit properties from their constructor's prototype chain. When an application attempts to set a property on an object that does not explicitly define it, the engine traverses up the prototype chain until it finds or creates the property at the top level of Object.prototype. This vulnerability arises because isomorphic-git processes ref names advertised by remote Git servers without sufficiently sanitizing path segments such as _proto_, constructor, or proto. By accepting these special keys as part of a URL path during the negotiation phase, the library inadvertently allows an attacker to inject properties directly into the global object prototype, affecting all subsequent instances and operations within the same execution context.
The operational impact of this vulnerability is severe due to its potential for credential theft and network redirection. A malicious Git server operator can advertise a crafted ref name containing _proto_ followed by specific path segments like corsProxy during the initial handshake or negotiation process. When the victim application invokes getRemoteInfo with a URL supplied or influenced by the attacker, the library parses this input and assigns properties to Object.prototype based on the polluted keys. This manipulation effectively reroutes subsequent network operations through an intermediary controlled by the attacker. Consequently, when isomorphic-git attempts to fetch data or authenticate against what it believes to be the legitimate remote repository, the traffic is intercepted. The victim's authentication credentials are then transmitted directly to the attacker-controlled proxy rather than the intended destination, leading to a complete compromise of user identity and potentially sensitive code repositories stored within those projects.
From a classification perspective, this issue aligns with CWE-1325, which covers Incorrect Check for Unusual or Exceptional Conditions, specifically regarding the failure to validate input that could lead to prototype pollution. It also maps closely to CWE-915, Improper Modification of Object Attributes, as it involves altering the structure and behavior of core JavaScript objects in an unintended manner. In terms of offensive security frameworks, this vulnerability facilitates techniques associated with MITRE ATT&CK T1078, Valid Accounts, by allowing attackers to harvest credentials through man-in-the-middle interception enabled by prototype pollution. The attack vector is primarily remote via network interaction with a compromised or malicious Git server, requiring no local access but relying on the victim initiating a connection using vulnerable library versions.
Mitigation strategies must focus strictly on upgrading the underlying dependency and implementing defensive coding practices for any custom implementations. Users of isomorphic-git should immediately upgrade to version 1.42.0 or later, where this specific prototype pollution vector has been addressed through stricter input validation and path normalization that prevents special JavaScript keys from being interpreted as object properties during URL parsing. For developers maintaining their own Git client integrations, it is essential to sanitize all inputs derived from remote sources before using them in object construction or property assignment. Utilizing Object.create(null) for creating new objects instead of default object literals can also mitigate the risk of prototype pollution by removing the inheritance chain entirely. Additionally, implementing Content Security Policy headers and validating certificate chains during network operations can provide an additional layer of defense against man-in-the-middle attacks that exploit such protocol-level vulnerabilities.