CVE-2026-63476 in isomorphic-git
Summary
by MITRE • 11/03/2026
isomorphic-git is a pure JavaScript implementation of Git for Node.js and browsers. Prior to 1.38.6, isomorphic-git reads a server-controlled targetSize while applying a packfile delta in src/utils/applyDelta.js and passes it without a bound to Buffer.alloc(targetSize). A malicious or man-in-the-middle-controlled remote can send one delta header that causes git.clone() or git.fetch() to allocate approximately 2 GB before validating the actual delta output. GitPackIndex.fromPack swallows the later size-mismatch error, so a host with sufficient memory may retain the resource waste without a visible failure, while containers, serverless functions, and small virtual machines may terminate. Node.js heap limits do not mitigate the off-heap Buffer allocation. This issue is fixed in version 1.38.6.
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Analysis
by VulDB Data Team • 11/03/2026
The vulnerability identified in isomorphic-git versions prior to 1.38.6 represents a critical resource exhaustion flaw rooted in improper input validation during packfile delta application. As a pure JavaScript implementation of Git designed for Node.js and browser environments, the library processes remote repository data which includes binary packfiles containing commit history and object deltas. The core technical failure occurs within the src/utils/applyDelta.js module where the system reads a targetSize parameter directly from the server-controlled delta header without performing any bounds checking or sanity validation before memory allocation. This untrusted value is passed immediately to Node.js Buffer.alloc, which reserves heap space corresponding exactly to the specified size. Because this allocation happens prior to the actual cryptographic and structural verification of the delta data, an attacker can manipulate a single field in the remote packfile header to request massive amounts of memory, effectively bypassing standard application-level logic that might otherwise limit resource consumption based on expected repository sizes or historical patterns.
The operational impact of this flaw is severe due to the specific behavior of Node.js memory management and how isomorphic-git handles subsequent errors. When a malicious actor sends a delta header with an inflated targetSize, such as one requesting approximately two gigabytes of RAM, the application attempts to allocate that contiguous block of memory immediately. In environments with sufficient physical or virtual memory, this allocation may succeed initially, but it creates significant performance degradation and potential instability for other processes sharing the host resources. More critically, if the subsequent validation of the delta output fails due to size mismatch or corruption, isomorphic-git swallows the resulting error rather than propagating a clear exception or releasing the allocated buffer efficiently in all contexts. This means that on hosts with ample memory, the resource waste may persist without an immediate visible failure, leading to gradual degradation known as a slow-kill denial of service. However, for constrained environments such as containers, serverless functions, and small virtual machines which typically operate under strict memory limits or have limited swap space, this action triggers an out-of-memory condition that causes the process to terminate abruptly. This results in a reliable remote denial of service attack vector where any client performing git.clone() or git.fetch() against a compromised or maliciously configured repository can crash their application instance.
This vulnerability is categorized under CWE-400 Uncontrolled Resource Consumption, as it allows an external actor to dictate the amount of system resources consumed by the victim process through crafted input data. Furthermore, from a threat modeling perspective aligned with MITRE ATT&CK techniques, this aligns with T1496 Resource Hijacking or potentially T1498 Network Denial of Service depending on whether the goal is simply disruption or resource theft for cryptomining in broader contexts, though here it primarily serves as an availability attack. The fact that Node.js heap limits do not mitigate off-heap Buffer allocations exacerbates the risk because traditional memory protection mechanisms within the JavaScript runtime environment are bypassed by direct native buffer allocation requests triggered by unvalidated user input. This highlights a common pitfall in libraries interacting with low-level system resources where trust boundaries must be strictly enforced at the point of data ingestion rather than relying on downstream validation or environmental constraints to prevent abuse.
Mitigation for this vulnerability requires an immediate upgrade to isomorphic-git version 1.38.6 or later, which implements proper bounds checking and validation logic before allocating memory based on delta headers. Developers integrating older versions into their codebases should consider implementing network-level filtering if possible, such as limiting the size of incoming HTTP responses from Git servers, although this is less effective against sophisticated attacks that might use multiple smaller requests to exhaust resources cumulatively. Additionally, running these applications in environments with strict memory quotas and monitoring tools can help detect anomalous spikes in RAM usage indicative of such an attack, allowing for automated restarts or alerts before total system failure occurs. Security teams should also audit other areas of the codebase where external binary data is processed to ensure similar patterns of unvalidated size parameters are not present in other modules handling packfiles or delta operations.