# CVE-2026-63476: isomorphic-git unbound buffer allocation in packfile delta

Source: https://junglewise.ai/threats/cve-2026-63476-isomorphic-git-is-a-pure-javascript-implementation-of-git-for  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-11-03. Updated: 2026-11-03  
License: CC BY 4.0 with a link back

CVE-2026-63476 · Severity: medium · CVSS 4.3 · Published 2026-11-03

isomorphic-git is a JavaScript implementation of Git for Node.js and browsers. Before version 1.38.6, it allocates up to 2 GB of memory without validation when a remote server sends a malicious packfile delta during git.clone() or git.fetch(). This can exhaust memory and crash containerized applications, serverless functions, and small virtual machines.

Technologies: [Isomorphic-Git](https://junglewise.ai/threats/technologies/isomorphic-git-isomorphic-git). Vendors: [Isomorphic-Git](https://junglewise.ai/threats/vendors/isomorphic-git).

## Quick answers

**Is CVE-2026-63476 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-63476 being exploited in the wild.

**How severe is CVE-2026-63476?** Medium severity, with a CVSS score of 4.3, published 3 November 2026.

**Which products and versions are affected?** Isomorphic-Git before 1.38.6.

**Is there a fix for CVE-2026-63476?** Yes: fixed in 1.38.6. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-63476 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-63476?** Check whether you run Isomorphic-Git at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- Fixed in 1.38.6

## Technical details

The vulnerability exists in src/utils/applyDelta.js where a server-controlled targetSize value from a delta header is passed directly to Buffer.alloc() without bounds checking. An attacker in a network position (man-in-the-middle or controlling a malicious Git server) can craft a delta with an oversized targetSize to trigger allocation of approximately 2 GB before the actual delta output is validated. The error is later silently swallowed by GitPackIndex.fromPack, allowing resource exhaustion on memory-constrained systems.

## Affected products

- isomorphic-git isomorphic-git before 1.38.6

## Timeline

- 2026-11-03: disclosed
- 2026-07-01: patched

## References

- [https://github.com/isomorphic-git/isomorphic-git/commit/823843d1cd19182df79bc907c767f5bcecb39a0b](https://github.com/isomorphic-git/isomorphic-git/commit/823843d1cd19182df79bc907c767f5bcecb39a0b)
- [https://github.com/isomorphic-git/isomorphic-git/pull/2370](https://github.com/isomorphic-git/isomorphic-git/pull/2370)

## Related threats

- [isomorphic-git path traversal in tree parser](https://junglewise.ai/threats/cve-2026-63478-isomorphic-git-is-a-pure-javascript-implementation-of-git-for)
- [isomorphic-git circular symref denial of service](https://junglewise.ai/threats/cve-2026-63475-isomorphic-git-is-a-pure-javascript-implementation-of-git-for)
- [isomorphic-git symlink path traversal in checkout](https://junglewise.ai/threats/cve-2026-63395-isomorphic-git-is-a-pure-javascript-implementation-of-git-for)