CVE-2026-47734 — dulwich
MEDIUMCVE-2026-47734 is a medium-severity (CVSS 5.7) Uncontrolled Resource Consumption vulnerability in dulwich. A fix is available for dulwich — see the affected versions and patch details below.
Dulwich has unbounded memory allocation in receive-pack from crafted thin packs
Exploitation Status
No confirmed exploitation observed yet
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-47734.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2026-47734 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 391,915 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
dulwichReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
Impact
An uncontrolled-resource-consumption (memory exhaustion) denial-of-service vulnerability (CWE-400 / CWE-789).
A client with push access could push a tiny crafted thin pack (~174 bytes) whose delta header declares a huge dest_size. When dulwich ingested it via add_thin_pack / apply_delta, it would allocate hundreds of MB of memory based on that attacker-controlled size, with no relationship to the actual bytes received.
Who is impacted: Operators running a Dulwich-based Git server that exposes git-receive-pack (i.e. accepts pushes) - for example via dulwich.server functionality, the HTTP smart server, or anything built on ReceivePackHandler.
Patches
Patched in 1.2.5.
add_thin_pack now accepts a max_input_size keyword (bytes; 0/None = unlimited, matching git's semantics), and ReceivePackHandler reads receive.maxInputSize from the repository config and passes it through. Wire reads are counted and a PackInputTooLarge exception is raised once the cap is exceeded - equivalent to git index-pack --max-input-size.
Users should upgrade to Dulwich 1.2.5 or later and set receive.maxInputSize in their server's repository config to a sane bound for their environment.
Workarounds
On unpatched versions, receive.maxInputSize has no effect, so it cannot be used as a workaround. Until upgrading, operators should:
- Restrict dulwich-receive-pack (push) access to trusted, authenticated clients only, or disable it entirely on servers that only need to serve fetches.
- Run the server under an OS-level memory limit (e.g. ulimit, cgroups/MemoryMax, or a container memory limit) so a malicious push is killed rather than taking down the host.
Resources
- git's receive.maxInputSize / git index-pack --max-input-size documentation
- Reported by Liyi, Ziyue, Strick, Maurice and Chenchen @ University of Sydney
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | dulwich | ≥ 0.1.0&&< 1.2.5 | 1.2.5pip install --upgrade 'dulwich==1.2.5' |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for dulwich, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update dulwich to 1.2.5 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-47734 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
This Moderate impact denial of service flaw in Dulwich affects Red Hat products utilizing Dulwich-based Git server functionalities. An authenticated attacker with push access could send a crafted Git thin pack, causing excessive memory allocation and disrupting service availability. The vulnerability requires prior…
This flaw is fixed in Dulwich 1.2.5, which adds a max_input_size limit to add_thin_pack and makes ReceivePackHandler honor the receive.maxInputSize repository configuration setting (equivalent to git index-pack --max-input-size). Upgrade to Dulwich 1.2.5 or later and set receive.maxInputSize in the Git server's repository configuration to a sensible bound for the environment. On unpatched versions, receive.maxInputSize has no effect and cannot be used as a workaround. Until the fixed version is deployed, operators of Dulwich-based Git servers that accept pushes can restrict git-receive-pack (push) access to trusted, authenticated clients only, or disable push access entirely on servers that…Source: Red Hat security advisory for CVE-2026-47734 (CC BY 4.0)
Frequently Asked Questions
Is CVE-2026-47734 in your dependencies?
Find it across PyPI, including transitive dependencies.