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Although NTLM authentication is considered weak (extending even to
NTLMv2, which purportedly allows brute-forcing reasonably complex
8-character passwords in a matter of days, given ample compute
resources), it _is_ one of the authentication methods supported by
libcurl.
Note: The added test case *cannot* reuse the existing `custom_auth`
facility. The reason is that that facility is backed by an NPH script
("No Parse Headers"), which does not allow handling the 3-phase NTLM
authentication correctly (in my hands, the NPH script would not even be
called upon the Type 3 message, a "200 OK" would be returned, but no
headers, let alone the `git http-backend` output as payload). Having a
separate NTLM authentication script makes the exact workings clearer and
more readable, anyway.
Co-authored-by: Matthew John Cheetham <mjcheetham@outlook.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
NTLM authentication is relatively weak. This is the case even with the
default setting of modern Windows versions, where NTLMv1 and LanManager
are disabled and only NTLMv2 is enabled: NTLMv2 hashes of even
reasonably complex 8-character passwords can be broken in a matter of
days, given enough compute resources.
Even worse: On Windows, NTLM authentication uses Security Support
Provider Interface ("SSPI"), which provides the credentials without
requiring the user to type them in.
Which means that an attacker could talk an unsuspecting user into
cloning from a server that is under the attacker's control and extracts
the user's NTLMv2 hash without their knowledge.
For that reason, let's disallow NTLM authentication by default.
NTLM authentication is quite simple to set up, though, and therefore
there are still some on-prem Azure DevOps setups out there whose users
and/or automation rely on this type of authentication. To give them an
escape hatch, introduce the `http.<url>.allowNTLMAuth` config setting
that can be set to `true` to opt back into using NTLM for a specific
remote repository.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
The new default of Git is to disable NTLM authentication by default. To help users find the escape hatch of that config setting, should they need it, suggest it when the authentication failed and the server had offered NTLM, i.e. if re-enabling it would fix the problem. Helped-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
The previous commits disabled NTLM authentication by default due to its cryptographic weaknesses. Users can re-enable it via the config setting http.<url>.allowNTLMAuth, but this requires manual intervention. Credential helpers may have knowledge about which servers are trusted for NTLM authentication (e.g., known on-prem Azure DevOps instances). To allow them to signal this trust, introduce a simple negotiation: when NTLM is suppressed and the server offered it, Git advertises ntlm=suppressed to the credential helper. The helper can respond with ntlm=allow to re-enable NTLM for this request. This happens precisely at the point where we would otherwise warn the user about NTLM being suppressed, ensuring the capability is only advertised when relevant. Helped-by: Matthew John Cheetham <mjcheetham@outlook.com> Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
This topic branch addresses the following vulnerability: - **CVE-2025-66413**: When a user clones a repository from an attacker-controlled server, Git may attempt NTLM authentication and disclose the user's NTLMv2 hash to the remote server. Since NTLM hashing is weak, the captured hash can potentially be brute-forced to recover the user's credentials. This is addressed by disabling NTLM authentication by default. (GHSA-hv9c-4jm9-jh3x) Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Git for Windows v2.53.0(2)
Changes since Git for Windows v2.53.0 (February 2nd 2026):
This is a security fix release, addressing CVE-2025-66413.
* CVE-2025-66413, Git for Windows: When a user clones a repository
from an attacker-controlled server, Git may attempt NTLM
authentication and disclose the user's NTLMv2 hash to the remote
server. Since NTLM hashing is weak, the captured hash can
potentially be brute-forced to recover the user's credentials. This
is addressed by disabling NTLM authentication by default.
dscho
approved these changes
Mar 11, 2026
derrickstolee
approved these changes
Mar 11, 2026
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Merging in the
v2.53.0.windows.2tag containing the security fixes for CVE-2025-66413.