Security Bulletin
Summary
Multiple vulnerabilities in the curl/libcurl are related to use-after-free, reusing an invalid connection, bypass Public Suffix List checks, improper host key validation and invalid authorization.
Vulnerability Details
CVEID: CVE-2026-10536
DESCRIPTION: A use-after-free vulnerability exists in libcurl when an application
configures an HTTP/2 stream-dependency tree via `CURLOPT_STREAM_DEPENDS` or
`CURLOPT_STREAM_DEPENDS_E`, subsequently invokes `curl_easy_reset()`, and
finally terminates the handle with `curl_easy_cleanup()`. During this final
cleanup phase, libcurl attempts to access and modify an internal structure
that was already freed during the reset operation.
CWE: CWE-416: Use After Free
CVSS Source: CISA ADP
CVSS Base score: 9.8
CVSS Vector: (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H)
CVEID: CVE-2026-11856
DESCRIPTION: Successfully using libcurl to do a transfer to a specific HTTP origin
(`hostA`) with **Digest** authentication and then changing the origin to a
different one (`hostB`) for a second transfer, reusing the same handle, makes
libcurl wrongly pass on the `Authorization:` header field meant for `hostA`,
to `hostB`.
CWE: CWE-294: Authentication Bypass by Capture-replay
CVSS Source: CISA ADP
CVSS Base score: 9.8
CVSS Vector: (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H)
CVEID: CVE-2026-8286
DESCRIPTION: A vulnerability exists where a new transfer that uses STARTTLS to upgrade the
connection might reuse an existing live connection even though the TLS
configuration mismatches so it should not.
CWE: CWE-295: Improper Certificate Validation
CVSS Source: CISA ADP
CVSS Base score: 8.1
CVSS Vector: (CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N)
CVEID: CVE-2026-8458
DESCRIPTION: libcurl might in some circumstances reuse the wrong connection when asked to
do Negotiate-authenticated ones, even when they are set to use different
'services'.
libcurl features a pool of recent connections so that subsequent requests can
reuse an existing connection to avoid overhead.
When reusing a connection a range of criteria must be met. Due to a logical
error in the code, a request that was issued by an application could
wrongfully reuse an existing connection to the same server that was
authenticated using different services.
CVSS Source: CISA ADP
CVSS Base score: 6.5
CVSS Vector: (CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:N)
CVEID: CVE-2026-8924
DESCRIPTION: A flaw in curl’s cookie parsing logic allows a malicious HTTP server to set
'super cookies' that bypass the Public Suffix List check. This enables an
attacker-controlled origin to inject cookies that curl subsequently scopes and
transmits to unrelated third-party domains.
CVSS Source: CISA ADP
CVSS Base score: 9.1
CVSS Vector: (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N)
CVEID: CVE-2026-8927
DESCRIPTION: When reusing a libcurl handle for sequential transfers driven by
environment-variable proxy configuration, libcurl fails to clear the proxy
authentication state between requests. Specifically, if the initial transfer
authenticates against `proxyA` using Digest auth, a subsequent transfer routed
through `proxyB` erroneously leaks the `Proxy-Authorization:` header intended
solely for `proxyA`.
CWE: CWE-294: Authentication Bypass by Capture-replay
CVSS Source: CISA ADP
CVSS Base score: 9.1
CVSS Vector: (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N)
CVEID: CVE-2026-8932
DESCRIPTION: libcurl would reuse a previously created connection even when some mTLS config
related option had been changed that should have prohibited reuse.
libcurl keeps previously used connections in a connection pool for subsequent
transfers to reuse if one of them matches the setup. However, some TLS
settings related to client certificates were left out from the configuration
match checks, making them match too easily. In particular options related to
the private key.
CVSS Source: CISA ADP
CVSS Base score: 7.5
CVSS Vector: (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N)
CVEID: CVE-2026-9547
DESCRIPTION: When a libcurl-based application performs transfers via `SCP://` or `SFTP://`
and utilizes the `CURLOPT_SSH_KEYFUNCTION` callback, it may silently accept an
untrusted server. This vulnerability occurs when a server presents a host key
type that does not match the specific key type already recorded for that host
in the `known_hosts` file. Instead of rejecting the mismatch, the callback
mechanism fails to properly enforce the restriction, allowing the connection
to succeed without warning and risking a potential man-in-the-middle attack.
CVSS Source: CISA ADP
CVSS Base score: 7.4
CVSS Vector: (CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N)
Affected Products and Versions
| Affected Product(s) | Version(s) |
| AIX | 7.3 |
The following fileset levels are vulnerable:
key_fileset = aix
Fileset Lower Level Upper Level KEY
---------------------------------------------------------
oss.lib.libcurl 7.79.1.0 7.79.1.0 key_w_fs
oss.lib.libcurl 8.1.2.0 8.1.2.0 key_w_fs
oss.lib.libcurl 8.5.0.0 8.5.0.2 key_w_fs
Note: This bulletin does not apply to versions of curl installed from
the AIX Toolbox.
To find out whether the affected filesets are installed
on your systems, refer to the lslpp command found in AIX user's guide.
Example: lslpp -L | grep -i oss.lib.libcurl
Remediation/Fixes
A. FIXES
IBM strongly recommends addressing the vulnerability now.
AIX fixes are available.
The AIX fixes can be downloaded via https from:
https://aix.software.ibm.com/aix/efixes/security/curl_fix11.tar
The link above is to a tar file containing this signed
advisory, fix packages, and OpenSSL signatures for each package.
The fixes below include prerequisite checking. This will
enforce the correct mapping between the fixes and AIX
Technology Levels.
Note that the tar file contains Interim fixes that are based on
Curl version, and AIX Curl fixes are cumulative.
AIX Level Interim Fix (*.Z) KEY
--------------------------------------------------
7.3 10536ma.260730.epkg.Z key_w_fix
Please reference the Affected Products and Version section above
for help with checking installed fileset levels.
To extract the fixes from the tar file:
tar xvf curl_fix11.tar
cd curl_fix11
Verify you have retrieved the fixes intact:
The checksums below were generated using the
"openssl dgst -sha256 [filename]" command as the following:
openssl dgst -sha256 filename KEY
-----------------------------------------------------------------------------------------------------
f48598dcb663bc879bef600b33c8d456fb3f2275cd5790826e0da376b52e4876 10536ma.260730.epkg.Z key_w_csum
The checksums below were generated using the
"openssl dgst -sha512 [filename]" command as the following:
openssl dgst -sha512 filename
KEY
---------------------------------------------------------------------------------------------------------------------------------------------------
---------------------
766433a6b680f316b5bf66d826f3f6b651fb5b9ba940f1dd9c8a861adf207deb68d8579108c57dbcc8561cd9428cea9a45acd7af7fd2da130977006d96bdbb40 10536ma.260730.epkg.Z key_4K_w_csum
These sums should match exactly. The OpenSSL signatures in the tar
file and on this advisory can also be used to verify the integrityi
of the fixes. If the sums or signatures cannot be confirmed, contact
IBM Support at http://ibm.com/support/ and describe the discrepancy.
openssl dgst -sha256 -verify [pubkey_file] -signature [advisory_file].sig [advisory_file]
openssl dgst -sha256 -verify [pubkey_file] -signature [ifix_file].sig [ifix_file]
Published advisory OpenSSL signature file location:
https://aix.software.ibm.com/aix/efixes/security/curl_advisory11.asc.sig
B. FIX AND INTERIM FIX INSTALLATION
If possible, it is recommended that a mksysb backup of the system
be created. Verify it is both bootable and readable before
proceeding.
To preview a fix installation:
installp -a -d fix_name -p all # where fix_name is the name of the
# fix package being previewed.
To install a fix package:
installp -a -d fix_name -X all # where fix_name is the name of the
# fix package being installed.
Interim fixes have had limited functional and regression
testing but not the full regression testing that takes place
for Service Packs; however, IBM does fully support them.
Interim fix management documentation can be found at:
https://www.ibm.com/support/pages/managing-interim-fixes-aix
To preview an interim fix installation:
emgr -e ipkg_name -p # where ipkg_name is the name of the
# interim fix package being previewed.
To install an interim fix package:
emgr -e ipkg_name -X # where ipkg_name is the name of the
# interim fix package being installed.
Workarounds and Mitigations
None
Get Notified about Future Security Bulletins
References
Acknowledgement
Change History
29 Jul 2026: Initial Publication
Second Issued: Thu Jul 30 07:54:12 CDT 2026 (Fix is updated for dependency on openssl.)
*The CVSS Environment Score is customer environment specific and will ultimately impact the Overall CVSS Score. Customers can evaluate the impact of this vulnerability in their environments by accessing the links in the Reference section of this Security Bulletin.
Disclaimer
According to the Forum of Incident Response and Security Teams (FIRST), the Common Vulnerability Scoring System (CVSS) is an "industry open standard designed to convey vulnerability severity and help to determine urgency and priority of response." IBM PROVIDES THE CVSS SCORES ""AS IS"" WITHOUT WARRANTY OF ANY KIND, INCLUDING THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. CUSTOMERS ARE RESPONSIBLE FOR ASSESSING THE IMPACT OF ANY ACTUAL OR POTENTIAL SECURITY VULNERABILITY. In addition to other efforts to address potential vulnerabilities, IBM periodically updates the record of components contained in our product offerings. As part of that effort, if IBM identifies previously unidentified packages in a product/service inventory, we address relevant vulnerabilities regardless of CVE date. Inclusion of an older CVEID does not demonstrate that the referenced product has been used by IBM since that date, nor that IBM was aware of a vulnerability as of that date. We are making clients aware of relevant vulnerabilities as we become aware of them. "Affected Products and Versions" referenced in IBM Security Bulletins are intended to be only products and versions that are supported by IBM and have not passed their end-of-support or warranty date. Thus, failure to reference unsupported or extended-support products and versions in this Security Bulletin does not constitute a determination by IBM that they are unaffected by the vulnerability. Reference to one or more unsupported versions in this Security Bulletin shall not create an obligation for IBM to provide fixes for any unsupported or extended-support products or versions.
Document Location
Worldwide
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Document Information
Modified date:
30 July 2026
Initial Publish date:
29 July 2026
UID
ibm17281743