Security readout for executives and security teams
Plain-English summary
CVE-2026-1299 affects CPython’s email serialization. In limited circumstances, software that builds email headers using LiteralHeader could allow unintended extra headers when messages are serialized. This can affect message integrity, routing, or policy decisions. It is not listed as actively exploited in KEV, and exposure depends on specific application usage.
Executive priority
Treat as a targeted medium-priority engineering fix. Prioritize systems that generate outbound email from user-controlled data, such as support, CRM, notification, or workflow platforms. No cited source indicates active exploitation, so handle through normal patch and code-review cycles unless critical mail workflows are exposed.
Technical view
The CPython email.BytesGenerator class did not properly quote newlines in headers during serialization. The issue is CWE-93 header injection and applies when LiteralHeader is used to write headers that violate email folding rules. The updated behavior rejects incorrectly folded headers. CVSS v4.0 is 6.0 with network attack vector, low complexity, privileges required, and high integrity impact.
Likely exposure
Exposure is likely limited to Python applications or services using CPython’s email module with LiteralHeader and untrusted or poorly validated header content. The source bundle identifies CPython 3.11, 3.12, 3.13, 3.14, and 3.15.0a1 lines, but exact fixed release mapping is not fully provided.
Exploitation context
The attacker needs some level of privilege or application access to influence email header content. The CVSS vector lists attack requirements present and no user interaction. Sources do not state public exploitation, weaponized exploit availability, or CISA KEV inclusion.
Researcher notes
The core condition is narrow: BytesGenerator plus LiteralHeader misuse with headers that do not respect folding rules. The record references CVE-2024-6923 as related. Version and release-status details should be confirmed from PSF and downstream package advisories before declaring remediation complete.
Mitigation direction
- Review Python Software Foundation advisory and CPython issue for current fixed releases.
- Apply vendor-provided CPython updates or downstream distribution patches when available.
- Avoid using LiteralHeader with untrusted input unless strictly validated.
- Reject or normalize header values containing invalid newline folding.
- Add regression tests for email header serialization behavior.
Validation and detection
- Inventory services using CPython email.BytesGenerator or LiteralHeader.
- Identify Python versions across affected 3.11 through 3.15 alpha lines.
- Review code paths where users can influence email headers.
- Confirm deployed Python includes the referenced CPython patch set or vendor backport.
- Test that incorrectly folded headers are rejected, not serialized.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.
CWE-93: Exact CWE lookup
Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.
Open ATT&CK lookupCVE-2026-1299 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
Open ATT&CK lookup- Severity
- Medium
- CVSS
- 6 (4.0)
- Known Exploited
- No
- Published
Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:L/VI:H/VA:N/SC:N/SI:N/SA:N
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:L/VI:H/VA:N/SC:N/SI:N/SA:N——Primary CVE scoreVulnerability scoring details
Base CVSS 4.0 score
6MediumVector: CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:L/VI:H/VA:N/SC:N/SI:N/SA:N
Source materials
- CVE List V5 sourceCVE List V5
- https://github.com/python/cpython/pull/144126CVE reference · patch
- https://github.com/python/cpython/issues/144125CVE reference · issue-tracking
- https://cve.org/CVERecord?id=CVE-2024-6923CVE reference · related
- https://mail.python.org/archives/list/security-announce@python.org/thread/6ZZULGALJTITEAGEXLDJE2C6FORDXPBT/CVE reference · vendor-advisory
- https://github.com/python/cpython/commit/052e55e7d44718fe46cbba0ca995cb8fcc359413CVE reference · patch
- https://github.com/python/cpython/commit/0a925ab591c45d6638f37b5e57796f36fa0e56d8CVE reference · patch
- https://github.com/python/cpython/commit/7877fe424415bc4a13045e62a90a7277413d8cb9CVE reference · patch
- https://github.com/python/cpython/commit/842ce19a0c0b58d61591e8f6a708c38db1fb94e4CVE reference · patch
- https://github.com/python/cpython/commit/8cdf6204f4ae821f32993f8fc6bad0d318f95f36CVE reference · patch
- https://github.com/python/cpython/commit/e417f05ad77a4c30ddc07f99e90fc0cef43e831aCVE reference · patch
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Improper Neutralization of CRLF Sequences ('CRLF Injection')
Improper Neutralization of CRLF Sequences ('CRLF Injection') represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.