Security readout for executives and security teams
Plain-English summary
Affected curl/libcurl versions can accidentally send usernames or passwords embedded in a URL to another server through the HTTP Referer header. This is an information exposure issue, not code execution. Business impact is highest where applications still use credential-bearing URLs and automatic Referer behavior.
Executive priority
Treat this as a targeted confidentiality cleanup, not an emergency outage event. Prioritize internet-facing services, automation, and appliances that may embed credentials in URLs. Remediate through normal patch cycles unless evidence shows credential leakage in logs or third-party requests.
Technical view
curl 7.1.1 through 7.75.0 can leak URL user credentials when libcurl automatically populates the Referer header for a subsequent HTTP request. The issue is classified as CWE-359 with CVSS 3.1 score 5.3, affecting confidentiality only. Public sources do not show CISA KEV listing.
Likely exposure
Exposure is likely in older operating systems, appliances, containers, or applications bundling curl/libcurl 7.1.1 through 7.75.0. Risk requires credentials to be present in URLs and Referer auto-population to occur. Products from several vendors issued advisories, so transitive exposure is plausible.
Exploitation context
The source bundle does not support active exploitation claims. The practical risk is passive credential disclosure to the destination server receiving the second HTTP request, or to systems logging that request. No exploit steps are needed to understand the exposure pattern.
Researcher notes
Key questions are whether the local build uses affected curl/libcurl, whether applications allow credentials in URL userinfo, and whether automatic Referer generation is enabled. Evidence in the bundle supports credential leakage via Referer, but not active exploitation or broader impact beyond confidentiality.
Mitigation direction
- Upgrade curl/libcurl beyond the affected version range using vendor-supported packages.
- Apply relevant OS or product security advisories from affected vendors.
- Remove credentials from URLs wherever applications or scripts construct requests.
- Disable automatic Referer population where credential-bearing URLs may be used.
- Review vendor guidance before changing behavior in embedded or appliance-managed curl builds.
Validation and detection
- Inventory curl and libcurl versions across hosts, containers, appliances, and embedded products.
- Flag versions from 7.1.1 through 7.75.0 as affected.
- Identify applications using credential-bearing URLs with curl/libcurl HTTP requests.
- Review HTTP logs for Referer headers containing URL userinfo credentials.
- Confirm vendor advisory applicability for packaged products such as OS distributions or appliances.
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-359: 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-2021-22876 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
- 5.3 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A: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:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N3.91.4Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
5.3MediumVector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
Source materials
- CVE List V5 sourceCVE List V5
- https://hackerone.com/reports/1101882CVE reference · x_refsource_MISC
- https://curl.se/docs/CVE-2021-22876.htmlCVE reference · x_refsource_MISC
- FEDORA-2021-cab5c9befbCVE reference · vendor-advisory, x_refsource_FEDORA
- FEDORA-2021-065371f385CVE reference · vendor-advisory, x_refsource_FEDORA
- FEDORA-2021-26a293c72bCVE reference · vendor-advisory, x_refsource_FEDORA
- [debian-lts-announce] 20210517 [SECURITY] [DLA 2664-1] curl security updateCVE reference · mailing-list, x_refsource_MLIST
- GLSA-202105-36CVE reference · vendor-advisory, x_refsource_GENTOO
- https://www.oracle.com//security-alerts/cpujul2021.htmlCVE reference · x_refsource_MISC
- https://security.netapp.com/advisory/ntap-20210521-0007/CVE reference · x_refsource_CONFIRM
- https://cert-portal.siemens.com/productcert/pdf/ssa-389290.pdfCVE reference · x_refsource_CONFIRM
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.